multiarea ospf slides
TRANSCRIPT
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Ch.6 OSPFPart 2 of 2: Multi-Area OSPF
CCNP version 3.0
Rick Graziani
Cabrillo College
* Updated by HoonJae Lee (Dongseo Univ.)
2006. 3. 3
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Note
Optional: This lab coincides with my Multi-area OSPF Labhandout, Rick's OSPF Scenarios (Word doc)
Most of the slides in this presentation do not come directlyfrom CCNP 1 version 3.0, OSPF, but slides which containa little more detail and explanation of OSPF.
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Optional: Ricks OSPF Scenarios
Optional: We will be using the following handout for this presentation:
Ricks OSPF Handout:
1. OSPF Multi-Area - All Normal Areas
2. OSPF Multi-Area - Stub Area
3. OSPF Multi-Area - Totally Stubby Area
This handouts can be downloaded from (Word doc):
http://www.cabrillo.cc.ca.us/ciscoacad/curriculum/presentations/semester5/OSPF_Scenario_Handout.doc
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Part I - LSAs using all normal areas
Multi Area OSPF
Normal Areas
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Suggestion to Instructors:Draw this network on thewhite-board as it will be usedfor discussion throughoutthese slides.
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Topics
Areas LSAs show ip ospf database (summary of link state database) show ip route Stub Areas Totally Stubby Areas E1 and E2 routes Default Routes Route Summarization NSSA (Not So Stubby Areas) Multiple ABR Scenario
Multiple ASBR Scenario Virtual Links Load Balancing show commands
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Issues with large OSPF nets
Large link-state table
Each router maintains a LSDB for all links in the area
The LSDB requires the use of memory
Frequent SPF calculations
A topology change in an area causes each router to re-run SPF to
rebuild the SPF tree and the routing table.
A flapping link will affect an entire area.
SPF re-calculations are done only for changes within that area.
Large routing table
Typically, the larger the area the larger the routing table.
A larger routing table requires more memory and takes more time toperform the route look-ups.
Solution: Divide the network into multiple areas
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OSPF uses Areas
Hierarchical routing enables you to separate large internetworks (autonomoussystems) into smaller internetworks that are called areas.
With this technique, routing still occurs between the areas (called inter-arearouting), but many of the smaller internal routing operations, such asrecalculating the database re-running the SPF algorithm, are restricted withinan area.
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Question: I understand the routing table is recalculated every time the routerreceives an new version of an LSA. Does OSPF recalculate its routing tablewhen their is a topology change in another area? show ip ospf displays nochange in SPF execution, but show ip ospf database shows a change in thetopology?
Answer: Good question! OSPF areas are designed to keep issues like flappinglinks within an area. SPF is not recalculated if the topology change is in
another area. The interesting thing is that OSPF distributes inter-area (betweenareas) topology information using a distance-vector method. OSPF uses link-state principles only within an area. ABRs do not announce topologicalinformation between areas, instead, only routing information is injected intoother areas. ABRs relay routing information between areas via distance vectortechnique similar to RIP or IGRP. This is why show ip ospf does not show achange in the number of times SPF has been executed when the topologychange is in another area.
Note: It is still a good idea to perform route summarization between areas,announcing multiple routes as a single inter-area route. This will hide anychanges in one area from affecting routing tables in other areas.
An advantage of Multiple Areas
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OSPF uses Areas
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OSPF Router Types
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OSPF Router Types
InternalInternal: Routers with all their interfaces within the same area
BackboneBackbone: Routers with at least one interface connected to area 0
ASBRASBR(Autonomous System Boundary Router): Routers that have at leastone interface connected to an external internetwork (anotherautonomous system)
ABRABR(Area Border Router): Routers with interfaces attached to multipleareas.
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OSPF Packet Types
Last week we discussed various OSPF packets, used for: Means for dynamic neighbor discovery
Detect unreachable neighbors within a finite period of time
Ensure two-way communications between neighbors
Ensure correctness of basic interace parameters between neighbors
Provide necessary information for the election of the Designated andBackup Designated routers on a LAN segement
Request link state information from another router
Sharing data base summary and detailed information
Acknowledge the receipt of an OSPF packet
OSPF packet typesOSPF Type-2 (DBD)
OSPF Type-3 (LSR)
OSPF Type-4 (LSU)
OSPF Type-5 (LSAck)
OSPF Type-1 (Hello)
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OSPF Type 4 - Link State Advertisements
This week we will look at OSPF Type 4 packets more closely
OSPF packet types
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OSPF packet types (Extra)
OSPF Type-4 packets have 7 LSA packets (later)
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LSAs used for discovering routes and reaching Full State, along withMaintain Routes
LSA Types
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LSA Types
LSA Types 1 through 5
We will look at these in detail as we discuss areas in this chapter.
LSA Type 6 MOSPF (Multicast OSPF)
Not supported by Cisco. MOSPF enhances OSPF by letting routers use their link-state
databases to build multicast distribution trees for the forwarding ofmulticast traffic.
LSA Type 7 NSSA External Link Entry
Originated by an ASBR connected to an NSSA. Type 7 messages can be flooded throughout NSSAs and translated
into LSA Type 5 messages by ABRs.
Routes learned via Type-7 LSAs are denoted by either a N1 or andN2 in the routing table. (Compare to E1 and E2).
We will discuss this more later when we look at NSSA areas.
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Area Types
Standard or Normal AreasBackbone
Non-Backbone
StubStub Area
Totally Stubby Area (TSA)
Not-so-stubby-area (NSSA)
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Area Types
(Standard area):(Backbone area):, 0., OSPFOSPF.(Stubby area): OSPF OSPF,ASdefault route(0.0.0.0/0).(TSA, Totally Stubby area):ASAS., 0.0.0.0/0.. NSSA(Not-So-Stubby area): 7 LSA, 7 LSA 5 LSA.
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Part I - LSAs using all normal areas
Multi Area OSPF
Normal Areas
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Routes Received on all OSPF Routers
Overview of Normal Areas This will all be explained!
Receives all routes from within A.S.:
Within the local area LSA 1 and LSA 2
From other areas (Inter-Area) LSA 3, LSA 4, LSA 5
Receives all routes from External A.S.s (External AS means routes not from thisOSPF routing domain):
From external ASs LSA 5 As long as routes are being redistributed by the ASBR (more later)
Default Routes
Received only if default-information-originate command was used (later) If default-information-originate command is not used, then the default
route is not received
Part I - LSAs using all normal areas
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Your Turn - In groups, examine running-configs
Look at the running-configs for 1. OSPF Multi-Areas- All Normal Areas
Look at the OSPF network statements!
Part I - LSAs using all normal areas
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1. OSPF Multi-Areas - All Normal Areas
ASBRrouter ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
!
ip classless
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0ip route 13.0.0.0 255.0.0.0 Null0
ABR-1interface FastEthernet0/0
ip address 172.16.1.2 255.255.255.0
ip ospf priority 200
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.51.0 0.0.0.255 area 51
ABR-2interface FastEthernet0
ip address 172.16.1.3 255.255.255.0
ip ospf priority 100
!
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.10.4 0.0.0.3 area 1
Internalrouter ospf 1
network 172.16.0.0 0.0.255.255 area 1
ABR contains networkstatements for each areait belongs to, using theproper area value.
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Understanding LSAs
show ip ospf database This is not the link state database, only a summary.
It is a tool to help determine what routes are included in the routing table.
We will look at this output to learn the tool as well as become familiar withthe different types of LSAs.
To view the link state database use: show ip ospf database[router|network|]
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LSA Header
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LSA 1 Router LSA Generated by each router for each area it belongs to. Describes the states of the links in the area to which this router belongs.
Tells the other routers in the area about itself and its links to adjacent OSPFrouters, and leaf networks.
Flooded only within the area. On multi-access networks, sent to the DR. Denoted by just an O in the routing table or C if the network is directly
connected.
ABR will include a set of LSA 1s for each area it belongs to.
A C
D
2
5
B
15 Router As LSA 1swhich are flooded to all
other routers in thisarea.
Leaf network
LSA 1 - Router Link States
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LSA 1 - Router Link States
When a new LSA 1 is received and installed in the LSDB, the routerforwards that LSA, using hop-by-hop or asynchronous flooding.
The LSA is sent out all OSPF interfaces that are in the Exchange Stateor a higher state.
For interfaces in Exstart or lesser state, the router will wait until it is outof Exstart.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
LSA 1 - Router Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID
192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s being sentwithin Area 0
LSA 1LSA 1LSA 1
LSA 1 - Router Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID
192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s being sentwithin other areas
LSA 1
LSA 1
LSA 1
LSA 1
LSA 1 - Router Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1Originated
LSA 1s are flooded outother interfaces withinthe same area.
LSA 1flooded
LSA 1 - Router Link States
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LSA 1 - Router Link States
For Router Links:
The Link State ID is always the same as the Advertising Router Advertising Router is the Router ID of the router that created this LSA 1
Internal#show ip ospf data
OSPF Router with ID (192.168.4.1) (Process ID 1)
Router Link States (Area 1) my one area
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ABR-2#show ip ospf data
OSPF Router with ID (192.168.3.1) (Process ID 1)
Router Link States (Area 1) my one area
LSA 1 - Router Link States
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ABR-2 - show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0
C 172.16.10.4/30 is directly connected, Serial0
C 172.16.1.0/24 is directly connected, FastEthernet0O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
O E2 13.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
192.168.3.0/32 is subnetted, 1 subnets
C 192.168.3.1 is directly connected, Loopback1
LSA 1 - Router Link States
Denoted by just an O in the routing table, or a C Why is there only just anO for this network and not the other
networks?
Directly connected or via another area.
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Your Turn -Discuss in groups (LSA 1s)
Using the Multi-area OSPF Lab Handout: Lab 1. OSPFNormal Areas verify these results.
Look at the link state database summary (show ip ospfdatabase) commands and the Router Links States (LSA1s) foreach router.
Look at the routing tables (show ip route) and notice the routeswithin that routers area.
Why do some routers have more than one set of Router LinksStates?
Where does show ip ospf database tell you the RouterID. Where does show ip ospf database tell you the Area.
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LSA 2 Network LSA Generated by the DR on every multi-access network Denoted by just an O in the routing table or C if the network is
directly connected.
Flooded only within the originating area. LSA 2s are in link state database for all routers within area, even those
routers on not on multi-access networks or DRs on other multi-accessnetworks in the same area.
ABR may include a set of LSA 2s for each area it belongs to.
LSA 2 - Network Link States
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
LSA 2 - Network Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID
192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 2
No LSA 2s for ABR-1in Area 51, or forInternal because noother routers on multi-access segment.
flooded
DR
DR
LSA 2
LSA 2flooded
LSA 2
LSA 2
LSA 2 - Network Link States
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ASBR#show ip ospf data
OSPF Router with ID (192.168.1.1) (Process ID 1)
Net Link States (Area 0)
(LSA 2 - Generated by the DR)
Link ID ADV Router Age Seq# Checksum
172.16.1.2 192.168.2.1 201 0x8000000D 0xCFE8
LSA 2 - Network Link States
Link ID 172.16.1.2 = IP address of DR on MultiAccess Network ADV Router 192.168.2.1 = Router ID of DR Bottom line: Net Link States (LSA2s) should display the RouterIDs of
the DRs on all multi-access networks in the area and their IPaddresses.
Ricks reminder: LSA 2 -> Ethernet = Layer 2 or D R1 2
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Using the Multi-area OSPF Lab Handout: Lab 1. OSPFNormal Areas verify these results.
Look at the link state database summary (show ip ospfdatabase) commands and the Net Links States (LSA2s) foreach router.
Look at the routing tables (show ip route) and notice the multi-access routes within that routers area.
Could a router have more than one entry in its listing of NetLinks States?
Could an area with a broadcast segment, still have no LSA 2s?
Your Turn -Discuss in groups (LSA 2s)
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LSA 3 Summary LSA Originated by the ABR. Describes links between ABR and Internal Routers of the Local Area ABR will include a set of LSA 3s for each area it belongs to. LSA 3s are flooded throughout the backbone (Area 0) and to other
ABRs.
Routes learned via LSA type 3s are denoted by an IA (Inter-area) inthe routing table.
LSA 3 Summary Net Link States
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
LSA 3 Summary Net Link States
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LSA 3 Summary Net Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID
192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s are sent asLSA 3s into otherareas by the ABRs.
LSA 1LSA 1
LSA 3LSA 3
LSA 1
LSA 3 Summary Net Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s are sent asLSA 3s into otherareas by the ABRs.
LSA 1
LSA 1
LSA 1
LSA 3
LSA 3
LSA 3
LSA 3
LSA 1LSA 3
LSA 3
LSA 3 Summary Net Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1
LSA 1
LSA 1
LSA 3
LSA 3
LSA 3
LSA 3
LSA 1
LSA 3
LSA 3
Area 1
LSA 1s are sent as LSA 3sinto other areas by theABRs.
Routers only see the topology of the area they belong to. When a link in one area changes, the adjacent routers originate in LSA 1s and
flood them within the area, causing intra-area (internal) routers to re-run theSPF and recalculating the routing table.
ABRs do not announce topological information between areas. ABRs only inject routing information into other areas, which is basically a
distance-vector technique.
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s are sent as LSA3s into other areas by the
ABRs.
LSA 1
LSA 1
LSA 1
LSA 3
LSA 3
LSA 3
LSA 3
LSA 1
LSA 3
LSA 3
Area 1
ABRs calculate intra-area routes for directly attached areas and announcethem to all other areas as inter-area routes, using LSA 3s.
OSPF ABRs will only announce inter-area routes that were learned from thebackbone area, area 0.
The backbone area serves as a repository for inter-area routes. This keeps OSPF safe from routing loops.
46
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID
192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Dont forget about the LSA1s from Area 0.
Multi Area OSPF
Normal Areas
LSA 1LSA 1LSA 3
LSA 3
LSA 1
LSA 3 Summary Net Link States
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Area 0Backbone Area
Area 51Area 1
RTA RTB
RTC
LSA 3 LSA 3
Normal AreasLSA 1s
Not ABR
ABRs calculate intra-area routes for directly attached areas andannounce them to all other areas as inter-area routes, using LSA 3s.
In normal operation, OSPF ABRs will only announce inter-arearoutes that were learned from the backbone area, area 0. RTC does not forward LSA 3s from Area 1 to Area 51, and does not
forward LSA 3s from Area 51 to Area 1.
The backbone area serves as a repository for inter-area routes. This keeps OSPF safe from routing loops.
48
Area 0Backbone Area
Area 51Area 1
RTA RTB
RTC
LSA 3
LSA 3
Normal Areas
LSA 1s
Not ABR
Example of an LSA 1 originated in Area 1, sent to Area 0 as an LSA 3,and the sent to Area 51 as an LSA 3.
RTC does not forward the LSA 3s back into Area 1, or routingloops may develop.
Again, in normal operations, OSPF ABRs will only announce inter-arearoutes that were learned from the backbone area, area 0.
Note: RTC will create LSA 1s and flood them within the appropriatearea.
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Area 0Backbone Area
Area 51Area 1
RTA RTB
RTC
Normal Areas
LSA 1s LSA 1s
LSA 3 LSA 3
LSA 3LSA 3
Not ABR
RTC does not forward LSA 1s from Area 1 as LSA 3s into Area 51.
RTC does not forward LSA 1s from Area 51 as LSA 3s into Area 1. Any LSA 3s from RTC are not forwarded into Area 0 by RTA or RTB. OSPF specification states that ABRs are restricted to considering LSA
3s only from the backbone area to avoid routing information loops.
50
Area 0Backbone Area
Area 51Area 1
RTA RTB
RTC
Normal Areas
LSA 1s
LSA 3
LSA 3
X
Area 1 routers re-runSPF, creates new SPF
tree and updatesrouting table.
Update is sent to Area 0 and Area51 routers using a distancevector update technique. SPF notre-run, but routers update routingtable.
Topology Change: Down Link
When a router detects a topology change it immediately sends out LSA1s (Router LSAs) with the change.
In the case of a down link, the age of the LSA is set to MaxAge (3,600seconds) Routers that receive LSAs with the age equal to MaxAgeremove this entry from their LSDB (Link State Data Base).
Routers that receive the LSA 1s, within the area of the change, re-runtheir SPF algorithm, to build a new SPF tree and then make thechanges to their IP routing tables. (Continued next slide)
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Area 0Backbone Area
Area 51Area 1
RTA RTB
RTC
Normal Areas
LSA 1s
LSA 3
LSA 3
X
Area 1 routers re-runSPF, creates new SPFtree and updatesrouting table.
Update is sent to Area 0 and Area51 routers using a distance
vector update technique. SPF notre-run, but routers update routingtable.
Topology Change: Down Link
ABR RTA receives the LSA 1 and recalculate their SPF for that area,Area 1.
RTA floods the change as a LSA 3 within its other area, Area 0. RTB receives the LSA 3 and floods it within Area 51. Area 0 and Area 51 routers do not recalculate their SPFs, but inject the
change into their routing tables.
Note: LSA 3s (and other Inter-Area routes) are viewed as leaf nodes inthe SPF tree.
52
Multi Area OSPF
Normal Areas
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID
192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
LSA 3 Summary Net Link States
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ASBRASBR# show ip ospf database
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
(Area 1 networks - Advertising Router ABR-2)
172.16.10.4 192.168.3.1 278 0x80000001 0xD126
172.16.20.0 192.168.3.1 278 0x80000001 0xA746
(Area 51 networks - Advertising Router ABR-1)
172.16.51.1 192.168.2.1 206 0x80000005 0xA832
LSA 3 Summary Net Link States
Link ID = IP network addresses of networks in other areas ADV Router = ABR Router ID sending the LSA-3 Divided by ABR Bottom line: Should see networks in other areas and the ABR advertising that
route.
Ricks reminder: LSA 3 -> networks sent by the A B R1 2 3
54
ASBRASBR# show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:54, FastEthernet0/0
O IA 172.16.20.0/24 [110/783] via 172.16.1.3, 00:02:54, FastEthernet0/0
O IA 172.16.10.4/30 [110/782] via 172.16.1.3, 00:02:54, FastEthernet0/0
C 172.16.1.0/24 is directly connected, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.0.0 is directly connected, FastEthernet0/1
S 11.0.0.0/8 is directly connected, Null0
S 12.0.0.0/8 is directly connected, Null0
192.168.1.0/32 is subnetted, 1 subnets
C 192.168.1.1 is directly connected, Loopback0
S 13.0.0.0/8 is directly connected, Null0
LSA 3 Summary Net Link States
Routes learned via LSA type 3s are denoted by an IA (Inter-AreaRoutes) in the routing table.
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Another example: non-area 0 router, Internal
Internal# show ip ospf database
LSA 3 - Generated by the ABR. Describes links between ABR and
Internal Routers of the Local Area
(Area 51 networks - Advertising Router ABR-2)
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
172.16.1.0 192.168.3.1 848 0x80000005 0xD339
172.16.51.1 192.168.3.1 843 0x80000001 0xB329
Internal# show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0
C 172.16.20.0/24 is directly connected, FastEthernet0
C 172.16.10.4/30 is directly connected, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0192.168.4.0/32 is subnetted, 1 subnets
C 192.168.4.1 is directly connected, Loopback0
O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0
56
Using the Multi-area OSPF Lab Handout: Lab 1. OSPF Normal Areas verify these results.
Look at the link state database summary (show ipospf database) commands and the Summary NetLinks States (LSA3s) for each router.
Look at the routing tables (show ip route) and noticethe Interarea (IA) routes.
Why do some routers have more than one set ofSummary Net Links States?
Your Turn -Discuss in groups (LSA 3s)
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LSA 4 ASBR Summary LSA Originated by the ABR. Flooded throughout the backbone area to the other
ABRs.
Describes the reachability to the ASBRs Advertises an ASBR (Router ID) not a network Included in routing table as an IA route. Same format as a LSA 3 - Summary LSA, except LSA 4
ASBR Summary LSA the Network Mask field is always 0
Exceptions
Not flooded to Stub and Totally Stubby networks. More on this later
LSA 4 ASBR Summary Link States
58
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
LSA 4 ASBR Summary Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Normal Areas
LSA 4
LSA 4
LSA 4
LSA 5s flooded
LSA 4
Area 1
Flooded throughout the backbone area to the other ABRs. Describes the reachability to the ASBRs How do the ABRs know about the ASBR? I am still researching this,
but I believe when routers receive an LSA 5 (AS External LSA) withexternal route information, the routers denote the Router ID being theASBR.
60
ABR-2
ABR-2# show ip ospf database
Summary ASB Link States (Area 1)
LSA 4 - Reachability to ASBR.
Not flooded to Stub and Totally Stubby networks.
Link ID ADV Router Age Seq# Checksum
192.168.1.1 192.168.3.1 801 0x80000003 0x93CC
LSA 4 ASBR Summary Link States
Link ID 192.168.1.1 = Router ID of ASBR ADV Router 192.168.3.1 = Router ID ABR advertising route
Bottom line: Routers in non-area 0, should see Router ID of ASBRand its ABR to get there .
Ricks reminder: LSA 4 -> Reachability to the A S B R1 2 3 4
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Internal
Internal# show ip ospf database
Summary ASB Link States (Area 1)
LSA 4 - Reachability to ASBR.
Not flooded to Stub and Totally Stubby networks.
Link ID ADV Router Age Seq# Checksum
192.168.1.1 192.168.3.1 912 0x80000003 0x93CC
LSA 4 ASBR Summary Link States
Link ID 192.168.1.1 = Router ID of ASBR
ADV Router 192.168.3.1 = Router ID ABR advertising route Note: No LSA 4s for Area 0 on Router B
62
Using the Multi-area OSPF Lab Handout: Lab 1. OSPFNormal Areas verify these results.
Look at the link state database summary (show ip ospfdatabase) commands and the Summary Net Links States(LSA4s) for each router.
Why do some routers have more than one set of Summary ASBLinks States and others may not (like RouterA and ASBR)?
Which Area 0 routers have LSA 4s in their LSDB? Why dont some Area 0 routers have LSA 4s in their LSDB?
Your Turn -Discuss in groups (LSA 4s)
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
Normal Areas
66
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Normal Areas
LSA 5
LSA 5
LSA 5LSA 5
LSA 5s floodedLSA 5
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
Redistribute command creates an ASBR router. Originated by the ASBR. Describes destination networks external to the OSPF Routing Domain Flooded throughout the OSPF AS except to stub and totally stubby
areas
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LSA 5 - AS External Link States
ABR-2
ABR-2# show ip ospf databaseAS External Link States O T H E R networks1 2 3 4 5
68
ABR-2ABR-2# show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0
C 172.16.10.4/30 is directly connected, Serial0
C 172.16.1.0/24 is directly connected, FastEthernet0
O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
O E2 13.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0
192.168.3.0/32 is subnetted, 1 subnets
C 192.168.3.1 is directly connected, Loopback1
Designated by E2 Notice that the cost is 20 for all three routes, we will see why later. It has to do with E2 routes and where the default cost is 20.
Redistribute command (Route Optimization chapter): If a value is notspecified for the metric option, and no value is specified using the default-metric command, the default metric value is 0, except for OSPF wherethe default cost is 20.
LSA 5 - AS External Link States
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Internal# show ip ospf database
Type-5 AS External Link States
LSA 5 - External Networks originated by the ASBR,
Flooded throughout A.S. except to Stub and Totally StubbyLink ID ADV Router Age Seq# Checksum Tag
11.0.0.0 192.168.1.1 1191 0x80000001 0x3FEA 0
12.0.0.0 192.168.1.1 1191 0x80000001 0x32F6 0
13.0.0.0 192.168.1.1 1191 0x80000001 0x2503 0
LSA 5 - AS External Link States
Internal# show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0
C 172.16.20.0/24 is directly connected, FastEthernet0
C 172.16.10.4/30 is directly connected, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0
192.168.4.0/32 is subnetted, 1 subnets
C 192.168.4.1 is directly connected, Loopback0
O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0
O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0
70
E1 vs. E2 External Routes
External routes fall under two categories:
external type 1
external type 2 (default)
The difference between the two is in the way the cost (metric) of theroute is being calculated.
The cost of a type 2 route is always the external cost, irrespective ofthe interior cost to reach that route.
A type 1 cost is the addition of the external cost and the internal costused to reach that route.
A type 1 route is always preferred over a type 2 route for the samedestination.
More later
LSA 5 - AS External Link States
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Using the Multi-area OSPF Lab Handout: Lab 1. OSPFNormal Areas verify these results.
Look at the link state database summary (show ip ospfdatabase) commands and the AS External Links States (LSA5s)for each router.
Also, look at the routing tables for each router.
How many sets of LSA 5s does the ABRs have in their link statesummary database? Notice the ASBRs entries.
Your Turn -Discuss in groups (LSA 5s)
72
Stub Areas
Considerations for both Stub and Totally Stubby Areas
An area could be qualified a stub when:
There is a single exit point (a single ABR) from that area. Morethan one ABR can be used, but be ready to accept non-optimalrouting paths.
If routing to outside of the area does not have to take an optimalpath.
The area is not needed as a transit area for virtual links (later).
The ASBR is not within the stub area
The area is not the backbone area (area 0)
Stub areas will result in memory and processing savings dependingupon the size of the network.
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID
192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Stub Area
74
Receives all routes from within A.S.:
Within the local area - LSA 1s and LSA 2s (if appropriate) From other areas (Inter-Area) - LSA 3s
Does not receive routes from External A.S. (External Routes).
ABR: ABR blocks all LSA 4s and LSA 5s. If LSA 5s are not known inside an area, LSA 4s are not necessary. LSA 3s are propagated by the ABR.
Note: Default route is automatically injected into stub area by ABR
External Routes: Once the ABR gets a packet headed to a default route, itmust have a default route, either static or propagated by the ASBR viadefault information originate (coming!)
Configuration:
All routers in the area must be configured as stub
Stub Areas
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ABR-2
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.10.4 0.0.0.3 area 1
area 1 stub
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Stub Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected
Changes in External routes no longer affect Stub Area routing tables.
78
Internal
Internal# show ip ospf database
Summary Net Link States (Area 1)
LSA 3 - Generated by the ABR.
Describes links between ABR and Internal Routers of the Local Area
Link ID ADV Router Age Seq# ChecksumDefault Route Advertised by ABR-1
0.0.0.0 192.168.3.1 243 0x80000001 0x8A46
Area 0 networks - Advertised by ABR-1
172.16.1.0 192.168.3.1 243 0x80000006 0xEF1E
Area 51 networks - Advertised by ABR-1
172.16.51.1 192.168.3.1 243 0x80000002 0xCF0E
Notice that there are no LSA 4s or LSA 5s for stub area routers. Default Route injected by ABR (LSA 3)
Stub Areas
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Internal
Internal# show ip route
Gateway of last resort is 172.16.10.5 to network 0.0.0.0
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:03:08, Serial0
C 172.16.20.0/24 is directly connected, FastEthernet0
C 172.16.10.4/30 is directly connected, Serial0
O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:03:08, Serial0
192.168.4.0/32 is subnetted, 1 subnets
C 192.168.4.1 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:08, Serial0
Stub Areas
NOTE on default route:
ABR will advertise a default route with a cost of 1 cost of 65 = 1 + 64 (serial link) Using bandwidth of 128K, not 64K: 782 = (100,000,000/128,000) + 1
80
ABR-2
ABR-2# show ip route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:01:59,FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:01:59, Serial0
C 172.16.10.4/30 is directly connected, Serial0C 172.16.1.0/24 is directly connected, FastEthernet0
O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:01:59, FastEthernet0
O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:01:59, FastEthernet0
O E2 13.0.0.0/8 [110/20] via 172.16.1.1, 00:01:59, FastEthernet0
192.168.3.0/32 is subnetted, 1 subnets
C 192.168.3.1 is directly connected, Loopback1
Stub Areas
Notice, there is no automatic default route on ABR, as there are with theinternal stub routers.
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81
Using the Multi-area OSPF Lab Handout: Lab 2. OSPF StubArea.
Look at the link state database summary (show ip ospfdatabase) commands and the Summary Net Links States (LSA3s).
Note: A Stub area may have more than one ABR, but because ofthe default route, the internal routers will not be able todetermine which router is the optimal gateway outside the ASand end up load balancing between the multiple ABRs.
Your Turn -Discuss in groups (Stub)
82
Totally Stubby Areas
Cisco proprietary, however the RFC does make some provisions for thisas an optional feature..
Same considerations as with Stub areas:
An area could be qualified a stub when there is a single exit point (asingle ABR) from that area or if routing to outside of the area does not
have to take an optimal path. The area is not needed as a transit area for virtual links (later).
The ASBR is not within the stub area
The area is not the backbone area (area 0)
Stub areas will result in memory and processing savings depending
upon the size of the network. - This is even more true with Totally
Stubby areas
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ASBR
ABR-1 ABR-2
Internal
Area 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Totally Stubby Area
84
Receives routes from within A.S.:
Only from within the local area - LSA 1s and LSA 2s (if appropriate) Does not receive routes from other areas (Inter-Area) - LSA 3s
Does not receive routes from External A.S. (External Routes)
ABR:
ABR blocks all LSA 4s and LSA 5s. ABR blocks all LSA 3s, except propagating a default route. Default route is injected into totally stubby area by ABR.
Configuring:
All routers must be configured as stub ABR must be configured as stub no-summary
Totally Stubby Areas
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85
ABR-2
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.10.4 0.0.0.3 area 1
area 1 stub no-summary
^^ Command: area area stub no-summary
Internal
router ospf 1
network 172.16.0.0 0.0.255.255 area 1
area 1 stub
^^ Command: area area stub
Totally Stubby Areas
86
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected Area 1
X
LSA 3s (Inter-Area routes) are blocked by the ABR. ABR blocks all LSA 4s (reachability to ASBR) and LSA 5s (External routes) The ABR injects a default route (LSA 3) into the stub area, pointing to the ABR.
(This does not mean the ABR has a default route of its own.)
Essentially, internal routers in a Totally Stubby Area only see the default routeto the ABR.
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected Area 1
X
Changes in any networks outside the Totally Stubby Area no longeraffects the Totally Stubby routing tables.
88
Internal
Internal# show ip ospf database
Summary Net Link States (Area 1)
LSA 3 - Generated by the ABR.
Describes links between ABR and Internal Routers of the Local Area
Link ID ADV Router Age Seq# ChecksumDefault Route Advertised by ABR-2
0.0.0.0 192.168.3.1 205 0x80000003 0x8648
Default Route injected by ABR (LSA 3)
Totally Stubby Areas
Default route is injected into totally stubby area by ABR for all other networks(inter-area and external routes)
Does not receive routes from other areas (Inter-Area) Does not receive routes from External A.S. (External Routes)
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Internal
Internal# show ip route
Gateway of last resort is 172.16.10.5 to network 0.0.0.0
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.20.0/24 is directly connected, FastEthernet0
C 172.16.10.4/30 is directly connected, Serial0
192.168.4.0/32 is subnetted, 1 subnets
C 192.168.4.1 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:09, Serial0
Default route is injected into totally stubby area by ABR for all other networks(inter-area and external routes)
Does not receive routes from other areas (Inter-Area) Does not receive routes from External A.S. (External Routes)
Totally Stubby Areas
90
ABR-2
ABR-2# show ip route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:35,FastEthernet0
O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:02:35, Serial0
C 172.16.10.4/30 is directly connected, Serial0C 172.16.1.0/24 is directly connected, FastEthernet0
O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:02:35, FastEthernet0
O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:02:35, FastEthernet0
O E2 13.0.0.0/8 [110/20] via 172.16.1.1, 00:02:35, FastEthernet0
192.168.3.0/32 is subnetted, 1 subnets
C 192.168.3.1 is directly connected, Loopback1
Totally Stubby Areas
ABR will forward Intra-Area routes (to other areas within AS) Notice, there is not an automatic default route in the ABRs routing
table like there is with the internal Totally Stubby routers.
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OSPF Design Tips
Different people have different approaches to designing OSPFnetworks.
The important thing to remember is that any protocol can failunder pressure.
The idea is not to challenge the protocol but rather to work withit in order to get the best behavior. CCO
OSPF design considerations
92
Number of Routers per Area
The maximum number of routers per area depends on several factors,including the following:
What kind of area do you have?
What kind of CPU power do you have in that area?
What kind of media?
Will you be running OSPF in NBMA mode?
Is your NBMA network meshed?
Do you have a lot of external LSAs in the network?
Are other areas well summarized?
For this reason, it's difficult to specify a maximum number of routers perarea.
OSPF design considerations
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Stub and Totally Stubby Areas:
An area could be qualified a stub when there is a single exit point (asingle ABR) from that area or if routing to outside of the area does nothave to take an optimal path.
The area is not needed as a transit area for virtual links (later).
The ASBR is not within the stub area
The area is not the backbone area (area 0)
Stub areas will result in memory and processing savings depending
upon the size of the network. - This is even more true with Totally
Stubby areas Totally Stubby areas is a Cisco enhancement.
OSPF design considerations
94
Quick Review
Areas LSAs Stub Area Totally Stubby Area
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95
LSA-1 - Router LSA
96
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID
192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s being sentwithin Area 0
LSA 1LSA 1 LSA 1
LSA 1 - Router Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s being sentwithin other areas
LSA 1
LSA 1
LSA 1
LSA 1
LSA 1 - Router Link States
98
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID
192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1Originated
LSA 1s are flooded outother interfaces withinthe same area.
LSA 1flooded
LSA 1 - Router Link States
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99
LSA-2 - Network LSA
100
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID
192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 2
No LSA 2s for ABR-1in Area 51, or forInternal because noother routers on multi-access segment.
flooded
DR
DR
LSA 2
LSA 2flooded
LSA 2
LSA 2
LSA 2 - Network Link States
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101
LSA-3 - Summary LSA
102
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID
192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s are sent asLSA 3s into otherareas by the ABRs.
LSA 1LSA 1
LSA 3LSA 3
LSA 1
LSA 3 Summary Net Link States
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ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Multi Area OSPF
Normal Areas
LSA 1s are sent asLSA 3s into otherareas by the ABRs.
LSA 1
LSA 1
LSA 1
LSA 3
LSA 3
LSA 3
LSA 3
LSA 1LSA 3
LSA 3
LSA 3 Summary Net Link States
104
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID
192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
Dont forget about the LSA1s from Area 0.
Multi Area OSPF
Normal Areas
LSA 1LSA 1LSA 3
LSA 3
LSA 1
LSA 3 Summary Net Link States
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105
LSA-4 ASBR Summary LSA
106
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Normal Areas
LSA 4
LSA 4
LSA 4
LSA 5s flooded
LSA 4
Area 1
Flooded throughout the backbone area to the other ABRs. Describes the reachability to the ASBRs
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107
LSA-5 - External LSA
108
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
Normal Areas
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109
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Normal Areas
LSA 5
LSA 5
LSA 5LSA 5
LSA 5s floodedLSA 5
ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
ip route 11.0.0.0 255.0.0.0 Null0
ip route 12.0.0.0 255.0.0.0 Null0
ip route 13.0.0.0 255.0.0.0 Null0
Redistribute command creates an ASBR router. Originated by the ASBR. Describes destination networks external to the OSPF Routing Domain Flooded throughout the OSPF AS except to stub and totally stubby
areas
110
Stub Area
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111
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Stub Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected
112
Totally Stubby Area
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113
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected Area 1
X
114
Next
E1 and E2 routes Default Routes Route Summarization NSSA (Not So Stubby Areas)
Multiple ABR Scenario Multiple ASBR Scenario Virtual Links Load Balancing show commands
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115
NSSA (Not So Stubby Area)
NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
116
Relatively new, standards based OSPF enhancement, RFC 1587. NSSA allows an area to remain a stub area, but carry external routing
information (Type 7 LSAs) from its stubby end back towards the OSPFbackbone.
ASBR in NSSA injects external routing information into the backbone
and the NSSA area, but rejects external routing information comingfrom the ABR.
The ABR does not inject a default route into the NSSA. This is true for a NSSA Stub, but a default route is injected for a
NSSA Totally Stubby area.
Note: RFC 1587, A default route must not be injected into the NSSAas a summary (type-3) LSA as in the stub area case.
What??? Following scenario is only example of how NSSA works. For the
purposes of learning about NSSAs, dont get hung up on the whys andwhat ifs.
NSSA (Not So Stubby Area)
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117
NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
NSSA Stub Area
Area 2 would like to be a stub network. RTH only supports RIP, so RTG will run RIP and redistribute those routes in
OSPF.
Unfortunately, this makes the area 2 router, RTG, an ASBR and thereforearea 2 can no longer be a stub area.
RTH does not need to learn routes from OSPF, a default route to RTG is all itneeds.
But all OSPF routers must know about the networks attached to the RIP router,RTH, to route packets to them.
118
NSSA Stub Area (cont.) NSSA allow external routes to be advertised into the OSPF AS while retaining
the characteristics of a stub area to the rest of the OSPF AS.
ASBR RTG will originate Type-7 LSAs to advertise the external destinations. These LSA 7s are flooded through the NSSA but are blocked by the NSSA ABR. The NSSA ABR translates LSA 7s into LSA 5s and flood other areas.
NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
LSA 7LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
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119
Type 7 LSA NSSA External Link Entry
Originated by an ASBR connected to an NSSA. Type 7 messages can be flooded throughout NSSAs and translated
into LSA Type 5 messages by ABRs.
Routes learned via Type-7 LSAs are denoted by either a defaultN1 or an N2 in the routing table. (Relative to E1 and E2).
NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
LSA 7LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
120
Configuring NSSA Stub Area
Configured for all routers in Area 2:
router ospf 1
network 172.16.2.0 0.0.0.255 area 2
area 2 nssa
NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
NSSA Generic
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NSSA Stub and NSSA Totally Stubby
There are two flavors in NSSA:
Stub
Totally Stubby
Area 2 routers may or may not receive Inter-area routes from RTA, dependingupon NSSA configuration
NSSA areas have take on the same characteristics as stub and totally stubbyareas, along with the characteristics of NSSA areas.
NSSA (Not So Stubby Area)
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NSSA Stub
NSSA stub areas:
NSSAs that block type 4 and 5, but allow type 3.
To make a stub area into an NSSA, use the following command underthe OSPF configuration.
This command must be configured on all routers in area 2.
router ospf 1
area 2 nssa
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NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
LSA 7LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
NSSA Stub Areas
0.0.0.0/0
LSA 3s
XRTHroutes:E1/E2
RTH routes:N1/N2LSA 4s & LSA 5s
X
Default route via RTG
NSSA Stub Area Routing Tables:
RTG: Area 2 routes, Area 0 routes (IA), RTH RIP routes No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation
Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), Area 0 routes (IA)
No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes if redistributed from
RTA ASBR (E1/E2)
RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes if redistributed fromRTA (E1/E2)
Note: Area 2 routers may or may not receive E1/E2 routes from RTA, depending upon NSSAconfiguration (next).
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Area 2 routers:
router ospf 1
network 172.16.2.0 0.0.0.255 area 2
area 2 nssa
NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
NSSA Stub Areas
0.0.0.0/0
LSA 3s
XRTHroutes:
E1/E2
RTH routes:N1/N2LSA 4s & LSA 5s
X
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NSSA Totally Stubby Area
NSSA totally stub areas: Allow only summary default routes and filters everythingelse.
To configure an NSSA totally stub area, use the following command under theOSPF configuration on the NSSA ABR:
router ospf 1
area 2 nssa no-summary
Configure this command on NSSA ABRs only.
All other routers in area 2 (internal area 2 routers):router ospf 1
area 2 nssa
After defining the NSSA totally stub area, area 2 has the following characteristics (in
addition to the above NSSA characteristics):
No type 3 or 4 summary LSAs are allowed in area 2. This means nointer-area routes are allowed in area 2.
A default route is injected into the NSSA totally stub area as a type 3summary LSA by the ABR.
NSSA Totally Stubby
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NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
NSSA Totally Stubby Areas
0.0.0.0/0
LSA 3s
XRTHroutes:
E1/E2
RTH routes: N1/N2LSA 4s & LSA 5s
X
RTB (ABR):
router ospf 1
network 172.16.1.0 0.0.0.255 area 0
network 172.16.2.0 0.0.0.255 area 2 ...
area 2 nssa no-summary
Area 2 routers:
router ospf 1
network 172.16.2.0 0.0.0.255 area 2
area 2 nssa
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NSSA
Area 2
Backbone Area
Area 0
ASBR
ABR(Possible
ASBR)
RIP
RTARTB
RTC
RTD
RTE
RTF
RTG
RTH
Default route via RTG
LSA 7LSA 7
LSA 7
LSA 7
LSA 7
LSA 7LSA 5
LSA 7sBlocked
NSSA Totally Stubby Areas
0.0.0.0/0
LSA 3s
XRTHroutes:
E1/E2
RTH routes: N1/N2LSA 4s & LSA 5s
X
NSSA Totally Stubby Area Routing Tables:
RTG: Area 2 routes, RTH RIP routes, 0.0.0.0/0 (IA) route from RTB (ABR)
Totally Stubby: No Area 0 routes or external routes from RTA Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), 0.0.0.0/0 (IA) route from RTB
(ABR)
Totally Stubby: No Area 0 routes or external routes from RTA
RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes from RTA ASBR(E1/E2) if redistributed by ASBR
RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes (E1/E2)
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Area 2NSSA Area 0
RTARTBRTCRTD
RTE
RIP
Default Route
172.16.3 .0/24 172.16.2.0/24 172.16.1.0 /24
10.0.0.0/8
200.200.200.0/24
222.222.222.0/24
For More on NSSA
For more examples
See NSSA document on my web site for more info.
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Virtual Links
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All areas in an OSPF autonomous system must be physicallyconnected to the backbone area (area 0).
In some cases where this is not possible, you can use a virtual link toconnect to the backbone through a non-backbone area.
As mentioned above, you can also use virtual links to connect twoparts of a partitioned backbone through a non-backbone area.
The area through which you configure the virtual link, known as atransit area, must have full routing information.
Must be configured between two ABRs.
The transit area cannot be a stub area.
Virtual Links
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A virtual link has the following two requirements: It must be established between two routers that share a common
area and are both ABRs.
One of these two routers must be connected to the backbone.
Doyle, should be used only as a temporary fix to an unavoidabletopology problem.
Virtual Links
132
The command to configure a virtual link is as follows:
area virtual-link RTA(config)#router ospf 1
RTA(config-router)#network 192.168.0.0 0.0.0.255 area 51
RTA(config-router)#network 192.168.1.0 0.0.0.255 area 3
RTA(config-router)#area 3 virtual-link 10.0.0.1
...
RTB(config)#router ospf 1
RTB(config-router)#network 192.168.1.0 0.0.0.255 area 3
RTB(config-router)#network 192.168.2.0 0.0.0.255 area 0
RTB(config-router)#area 3 virtual-link 10.0.0.2
Virtual Links
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OSPF allows for linking discontinuous parts of the backbone using avirtual link.
In some cases, different area 0s need to be linked together . This canoccur if, for example, a company is trying to merge two separateOSPF networks into one network with a common area 0.
In other instances, virtual-links are added for redundancy in casesome router failure causes the backbone to be split into two. (CCO)
Whatever the reason may be, a virtual link can be configured betweenseparate ABRs that touch area 0 from each side and having acommon area.
Virtual Links
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Virtual Links Another Example
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Route Summarization
Inter-Area Route Summarization - Area Range
By default ABRs do not summarize routes between areas.
Route summarization is the consolidation of advertised addresses.
This feature causes a single summary route to be advertised to other areas byan ABR.
In OSPF, an ABR will advertise networks in one area into another area.
If the network numbers in an area are assigned in a way such that they arecontiguous, you can configure the ABR to advertise a summary route thatcovers all the individual networks within the area that fall into the specifiedrange.
On the ABR (Summarizes routes before injecting them into different area)
Router(config-router)# area area-id range network-
address subnet-mask
area-id- Identifier of the area about which routes are tobe summarized. (From area)
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RTB is summarizing the range of subnets from 128.213.64.0 to 128.213.95.0into one range: 128.213.64.0 255.255.224.0.
This is achieved by masking the first three left most bits of 64 using a mask of255.255.224.0.
128.213.64.0/24 - 010 00000
128.213.95.0/24 010 11111
-----------------------------------------
128.213.64.0/19 - 01000000
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Route Summarization
In the same way, RTC is generating the summary address 128.213.96.0255.255.224.0 into the backbone.
Note that this summarization was successful because we have two distinctranges of subnets, 64-95 and 96-127.
128.213.96.0/24 - 011 00000
128.213.127.0/24 011 11111
-----------------------------------------
128.213.96.0/19 - 01100000
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128.213.64.0/24 - 010 00000
128.213.95.0/24 010 11111
-----------------------------------------
128.213.64.0/19 - 01000000
RTB
router ospf 100
area 1 range 128.213.64.0 255.255.224.0
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128.213.96.0/24 - 011 00000
128.213.127.0/24 011 11111
-----------------------------------------
128.213.96.0/19 - 01100000RTC
router ospf 100
area 2 range 128.213.96.0 255.255.224.0
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External Route Summarization - summary-address
When redistributing routes from other protocols into OSPF (later), each route isadvertised individually in an external l ink state advertisement (LSA).
However, you can configure the Cisco IOS software to advertise a single routefor all the redistributed routes that are covered by a specified network address
and mask. Doing so helps decrease the size of the OSPF link state database.
On the ASBR only (Summarizes external routes before injecting them into theOSPF domain.)
Router(config-router)# summary-address network-address
subnet-mask
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RTA
router ospf 100
summary-address 128.213.64.0 255.255.224.0
redistribute bgp 50 metric 1000 subnets (later)
RTD
router ospf 100
summary-address 128.213.96.0 255.255.224.0
redistribute bgp 20 metric 1000 subnets (later)
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Injecting Default Routes into OSPF
By default, 0.0.0.0/0 route is not propagated from the ASBR to otherrouters.
An autonomous system boundary router (ASBR) can be forced togenerate a default route into the OSPF domain.
As discussed earlier, a router becomes an ASBR whenever routes areredistributed into an OSPF domain.
However, an ASBR does not, by default, generate a default route intothe OSPF routing domain.
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The way that OSPF generates default routes (0.0.0.0) varies dependingon the type of area the default route is being injected into.
Stub and Totally Stubby Areas
For stub and totally stubby areas, the area border router (ABR) to thestub area generates a summary link-state advertisement (LSA) with thelink-state ID 0.0.0.0.
This is true even if the ABR doesn't have a default route.
In this scenario, you don't need to use the default-information
originate command.
Injecting Default Routes into OSPF
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All routers in the area must be configured as stub including the ABR:
router ospf 1
area 1 stub
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Stub Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected Area 1
Stub Area
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All routers in the area must be configured as stub except the ABR stub no summary:
ABR: router ospf 1 Other: router ospf 1
area 1 stub no-summary area 1 stub
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID
192.168.3.1/32Pri 100Pri 200
Lo - RouterID
192.168.4.1/32
Totally Stubby Area
LSA 4
LSA 4
X BlockedLSA 5
Blocked XLSA 5
LSA 3
LSA 3
Defaultroute toABRinjected Area 1
X
Totally Stubby Area
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Normal Areas
By default, in normal areas routers don't generate default routes.
To have an OSPF router generate a default route, use the default-information originate command.
This generates an external type-2 link with link-state ID 0.0.0.0 andnetwork mask 0.0.0.0.
This command should only be used on the ASBR.
Some documentation states this command works only on anASBR while other documentation states this command turns arouter into an ASBR.
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To have OSPF generate a default route use the following:
router ospf 10
default-information originate [always] [metric metric-
value] [metric-type type-value] [route-map map-name]
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There are two ways to generate a default.
1) default-information originate
If the ASBR already has the default route (ip route 0.0.0.0 0.0.0.0),you can advertise 0.0.0.0 into the area.
2) default-information originate always
If the ASBR doesn't have the route (ip route 0.0.0.0 0.0.0.0), you canadd the keyword always to the default-information originate
command, and then advertise 0.0.0.0.
You should be careful when using the always keyword. If your routeradvertises a default (0.0.0.0) inside the domain and does not have adefault itself or a path to reach the destinations, routing will bebroken.
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ASBRrouter ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
default-information originate
ip route 0.0.0.0 0.0.0.0 10.0.0.2
ASBR
ABR-1 ABR-2
Internal
Area 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
0.0.0.0/00.0.0.0/0
0.0.0.0/0
0.0.0.0/0 0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
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ASBR
router ospf 1
redistribute static
network 172.16.1.0 0.0.0.255 area 0
default-information originate always
ip route 0.0.0.0 0.0.0.0 10.0.0.2
ASBR
ABR-1 ABR-2
InternalArea 51
Area 1
Area 0
172.16.0.0/16
172.16.1.0/24
172.16.51.0/24
172.16.10.4/30
172.16.20.0/24
10.1.0.0/24
11.0.0.0/812.0.0.0/813.0.0.0/8
.1
.1
.2 .3
.5
.6
.1
Lo - RouterID192.168.2.1/32
Lo - RouterID192.168.1.1/32
Lo - RouterID192.168.3.1/32
Pri 100Pri 200
Lo - RouterID192.168.4.1/32
0.0.0.0/00.0.0.0/0
0.0.0.0/0
0.0.0.0/0 0.0.0.0/0
0.0.0.0/0
No 0.0.0.0/0 route, butpropagated anyway oralways
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E1 vs. E2 External Routes
External routes fall under two categories, external type 1 andexternal type 2.
The difference between the two is in the way the cost (metric) of theroute is being calculated.
A type 1 (E1) cost is the addition of the external cost and the internalcost used to reach that route.
The cost of a type 2 (E2) route is always the external cost,irrespective of the interior cost to reach that route.
Type 2 (E2) is the default!
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router ospf 1
redistribute routing-protocolmetric-type [1|2]
metric-type 1 - A type 1 cost is the addition of the external cost andthe internal cost used to reach that route.
redistribute rip [metric value] metric-type 1
metric-type 2 - The cost of a type 2 route is always the external cost,irrespective of the interior cost to reach that route.
redistribute rip [metric value] metric-type 2
We will look at this command, along with internal/external costs, laterin the chapter discussion route redistribution.
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Redistributing External Routes
metric-type 2 - The cost of a type 2 route is always the external cost,irrespective of the interior cost to reach that route.
redistribute rip [metric value]metric-type 2
More later, but here is a taste of the metric valueoption
If a value is not specified for the metric valueoption, and no value isspecified using th