routing3

Ch. 6-7– Routing Theory – Part 3Ch. 6-7– Routing Theory – Part 3

CCNA Semester 2CCNA Semester 2Originally by Rick Graziani, Instructor

Modified by Prof. Yousif

Link-State Routing Protocols The first type of routing protocol we discussed was distance vector. The second type of routing protocol that we will examine is link-state. In this presentation we will only examine the very basic concepts of

link-state routing protocols. In CCNP Advanced Routing we examine the link state routing protocol

OSPF in detail. I have added a presentation, Introduction to OSPF, which we will

discuss at the end of this semester.

Distance Vector Routing Protocols Distance vector routing protocols like RIP and IGRP do not

know the exact topology of a network. All distance vector routing decisions are made from information

from neighboring routers – routing by rumor. The only information the router has about a route is how far

away the network is in hops or using another cost (distance) and which interface to send forward the packet out of (vector).

The router has no way to make its own decision on which direction is ultimately the best way to send the packets.

Link-State Routing Protocols - History An IETF working group designed a routing protocol specifically

for IP routing, OSPF (Open Shortest Path First). For most network administrators they had two open-standard

routing protocols to choose from: RIP, simple but very limited, or OSPF, robust but more sophisticated to implement.– IGRP and EIGRP are Cisco proprietary

– IS-IS is used in IP networks, but not as common as OSPF

Theory of Link-State Routing Protocols In this presentation we will examine “some” of the theory behind link-

state routing protocols. This will only be a brief introduction to the link-state theory, requiring

much more time and perhaps even some requisite knowledge of algorithms.

At the end of this presentation will be some suggested resources for leaning more about the theory of link-state routing and Dijkstra’s algorithm.

Mathematical point of view Link-state routing is not based on IP addresses, subnets and

network information! Link-state routing has a mathematical point of view, looking at

the network as nothing more than a graph with vertices and the costs to these vertices.

Okay, I’m losing you and I said I wouldn’t get mathematical. Link-state routing is based on a very simple algorithm known as

Dijkstras’s algorithm, invented by Edsger Wybe Dijkstra This algorithm can and has been used in many areas of human

activity, not just for routing.

Link-State Theory The network is viewed as a graph, showing the complete topology of

the network. How do routers build this topology?

1 – Flooding of link-state information The first thing that happens is that each node, router, on the network

announces its own piece of link-state information to other all other routers on the network: who their neighboring routers are and the cost of the link between them.

Example: “Hi, I’m RouterA, and I can reach RouterB via a T1 link and I can reach RouterC via an Ethernet link.”

Each router sends these announcements to all of the routers in the network.

1 – Flooding of link-state information

2. Building a Topological Database Each router collects all of this link-state information from other routers

and puts it into a topological database.

3. Shortest-Path First (SPF), Dijkstra’s Algorithm Using this information, the routers can recreate a topology graph of the

network. Believe it or not, this is actually a very simple algorithm and I highly

suggest you look at it some time, or even better, take a class on algorithms. (Radia Perlman’s book, Interconnections, has a very nice example of how to build this graph – she is one of the contributers to the SPF and Spanning-Tree algorithms.)


2 – Building a Topological Database

3 – SPF Algorithm

4. Shortest Path First Tree This algorithm creates an SPF tree, with the router making itself the

root of the tree and the other routers and links to those routers, the various branches.

– Note: Just a reminder that the link-state algorithm and graph it creates is mathematically based and although we are mentioning routers and their links, it has nothing to do with IP addresses or other network information.

5. Routing Table Using this information, the router creates a routing table.

I bet you can create this tree given the link-state information!



3 – SPF Algorithm

4 – SPF Tree

5 – Routing Table

Exercise: From link-state flooding to routing tables - Lets try it… For this exercise we will not worry about the individual, leaf,

networks attached to each node or router (shown as a blank line), but focus on how the topology is built to find the the shortest path between each router.

In order to keep it simple, we will take some liberties with the actual process and algorithm, but you will get the basic idea!

You are RouterA and you have a link to RouterB with a cost of 15, a link to RouterC with a cost of 2, a link to RouterD with a cost of 5, and a leaf network “apple.”

This is your own link-state information, which you will flood to all other routers so they can do the same thing we will be doing for RouterA.

A C

D

2

5

B

15“Leaf” network apples

We now get the following link-state information from RouterB RouterB has a link to RouterA with a cost of 15. RouterB has a link to RouterE with a cost of 2. And information about its own “leaf” network “bananas.”

A

B

E

15 2

Now lets attach the two graphs…

A C

D

2

5

B

15

A

B

E

15 2

A C

D

2

5

B

E

15 2

+ =

bananas

We now get the following link-state information from RouterC RouterC has a link to RouterA with a cost of 2. RouterC has a link to RouterD with a cost of 2. And information about its own “leaf” network “cherries.”


A C

D

2

5

B

E

15 2

+ =

A C

D

2

2

A C

D

2

2

A C

D

2

5

B

E

15 2

2

cherries

We now get the following link-state information from RouterD RouterD has a link to RouterA with a cost of 5. RouterD has a link to RouterC with a cost of 2. RouterD has a link to RouterE with a cost of 10. And information about its own “leaf” network “donuts.”


A C

D

2

5

B

E

15 2

2

A C

D

5

E

102

A C

D

5

E

102A C

D

2

5

B

E

15 2

102+ =

donuts

We now get the following link-state information from RouterE RouterE has a link to RouterB with a cost of 2. RouterE has a link to RouterD with a cost of 10. And information about its own “leaf” network “eggs.”

Now lets attach the two graphs and we have all the nodes, their links between them and their and leafs!

A C

D

2

5

B

E

15 2

102

+ =

D

B

E

2

102

D

B

E

2

102

A C

D

2

5

B

E

15 2

102

eggs

Topology Using the topological information we listed, RouterA has now

built a complete topology of the network. The next step is for the link-state algorithm to find the best path

to each node and leaf network.

eggsA C

D

2

5

B

E

15 2

102

donuts

cherries

bananas

apples

eggsA C

D

2

5

B

E

15 2

102

donuts

cherries

bananas

apples

Choosing the best path Using the link-state algorithm RouterA can now proceed to find

the shortest path to each leaf network. Try doing it on your own!

Choosing the best path Now RouterA knows the best path to each network.

eggs

donuts

cherries

bananas

applesA C

D

2

5

B

E

15 2

102

Creating the Routing Table RouterA can now enter these paths into its routing table, with

network numbers, exit interfaces and costs to each network.

eggs

donuts

cherries

bananas

applesA C

D

2

5

B

E

15 2

102

i1i2

i3i0

i = interface

Network interface cost

Apples i0 conn. Bananas i1 15 Cherries i2 2 Donuts i2 4 Eggs i2 14 Other directly connected networks

And now you have seen and done the process! All of the routers in the network go through this same

process.



3 – SPF Algorithm

4 – SPF Tree

5 – Routing Table

Link-State Routing Protocols – Hello Messages and LSAs First of all small Hello messages are exchanged between routers to find

out who their neighbors are. – This is known as “forming adjacencies.” Once a link-state router knows who their adjacent neighbors are, the

actual information exchanged between the routers are known as LSAs (Link State Advertisements) to build and maintain their link state databases. (Topological database).

There are different types of LSAs for different types of information and different situations – all of which is discussed in CCNP Advanced Routing.

Once the routing tables are built and the network is converged, routers do not exchange routing tables periodically.

Instead, routers using link-state routing protocols exchange periodic Hello messages between immediate neighbors, to make sure they are still there and the link between them is still up.

Link-State Routing Protocols – Topology Change When there is a change in the network, link going down, new

link coming up, etc., the router(s) attached to that link floods out LSAs to all other routers in the network, containing only the changed link information.

All other routers enter this new information into their topological database, re-run the SPF algorithm, come up with a new SPF tree, and eventually a new routing table with possible new best paths to some networks.

From on-line curriculum Running link-state routing protocols in most situations requires that

routers use more memory and perform more processing than distance-vector routing protocols.

For link-state routing, their memory must be able to hold information from various databases, the topology tree, and the routing table.

Using Dijkstra's algorithm to compute the SPF requires a processing task proportional to the number of links in the internetwork, multiplied by the number of routers in the internetwork.

From on-line curriculum During the initial discovery process, all routers using link-state routing

protocols send LSA packets to all other routers. This action floods the internetwork as routers make their en masse

demand for bandwidth, and temporarily reduce the bandwidth available for routed traffic that carries user data.

After this initial flooding, link-state routing protocols generally require only minimal bandwidth to send infrequent or event-triggered LSA packets that reflect topology changes. (and Hello messages)

Just a few notes on this table: * When a link-state router boots up it will most likely need to exchange

complete database information with neighboring routers in order to synchronize their databases. (CCNP Advanced Routing)

** Distance vector routing protocols can also use triggered updates.

*

**

For more information on OSPF, link-state routing protocol, Dijkstra’s algorithm and routing in general, check out these sources.

Interconnections : Bridges and Routers by Radia Perlman

Cisco IP Routing: Packet Forwarding & Intra-domain Routing Protocols by Alex Zinin

Routing TCP/IP Volume I by Jeff Doyle

OSPF, Anatomy of an Internet Routing Protocol by John Moy (creator of OSPF)

http://www.amazon.com/exec/obidos/tg/stores/detail/-/books/1578700418/reader/2/102-1499200-2096936

Topics – (Continued)Part II. Routing Theory and Dynamic Routing Operations (continued) Hybrid Routing Protocols

– Concepts– EIGRP (not IS-IS)

Path Switching– Example: Host X to Host Y (with three routers in between)– LAN-to-LAN Routing– LAN-to-WAN Routing

Cisco Router Configuration Summary Topics (Review)

The balanced hybrid approach combines aspects of the link-state and distance-vector algorithms.

These are really distance-vector routing protocols which apply some of the advantages of a link-state routing protocols, and also known as advanced-distance-vector routing protocols.

EIGRP is known as balanced hybrid routing protocol.– EIGRP is covered in CCNP Advanced Routing but it uses many of the

concepts from IGRP which we discuss this semester. In the curriculum, IS-IS is described as a balanced hybrid, but it is more

often regarded as a link-state routing protocol.– “Examples of hybrid protocols are OSI's IS-IS (Intermediate System-to-

Intermediate System), and Cisco's EIGRP (Enhanced Interior Gateway Routing Protocol).” (On-line curriculum)

I also disagree with the following information in the on-line curriculum:

– “Balanced-hybrid routing protocols use distance vectors with more accurate metrics to determine the best paths to destination networks. However, they differ from most distance-vector protocols by using topology changes to trigger routing database updates.”

Balanced hybrid routing protocols don’t necessarily use more accurate metrics than a distance vector routing protocol. EIGRP’s metrics are more accurate than RIP, but not necessarily more accurate than IGRP.

RIP and IGRP both use triggered updates during topology changes to speed up network convergence, the same as a balanced hybrid.

The real difference is that a hybrid routing protocol like EIGRP does not pass entire routing table information periodically like RIP or IGRP and uses other mechanisms for loop free routing.

EIGRP also uses the DUAL algorithm which guarantees loop-free path selection.

Topics – (Continued)Part III. Routing Theory and Dynamic Routing Operations (continued) Hybrid Routing Protocols




Path Switching Host X has a packet(s) to send to Host Y A router generally relays a packet from one data link to another, using

two basic functions: 1. a path determination function - Routing 2. a switching function – Packet Forwarding

Let’s go through all of the stages these routers use to route and switch this packet.

See if you can identify these two functions at each router. Note: Data link addresses have been abbreviated.

Path Switching

192.168.1.10/24 192.168.4.10/24

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e0 e0s0s0 s0s1.1 .1 .1 .1.2 .2

Data link destination address Data link source address Other data link fields

IP Destination Address IP Source Address Other IP fields and data

X Y

Data Link Header IP (Network layer) Packet

Data Link Frame = Data Link Header + IP Packet

From Host X to Router RTA Host X begins by encapsulating the IP packet into a data link frame (in

this case Ethernet) with RTA’s Ethernet 0 interface’s MAC address as the data link destination address.

How does Host X know to forward to packet to RTA and not directly to Host Y? How does Host X know or get RTA’s Ethernet address?– Remember, it looks at the packet’s destination ip address does an

AND operation and compares it to its own ip address and subnet mask.

– It determines if the two ip addresses are on the same subnet or not.– If they are on the same subnet, it looks for the destination MAC

address of the packet in its ARP cache. – sending out an ARP request if it is not there.

– If they are on different subnets, it looks for the MAC address of the default gateway in its ARP cache – sending out an ARP request if it is not there.

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

00-10 0A-10 192.168.4.10 192.168.1.10

RTA to RTB1. RTA looks up the IP destination address in its routing table. 192.168.4.0/24 has next-hop-ip address of 192.168.2.2 and an exit-

interface of e1. Since the exit interface is on an Ethernet network, RTA must resolve the

next-hop-ip address with a destination MAC address.

2. RTA looks up the next-hop-ip address of 192.168.2.2 in its ARP cache. If the entry was not in the ARP cache, the RTA would need to send an

ARP request out e1. RTB would send back an ARP reply, so RTA can update its ARP cache with an entry for 192.168.2.2.

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

0B-31 00-20 192.168.4.10 192.168.1.10

1

2

3RTA Routing Table

Network Hops Next-hop-ip Exit-interface192.168.1.0/24 0 Dir.Conn. e0192.168.2.0/24 0 Dir.Conn e1192.168.3.0/24 1 192.168.2.2 e1192.168.4.0/24 2 192.168.2.2 e1

RTA ARP CacheIP Address MAC Address192.168.2.2 0B-31

RTA to RTB (continued)3. Data link destination address and frame encapsulation After finding the entry for the next-hop-ip address 192.168.2.2 in its ARP

cache, RTA uses the MAC address for the destination MAC address in the re-encapsulated Ethernet frame.

The frame is now forwarded out Ethernet 1 (as specified in RTA’s routing table. Notice, that the IP Addresses did not change. Also notice that the Routing table was used to find the next-hop ip address,

used for the data link address and exit interface, to forward the packet in a new data link frame.

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

0B-31 00-20 192.168.4.10 192.168.1.10

1

2

3RTA Routing Table

Network Hops Next-hop-ip Exit-interface192.168.1.0/24 0 Dir.Conn. e0192.168.2.0/24 0 Dir.Conn e1192.168.3.0/24 1 192.168.2.2 e1192.168.4.0/24 2 192.168.2.2 e1

RTA ARP CacheIP Address MAC Address192.168.2.2 0B-31

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

FFFF 192.168.4.10 192.168.1.10

1

RTB to RTC1. RTB looks up the IP destination address in its routing table. 192.168.4.0/24 has next-hop-ip address of 192.168.3.2 and an exit-

interface of s0 (serial 0). Since the exit interface not on an Ethernet network, RTA does not need

to resolve the next-hop-ip address with a destination MAC address. Remember, serial interfaces do not have MAC addresses.

RTB Routing TableNetwork Hops Next-hop-ip Exit-interface192.168.1.0/24 1 192.168.2.1 e0192.168.2.0/24 0 Dir.Conn e0192.168.3.0/24 0 Dir.Conn s0192.168.4.0/24 1 192.168.3.2 s0

2

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

FFFF 192.168.4.10 192.168.1.10

1

RTB to RTC

2. Data link destination address and frame encapsulation. When the interface is a point-to-point serial connection, the Routing Table

process does not even look at the next-hop IP address. Remember, a serial link is like a pipe - only one way in and only one way

out. RTA now encapsulates the IP packet into the proper data link frame,

using the proper serial encapsulation (HDLC, PPP, etc.). The data link destination address is set to a broadcast, since there is only

one other end of the pipe and the frame is now forwarded out serial 0.

RTB Routing TableNetwork Hops Next-hop-ip Exit-interface192.168.1.0/24 1 192.168.2.1 e0192.168.2.0/24 0 Dir.Conn e0192.168.3.0/24 0 Dir.Conn s0192.168.4.0/24 1 192.168.3.2 s0

2

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

0B-20 0C-22 192.168.4.10 192.168.1.10

RTC to Host Y1. RTC looks up the IP destination address in its routing table. 192.168.4.0/24 is a directly connected network with an exit-interface of e0. RTC realizes that this destination ip address is on the same network as one

of its interfaces and it can sent the packet directly to the destination and not another router.

Since the exit interface is on an directly connected Ethernet network, RTC must resolve the destination ip address with a destination MAC address.

2. RTC looks up the destination ip address of 192.168.4.10 in its ARP cache. If the entry was not in the ARP cache, the RTC would need to send an ARP

request out e0. Host Y would send back an ARP reply, so RTC can update its ARP cache with an entry for 192.168.4.10.

RTC ARP CacheIP Address MAC Address192.168.4.10 0B-20

RTC Routing TableNetwork Hops Next-hop-ip Exit-interface192.168.1.0/24 2 192.168.3.1 s0192.168.2.0/24 1 192.168.3.1 s0192.168.3.0/24 0 Dir.Conn s0192.168.4.0/24 0 Dir.Conn e0

13

2

192.168.1.10/240A-10

192.168.4.10/240B-20

RTA RTB RTC192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24

e000-10

e00C-22

e00B-31

e100-20

s0s0.1 .1 .1 .1.2 .2



X Y

0B-20 0C-22 192.168.4.10 192.168.1.10

RTC to Host Y (continued)3. Data link destination address and frame encapsulation After finding the entry for the destination ip address 192.168.4.10 in its ARP

cache, RTC uses the MAC address for the destination MAC address in the re-encapsulated Ethernet frame.

The frame is now forwarded out Ethernet 0 (as specified in RTA’s routing table.

RTC ARP CacheIP Address MAC Address192.168.4.10 0B-20

RTC Routing TableNetwork Hops Next-hop-ip Exit-interface192.168.1.0/24 2 192.168.3.1 s0192.168.2.0/24 1 192.168.3.1 s0192.168.3.0/24 0 Dir.Conn s0192.168.4.0/24 0 Dir.Conn e0

13

2

From Cisco on-line curriculum: When the router checks its routing table entries, it discovers that the

best path to destination Network 2 uses outgoing port To0, the interface to a token-ring LAN.

Although the lower-layer framing must change as the router passes packet traffic from Ethernet on Network 1 to token-ring on Network 2, the Layer 3 addressing for source and destination remains the same.

In the Figure, the destination address remains Network 2, Host 5, regardless of the different lower-layer encapsulations.

From Cisco on-line curriculum: Routers enable LAN-to-WAN packet flow by keeping the end-to-end source and

destination addresses constant while encapsulating the packet in data link frames, as appropriate, for the next hop along the path.

NOTE: Remember, when the interface is a point-to-point serial connection, the

Routing Table process does not even look at the next-hop IP address in the routing table, only the exit-interface.

Topics – (Continued)Part II. Routing Theory and Dynamic Routing Operations (continued) Hybrid Routing Protocols




Summary

We have covered a lot of topics and a lot of new concepts. These topics will be reinforced when we discuss the specific

routing protocols Understanding these concepts is necessary to understand to be able to design, implement, and troubleshoot networks.

As we will see, anyone can type in a few commands to enable routing on a router, but if you do not understand these concepts we discussed at best you may not be optimally routing packets in your network and at worst you may be creating routing loops, blackholes, and unreachable networks.

Understanding these concepts will also better prepare you for the CCNP Advanced Routing class.

Even if you do not take the CCNP Advanced Routing class, these concepts will help you general understanding networking and routing protocols.

Cabrillo College

Ch. 11 – Routing Basics – End of Part 3Ch. 11 – Routing Basics – End of Part 3

CCNA Semester 2CCNA Semester 2

Rick Graziani, Instructor

routing3

Technology

linkstate algorithm

linkstate information

linkstate flooding

routing theory

theory of link

flooding of link

ip routing

piece of link