IOS Features & Management

Multicast: What is the difference between an igmp filter and an igmp access-group?

After doing IE Mock Lab 5, and analyzing it over the last few days, I had an epiphany with regards to an igmp filtering question. The question required you to limit users joining a certain multicast group, and also place a limit on how many groups users in that vlan can join. I immediately thought “Oh cool, no problem ip igmp access-group with a standard access list, and an ip igmp limit and we are good to go! This stuff is easy”. Brrr! Wrong! :)

The ip igmp limit and ip igmp access-group commands apply to a layer 3 routed port! The question involved setting this up on a layer 2 port so this wouldn’t work properly. The correct answer involved ip igmp profile and ip igmp max-groups.

So, lets say you have a requirement where you want to limit users on vlan 10 from joining multicast groups 232.0.0.0 -> 239.255.255.255. The requirement also says to limit it so they can belong to only 5 multicast groups. How would you do this on a layer 2 port vs a layer 3 port? I’m glad you asked! :)

Solution on Layer 3 Switch:

Layer 2 Port (ie switchport)

int f0/10
 switchport mode access
 switchport access vlan 10
 ip igmp max-groups 5
 ip igmp filter 1
!
ip igmp profile 1
 deny
 range 232.0.0.0 239.255.255.255

Layer 3 Port (ie routed port)

int vlan 10
 ip igmp access-group DENY_THESE_GROUPS
 ip igmp limit 5
!
ip access-list standard DENY_THESE_GROUPS
 deny 232.0.0.0 7.255.255.255
 permit any

The Layer 3 port configuration would equally apply on a routed port (ie port with no switchport configured or a port on a router). Notice the differences in defining of the groups allowed. “Hey Ards, How about one of your famous tutorials on this so I can follow along in Dynamips?”. Sorry, no time :) My CCIE lab is in 30 days and I have a million practice labs to do! I’ll write one up when I’m done…hopefully with some CCIE digits next to my name!

© 2008 Arden Packeer. All rights reserved. The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway. This material is not sponsored or endorsed by Cisco Systems, Inc. Cisco, Cisco Systems, CCIE and the CCIE Logo are trademarks of Cisco Systems, Inc. and its affiliates