Eigrp Summarization problem

[fabriziospada]

Hello, i need to know if is possible to advertise via eigrp only a part of a connected subnet using the summarization command 

For example , i have an interface vlan /24 buy i want advertise to my neighbor only a /25 

Thank you 

[MHM Cisco World]

I dont think this possible.

Even if you config static route to null0 the traffic will drop becuase /25 is longer match than /24.

Sorry 

[Georg Pauwen]

Hello,

@MHM Cisco World is right, you cannot summarize to a longer prefix.

The only way I see is to redistribute a static null route, as in the example below:

router eigrp 1
redistribute static
!
ip route 192.168.1.0 255.255.255.128 Null0

[fabriziospada]

Hello, if i use a null 0 static i lose connectivity to end point belongs to /25 subnet

[Giuseppe Larosa]

Hello @fabriziospada ,

The only way I see is to use a secondary address but I am not able to test it now

Divide the original /24 prefix into two /25 prefixes and use secondary address for the secod subnet

at that point you can decide which of the two /25 you can advertise by applying a distribute-list on the upstream link(s) in that context you can use a prefix-list.

Being EIGRP derived from a pure DV protocol you have full control of what you send out each interface.

Warning: EIGRP neighborships can be built only on primary ip address so the proposed approach applies to end user facing VLAN/subnets.

For more complex scenario you can use a VRF, EIGRP VRF aware and by using also MP BGP you might  achieve what you want

Hope to help

Giuseppe

[paul driver]

Hello

You could advertise only routes with a prefix-length of /25 within that /24 subnet

ip prefix-list VLAN permit 100.100.100.0/24 ge 25 le 25

router eigrp xxx
no auto-summary
network 100.100.100.0
disbribute-list prefix vlan out

[fabriziospada]

Where should I use this prefix-list?
Thank you

[mlund]

Hi

You can try with

ip route 192.168.1.0 255.255.255.128 vlan<nr>

where <nr> is the same number that you have your /24 configured.

/Mikael

[paul driver]

Hello

If you just want advertise specific routes and no summary then really no need to summarise-especially with two /25 subnets -try the suggestion i posted it should provide the solution you are looking for.