F. Routing Concepts
1. Routing protocols job is to maintain routing tables and route packets
appropriately. Examples of routing protocols are RIP, IGRP, EIGRP, OSPF.
Routers can support multiple independent routing protocols and can update
and maintain routing tables for each protocol independently.
Routed protocols are used to transport user traffic from source node to destination node. Examples of routed protocols are IP, IPX, AppleTalk.
2. There are broadly three types of routing protocols:
1. Distance Vector (Number of hops) - Distance vector routing determines
the direction (vector) and distance to any link in the internetwork. Typically,
the smaller the metric, the better the path. EX: Examples of distance vector
protocols are RIP and IGRP. Distance vector routing is useful for smaller
networks. The limitation is that any route which is greater than 15 hops
is considered unreachable. One important thing that differentiates distance
vector with Link state is that distance vector listens to second hand information
to learn routing tables whereas, Link state builds its routing tables from
first hand information. Distance vector algorithms call for each router
to send its entire routing table to each of its adjacent neighbors.
2. Link State Routing: Link State algorithms are also known as Shortest Path First (SPF) algorithms. SPF recreates the exact topology of the entire network for route computation by listening at the first hand information. Link State takes bandwidth into account using a cost metric. Link State protocols only send updates when a change occurs, which makes them more attractive for larger networks. Bandwidth and delay are the most heavily weighed parts of the metric when using Link-State protocols. EX: OSPF and NLSP.
Benefits of Link State protocols:
· Allows for a larger scalable network
· Reduces convergence time
· Allows “supernetting”
3. Balanced Hybrid - Balanced Hybrid combines some aspects of Link State and Distance Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics to determine the best paths to destination networks. EX: EIGRP.
3. Distance vector protocol depends only on Hop count to determine the nearest next hop for forwarding a packet. One obvious disadvantage is that, if you have a destination connected through two hops via T1 lines, and if the same destination is also connected through a single hop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!
4. All RIP, RIP2, and IGRP use distance vector algorithms.
· RIP2 transmits the subnet mask with each route. This feature allows VLSM (Variable Length Subnet Masks) by passing the mask along with each route so that the subnet is exactly defined.
5. In RIP, after route convergence, all routes are distributed to the neighbors by a router.
6. Maximum hop count supported by RIP is 15. A hop count of 16 or greater is considered unreachable.
7. The command syntax for configuring RIP version1 on a router is:
· Router rip
Network <network number>
Note that the network numbers are specified as either class A, or Class B or Class C; and contain no subnetting information.
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