Need clarification of VLSM and EIGRP

[aneedell7]

Although EIGRP supports VLSM, its network command does not take a wildcard mask, as does OSPF. Instead, it advertises on classful boundaries. Then how would I advertise individual subnetted routes?

[Richard Burts]

Albert

I am not sure where you got your information from - but it is pretty outdated. In the early implementation of EIGRP the network command did not accept a wildcard mask and the network command would operate on classful assumptions. But for a very long time the improvements in EIGRP have taken a wildcard mask. And with or without a wildcard mask the protocol advertises individual subnet routes.

And the fact that the network statement would operate on classful assumptions does not have any bearing on whether the protocol supports VLSM. The essence of VLSM is the ability to have (and to advertise) subnets with different masks within the same major network. For example the ability to have /24 masks on LAN segments and to have /30 masks on serial links. And EIGRP has supported that since the beginning.

If I have not adequately understood something in your question and my answer does not address your question then perhaps you can clarify a bit so that I can provide better answers.

HTH

Rick

[royalblues]

Just to add to Rick's post, EIGRP by default summarizes at the classful boundaries.

So make sure you configure the no auto-summary command under the EIGRP configuration

HTH

Narayan

[aneedell7]

Hello Rick, thanks for your reply. The Cisco CCNA Study Guide gives the syntax as simply "network " (no wildcard mask). So, if my network statement is something like "network 192.168.1.192", does EIGRP advertise a /27 subnet? Thanks again.

[Richard Burts]

Albert

If that is what the CCNA study guide gives as the syntax then the guide is certainly due for a revision because the syntax changed quite a while back. If you enter the network without a mask then EIGRP will interpret it with the natural (default) mask.

Also to clarify: the mask that you enter (or do not enter) does not have anything to do with what EIGRP advertises. The network statement helps EIGRP decide which interfaces to include in the protocol. Once EIGRP has included an interface then it advertises the mask of the address on the interface.

HTH

Rick

[minumathur]

Hi

Old IOS only not support wildcard mask future, currently new IOS like 12.2ver and later version are supporting this future.

i hope this will clear your doubts

please rate this post.

-Minu