interior gateway protocol ospf --- open shortest path first · ospf— 8 lsdatabase • type 1....
TRANSCRIPT
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OSPF — 1
Interior Gateway Protocol OSPF — Open Shortest Path First
Martin Heusse
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OSPF — 2
Link States routing protocols
• Examples : OSPF (most widespread interior routing protocol);IS-IS ; PNNI
• Knowledge of the whole topology (with exceptions…); squarematrix with interconnection costs
• Basic principle :
Base de données
Table de routage
data base
routing
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OSPF — 3
OSPFv2 ; RFC 2328 (240 pages)
• LSA : quantum of information exchanged between routers
• Hello mechanism — Each system is identified by a router ID(lowest IP address by default)
• Reliable flooding of each topology change / refresh
✓ 1 router is in charge of sending information to all its neighbors ineach broadcast network
✓ On startup, synchronization mechanism
• Local shortest path computation on each router (SPFcalculation)
• Consistency check :
✓ Timestamping of the LSAs (time-out ; outdated LSAs arediscarded)
✓ LSAs have serial numbers (a LSA can only by replaced by a morerecent one)(when max value is reached, previous LSA must beflushed first)
• The network may be divided in areas
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OSPF — 4
Basic flooding
• Simplest way of sending information through a network
• No need to know the topology…
1. Re-send packet to all neighbors
2. they retransmit unless they’ve seen it already
→ Each packet travels on each link in both directions
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OSPF — 5
OSPF packets
• HELLO
✓ neighbor discovery, link check, multicast to all OSPF routers✓ Typically sent every 10s✓ Destination : AllSPFRouters : 224.0.0.5.
• DATABASE DESCRIPTION
✓ DB summary. Unicast, master-slave exchange
• LINK STATE REQUEST
• LINK STATE UPDATE
• LINK STATE ACK�
�
�
�No transport protocol (no TCP or UDP header)
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OSPF — 6
HELLO — neighbor discovery, check
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 1 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HelloInterval | Options | Rtr Pri |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RouterDeadInterval |
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OSPF — 7
HELLO — neighbor discovery, check(cont.)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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OSPF — 8
LS Database
• Type
1. router-LSAs - states of the router’s interfaces2. network-LSAs - the set of routers attached to the network3. summary-LSAs - summary routes to networks (émis par les ABR)4. summary LSAs - summary routes to AS boundary routers5. AS-external-LSAs - routes to destinations external to the
Autonomous System
• LS ID (par type)
1. The originating router’s Router ID.2. The IP interface address of the network’s DR3. The destination network’s IP address4. The Router ID of the described AS boundary router5. The destination network’s IP address
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OSPF — 9
Database
R1 R2
R3 R4
N1
R1+L1 R2
R3 R4
1020
1010
0
20 0
00
10
10 10
transit network
L1
}stub network
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OSPF — 10
OSPF database exampleSource : Cisco
Router ID 1.1.1.1 Router ID 2.2.2.2
4.0.0.1/8area 0cost 10
5.0.0.1/8area 0cost 10
5.0.0.2/8area 0cost 10
6.0.0.2/8area 0cost 10
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OSPF — 11
OSPF database exampleSource : Cisco (cont.)
r2.2.2.2#show ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 2)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 107 0x80000018 0x7966 2
2.2.2.2 2.2.2.2 106 0x80000015 0x6770 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
5.0.0.2 2.2.2.2 102 0x80000004 0x7E9D
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OSPF — 12
OSPF database exampleSource : Cisco (cont.)
r2.2.2.2#show ip ospf database router 1.1.1.1
OSPF Router with ID (2.2.2.2) (Process ID 2)
Router Link States (Area 0)
LS age: 147
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000018
Checksum: 0x7966
Length: 48
Number of Links: 2
Link connected to: a Transit Network
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OSPF — 13
OSPF database exampleSource : Cisco (cont.)
(Link ID) Designated Router address: 5.0.0.2
(Link Data) Router Interface address: 5.0.0.1
Number of TOS metrics: 0
TOS 0 Metrics: 10
Link connected to: a Stub Network
(Link ID) Network/subnet number: 4.0.0.0
(Link Data) Network Mask: 255.0.0.0
Number of TOS metrics: 0
TOS 0 Metrics: 10
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OSPF — 14
OSPF: Reliable flooding / designatedrouter
• LS updates are acknowledged
• Need to know the list of neighbors: HELLO (a neighbor is goneafter not receiving hellos for RouterDeadInterval)
• Problem: on a LAN an LS update may come from severaldirections
✓ Election of 1 designated router/bcast network (need at least 2routers…)(By default, highest ID. Keep the same DR until it’sgone)
✓ Any LS update goes first to the DR
I Sent to AllDRouters (224.0.0.6)I Re-sent to AllSPFRouters
I unicast acknowledgement
✓ The DR also originates the network-LSA of its (broadcast)network (i.e. link)
✓ There is one backup designated router (listens to AllDRouters)
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OSPF — 15
Designated router
• Spreads LSAs on the link
• Originates (creates) the network LSA
• Newcomers synchronize with it
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OSPF — 16
LS Update packet
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 4 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # LSAs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSAs |
+- +-+
| ... |
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OSPF — 17
Structure of a LSA header
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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OSPF — 18
Router-LSA
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 (== Link State ID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | metric |
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OSPF — 19
Router-LSA (cont.)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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OSPF — 20
Network-LSA
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 (DR interface addr.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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OSPF — 21
LSAs management
• LSAs are regenerated every 30 minutes (LSRefreshTime)
• LSA older than MaxAge are not used for SPF calculation (→ usedfor flushing)
• Sequence number : initially 0x80000001, maximum : 0x7fffffff
• LS Age incremented at each hop during flooding.• Network-LSA :
✓ If the DR changes (e.g. goes down), a new network-LSA iscreated (by new DR)
I No router is connected any more to the formernetwork-LSA !
I If previous DR is still alive, flushes network-LSA (sending anoutdated LSA)
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OSPF — 22
OSPF : initial synchronisation(New router ↔ DR)
• database descriptionmaster-slave protocol:
✓ flags Init, More, Master/slave✓ the master is the one with greater router ID
✓ the slave always repeat the seq. num. sent by master (ack), neversends a packet by itself
• Subsequently : LS request, LS update (unicast).
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OSPF — 23
OSPF : initial synchronisation(New router ↔ DR) (cont.)
Hello (DR=NULL, voisins=NULL)
Hello (DR=RT1, voisins=NULL)
RT1 RT2
DD (Seq=x,I,M,Master)
DD (Seq=y,I,M,Master)
DD (Seq=y,M,Slave))
DD (Seq=y+1,M,Master)
DD (Seq=y+1,M,Slave))
(...)
DD (Seq=y+n,Master)
DD (Seq=y+n,Slave))
LS Request
LS Update
(...)
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OSPF — 24
DD packet
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 2 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface MTU | Options |0|0|0|0|0|I|M|MS
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DD sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- An LSA Header -+
| ... |
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OSPF — 25
OSPF : areas
• Reduces the number of LS updates
• One backbone area, area 0 : all other areas are connected to it→ no inter-area routing
• LSA propagates within an area
• ABR (Area Border Router) originateSummary-LSAs, which bear a distanceto the network
• stub area (config. by user): one singleconnection to the backbone. Externalroutes are replaced by one defaultroute
Aire 2
Aire 1
Aire 0
R1 R2
R3 R4
R5
R6
R1R1
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OSPF — 26
Ce n’est pas tout…
• External LSA (LSA type 5)
• Not-So-Stubby-Area (LSA type 7)
• Authentification
• Equal-Cost Multipath