Question

(Intra Domain Routing)

a. Identify and briefly explain any three key features, a

distance vector protocol must incorporate in order to make it loop free?

(100 words)

b. In the case of inter-area routing, OSPF operates like a

distance vector routing protocol. Explain why? (100 words)

Answer 1

a. A brief explanation of three key features, a distance-vector protocol must incorporate in order to make it loop-free in Intra Domain Routing:

* In general, the distance vector protocol uses certain "algorithms" to make itself loop-free. Simply put, any and all packets "should not repeatedly be routed through same routers multiple times", ending up in an infinite loop or circle.

* At times, it requires distance vector protocol such as Enhanced Interior Gateway Routing Protocol (EIGRP) to suspend by forwarding to few or more destinations, at the same time, tables are in flux.

* A distance vector protocol can incorporate a "split horizon" feature making it loop-free. Split horizon stops a router from advertising of routing back onto the interface it was learned from. It is used with a poison reverse technique.
A successor router knows the routing update for a route coming from its predecessor router so it would not advertise the route back to its previous router it gained information or route information from.

* Another feature is called "route poisoning", where any router connected directly to an immediate route, or subnet, detects the route is down or has failed, it advertises that route with an infinite metric, thus poisoning the route, and hence the name. Any router receiving the advertisement or the update would be aware of the route been failed and would not use it in the future.

* The feature called "hold-down timer" is used preventing any router from learning any new information about an unreachable route. As soon as any router is aware of the information about the route been down, it starts a hold-down timer on its end.
This is a hold time to refuse route updates for that route for a few minutes, say, 180 seconds, until the timer expires or after a route retraction, thus preventing forming loops.

b. Explanation about the reason why Open Shortest Path First (OSPF) operates as a distance-vector routing protocol in the case of inter-area routing.

OSPF operates within a hierarchy. The autonomous system (AS) is the largest entity within this hierarchy. AS is a set of networks working under a common administration sharing a common routing strategy. OSPF is an intra-AS (interior gateway) routing protocol, even when it can receive routes from and send routes to other ASs. ASs are divided into a number of areas. These areas are groups of contiguous networks and attached hosts. Routers which are also called Area Border Routers which have multiple interfaces, participate in multiple areas, maintaining separate topological databases for each area.

Topological databases are overall pictures of networks with respect to routers. The topological database has a collection of Link-State Advertisements (LSAs) received from all routers in the same area. As routers in the same areas share and have the same information, they have the same topological databases.

An OSPF backbone shares routing information amongst areas. It has all Area Border Routers; the networks that are not completely held in any area; and their attached routers. Routers make up the backbone. The backbone itself makes up an OSPF area. Hence, backbone routers use the same procedures and algorithms maintaining routing information within the backbone any area router would.

This forms an internetwork, consisting of several areas. An OSPF AS would have multiple areas linked by several routers.

A domain is a part of the network of all routers with identical topological databases. Domain is referred to as AS.

Any area’s topology is not visible to the entities that are outside the area. Maintaining area topologies separate, OSPF passes less routing traffic than it would in case, the AS were not partitioned.

Area partitioning causes two different types of OSPF routing. If the source and the destination are in the same areas, intra-area routing occurs, and if they are in different areas, inter-area routing occurs.