Unit 11 – Interdomain routing
1. Previously we assumed that a router or a switch has enough information about the network to find
routes across it. In the case of virtual circuits the only problem is for the first packet (connection
request packet) that establishes the route and connections. Once established the packets will
willow on the same route
2. In a connectionless situation, a route has to be sought for each and every packet. In this case the
destination address of each packet is sought and the packet is routed
3. These decisions are taken by a router and a switch through internal routing tables
4. First we have to see how switches and routers populate their routing tables
5. Routing is the process by which forwarding tables are built
6. Routing, unlike forwarding, depends of an set of complex algorithms
7. Be careful, as forwarding and routing are sometimes used interchangeably
8. In forwarding a packet must contain enough info to complete the forwarding action
9. The table would have a MAC address of the next hop and a forwarding port
10. Routing tables are built before forwarding tables
11. Routing tables and forwarding tables are separate data structures
12. They are rarely, if ever, built in hardware
13. While routing table tells us how to reach a network, the forwarding table tells us how to forward a
packet to that next hop
14. The book at this point discusses, only the small to medium sized networks over one administrative
control regime
15. We can represent a network as a graph, where by routers are the nodes in the graph and
connections are the edges of the graph. Each edge has an associated cost to it.
16. Problems then reduce themselves to graph theory
17. The basic cost of routing is to find the path with the shortest cost to it
18. There are many factors that come into play when deciding on which is the best route, or algorithm
to choose, when to decide a path.
19. Algorithms are unusually distributed in nature and allow for network saleability
20. This means that there may be two different routers taking different decisions about the same route
21. Two main classes of routing protocols shall be discussed:
a. Distance vector
b. Link state
22. The Distance Vector
a. In this case each node updates a routing table from the information it receives from its
neighbour nodes
b. Unreachable nodes are marked by infinity
c. Nodes that can be reached through others are updated and the last cost added to the
resultant cost
d. But this can cause issues, as explained well in the book pg 272
e. The infinity problem arises when nodes fail to stabilise
23. Stability is a crucial problem in networks, and the quicker a network reaches stability the better
24. In this case networks are programmed to consider numbers that are reasonably high as infinity, and
hence a node marked as unreachable
MMM – Tutorial Notes
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Unit 11 – Interdomain routing
25. RIP – Routing interface protocol
a. Protocol famous because distributed with Unix
b. Built around the distance vector algorithm
c. Simple in scope
d. RIP normally advertises every 30 seconds,
e. It uses the same concept of allocating a cost of 1 to each node
f. But instead of node cost, it uses cost to reach networks
g. The lower cost to reach a network is always preferred
h. For mechanism see 276 in book
i. It uses 16 as infinity, thus this limits a network to 15 hops
26. Link State – OSFP (Open Shortest Path First)
a. This is the second class of intradomain routing protocol
b. We start with the same assumption as above, that we can learn the state of a link through
next neighbours
27. This is based on the knowledge that every node knows what it costs to reach its nearest neighbour
28. If this is disseminated to all on the network then we are able to have a picture of the whole network
29. Reliable flooding
a. This is to make sure that all participating nodes get a copy of the link-state info from the
other nodes
b. A node will send link information to all its other neighbours
c. The LSP contents are described on page 276 in the book
d. Follow the process from the book
30. LSPs are generated using a sequence number. When received the node compares sequence
numbers, it retains the newest one
31. Generating LSPs is a resource consuming process, so the less we consume the better.
32. Once we have an LSP from every other node then we can build a network map
33. Every so often hello packets are sent to the nearest neighbour to see if they are still active
a. If no response or hello then the route is marked as down
b. The update of the LSP is done only if a site is down
34. With this network map we can decide the best route towards delivering a packet
35. Dijkstra’s algorithm is a well-known algorithm
36. Follow the workings from the book P286
37. Open Shortest Part First (OSFP)
a. This is one of the most widely used link-state protocols
b. Open refers to non-proprietry
c. See the structure of the header on page 284
d. OSFP has a route authentication feature which enables the detection of mis-configured
router
e. Routing updates are authenticated
f. Early versions used an 8-bit password, but did not offer strong protection
g. OSFP has a hierarchy it partitions domains into areas
MMM – Tutorial Notes
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Unit 11 – Interdomain routing
h. Routers within a domain do not need to know how to reach every network, knowing how to
get to the right area is enough
i. This therefore reduces the amount of info needed to be carried by nodes
j. OSFP also has load balancing routines where various routes are supplied to take you to the
different nodes at a similar cost are loaded and then they load can be balanced across a
network
MMM – Tutorial Notes
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