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Drawbacks use Secondary IP?

What are the drawbacks in using Secondary IP?? Can anyone tell me?

Want to use secondary IP for migrating IP networks to a new IP plan. Have to use secondary IP for longer period because the migration process will take a while ( 6 months).

Kind regards.

Harry Gorter


Re: Drawbacks use Secondary IP?

One disadvantage i can think of, is that if you have routing protocols such as ospf, eigrp etc, you will have problems forming neighbor relationships, if you have eigrp or ospf neighbors on secondary subnets.


Re: Drawbacks use Secondary IP?

THe routing protocols will advertise the secondaries, they just won't build neighbor adjacencies across the secondaris. I would try and make the secondary the old addresses, and put the new addresses on the primary, so when you pull the secondaries, it won't impact routing as much....

Another disadvantage is the "route through the router between two hosts on the same wire" syndrom. If you have two hosts, say and, on the same wire, and you have the router configured as, with a secondary of, when wants to reach, it will actually forward the packet to the router, since the doesn't look like it's on the same segment. The router will switch the packet back out the same interface to the right host--so the traffic between two hosts on the same wire, but in two different subnets, will cross the wire twice.

Beyond these things, there might be issues with HSRP and/or GLBP that you could run in to, and it could make troubleshooting problems more difficult.

If using secondaries is the best way to convert addressing schemes, and in the end, the network is going to be more summarizable, etc, then it's fine to live with them for a while. It's also possible to use them in situations where none of the issues described thus far have any impact.


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