EIGRP summary route

[malithperera1]

Hi Guys,

I have the following topology, all lines shows the connections.

(ROUTER 1) -----------------------(ROUTER 2)

      |                                               |

      |                                               |

      |                                               |

      |                                               |

      |                                               |

(ROUTER 3)------------------------(ROUTER 4)

I have L0 - 172.16.0.1/24,

         L1 - 172.16.1.1/24,

         L2 - 172.16.2.1/24

         L3 - 172.16.3.1/24 on Router 3 and summarizing them as 172.16.0.0/22.

I added ip summary-address eigrp 172.16.0.0 255.255.252.0 on interface pointing to Router 1.

However when I check Router 2 routing table, I see the summarized route 172.16.0.0/22 learning through Router 1 and specific routes (172.16.1.0, 172.16.2.0 and 172.16.3.0) through router 4.

I did change bandwidth and delay on the interface on interface poining to router 4 on router 3 so it will be les used but Router 2 still learn specific routes.

Is there a way to fix this so I can only see one 172.16.0.0/22 will be learned through R1 and supress specific routes by default going out of interface towards Router 4 instead of adding another summary route to the interface pointing to Router 4 ?

Thank you in advance.

[Peter Paluch · Accepted solution]

Hello Malith,

Is there a way to fix this so I can only see one 172.16.0.0/22 will be 
learned through R1 and supress specific routes by default going out of 
interface towards Router 4 instead of adding another summary route to 
the interface pointing to Router 4 ?

You could use the distribute-list feature to filter routes that are announced or received via an interface. On R1, you could thus configure:

ip prefix-list EIGRPRoutes deny 172.16.0.0/22 ge 24 le 24
ip prefix-list EIGRPRoutes permit 0.0.0.0/0 le 32
router eigrp 1
 distribute-list prefix EIGRPRoutes out F0/0 ! assuming F0/0 leads to R4

Please note that your requirement on the configuration is rather unusual. If a summarization is configured on a router having multiple interfaces to neighbors, the summary route shall correctly be applied to all these interfaces. Otherwise, both the summary and the unsummarized component routes will be advertised to the surrounding network, and if the network is cyclic like yours, it will result in both summary route and the component routes being advertised throughout the entire network, not saving any space in your routing tables, on the contrary - making them even bigger than they were before configuring the summarization at all. Of course, because of longest-prefix-match rule, the routers will always use the best match to forward the packets to your networks, and these packets will follow the unsummarized route.

Best regards,

Peter

[malithperera1]

Hi Peter,

Thank you very much for the reply.

regards,