One major function of a routing protocol is to spread location information. When

Question

One major function of a routing protocol is to spread location information. When a new network is attached to a router, it will report the information to other routers. (a) In Fig. 5a, can router A still perform route aggregation and combine three networks into one and report it to B? (b) When a link failure is detected, a router needs to spread that information as well. But too many routing updates can make the network unstable. Suppose the link between 200.23.00010000.0/24 goes down. Does router A need to report this error to B? In other words, does it make any difference if A report that failure or not? (c) In figure 5b below, suppose 200.23.00010011.0/24 is our campus network and we use router C to connect to two different ISPs (Internet service providers) through router A and router D. This technique is usually called double-homing. The idea is that if one ISP goes down (such as the link C-A or C-D), we can still use the other ISP. Can double homing still work if route aggregation is performed by both router A and D?

Explanation / Answer

A) Yes, Router will perform route aggregation when new network is attched to the router. It will generalize all the prefixes and rather than advertising each and every prefix it will advertize a generalized prefix to router B.

For example, in our question, the IP Addresses of the networks connected to A are 200.23.16.0, 200.23.17, 200.23.18.0. So instead of advertizing individual prefix, Router A will advertize the generalized destination prefix as 200.23.16.0/30, (/30 for 4 users).

B) When a router advertizes a summary of the routing table, it hides failures and instabilities from other routers. This hiding prevents unnecessary routing protocol convergence and keeps the internetwork stable.So, when a link failure occurs, router advertizes its routing table to every other routers hiding the failure details. Failure details are not mandatory to be advertized, but new routing table after the link failure is must to be advertized.

As, in our question if the between 200.23.00010000/24. The router will stop advertizing the summary of this failed link to router B.

C) Yes, by configuring Dual-homed Inter-Board Link Aggregation in which two devices(Router A and Router D) are configured as primary and secondary upper layer devices.LACP(Link Aggregation Control Protocol) is used in this type of network, which also comes with LAG's (Link Aggregation) functionalities. We can use LAG function to form a logical point-to-point link known as LAG link or virtual link.

Thank You. If any queries, ask in comment section

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