Secondary address.

by arul on March 15, 2010

I want to configure EIGRP with ip secondary address on differents routers.What can be disavantage of ip secondary with EIGRP ?

{ 2 comments… read them below or add one }

karan March 15, 2010 at 4:44 am: There are 2 disadvantages I can think of, though neither are really specific to EIGRP.
1. Traffic between the 2 subnets has to pass through the router even though they are in the same broadcast domain. This “one-armed” routing is probably not a big deal unless there is a huge amount of traffic between the subnets.
2. You can’t use DHCP via ip helper addresses for PCs in the secondary subnet. The router will always use the primary subnet when it relays the DHCP request to the DHCP server.
Kumar March 15, 2010 at 4:44 am: There is another aspect of secondary addresses which you should be aware of and which does impact using secondary addresses with routing protocols including EIGRP. The routing updates (and the EIGRP HELLO messages) have their source address as the primary address and not the secondary address. So EIGRP can not form neighbor relationships using the secondary addresses. This means that all routers on that segment must have primary addresses in the same subnet, and configure EIGRP to use both the primary address and the secondary address. If you do not do this EIGRP will not be able to form neighbor relationship and will not advertise routes.

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