EIGRP Subnet Case

Projects-3Case:-  All of the three routers are running eigrp routing protocol . All of the subnets are advertised accurately i.e. the way they are described and summarization is disabled in every router. AS number is 100 in all of the eigrp router. There is no restriction applied anywhere to block any route. Now, as we can see that R6 is advertising 10.1.1.0/28 and R7 is advertising 10.1.1.0/24, so which of these subnets will enter in R5’s routing table? Whether 10.1.1.0/28 or 10.1.1.0/24 or both. Explain every case.

Solution: – Since EIGRP is running well and advertising all of the subnets, so in the above case, both routes will enter in R5 EIGRP routing table. I.e. Both 10.1.1.0/28 and 10.1.1.0/24.

Reason is 10.1.1.0/28 has range of 10.1.1.1 till 10.1.1.15. Range of 10.1.1.0/24 is 10.1.1.1 till 10.1.1.255

When Router 5 has to reach 10.1.1.13, it will have two options to refer from routing table i.e. 10.1.1.0/28 and 10.1.1.0/24. It will give precedence to longest prefix match  i.e. 28 over 24 and therefore, will use 10.1.1.0/28

When Router 5 has to reach 10.1.1.131, It will have only one option i.e. 10.1.1.0/24 so it will choose this one. It will not pick 10.1.1.0/28 because 10.1.1.131 does not come in the range of 10.1.1.0/28.

So R5 will have both subnets in its routing table.

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