Reducing Multicast traffic

angel-moon · ‎08-19-2008

All,

I don't have a lot of information but here goes. There is a situation where there are 2 offices connected by point to point circuit at 1.5Mbps. The IP PBX there kicks out a ton of constant multicast traffic whene Music on Hold is enabled. This multicast traffic is ALWAYS on, even when no one is using MOH (stinks right?). This is a 3Com NBX 100 system. There is a 2811 on each side of the P2P link. Just in general, are there ways of limiting the amount of bandwidth that can be used by Multicast traffic over the WAN? I know we are going to have to do general QoS for the system to work but I want to limit the amount a bandwidth used by MOH across the WAN as well. Possible?

All replies rated!

Giuseppe Larosa · ‎08-19-2008

Hello Angel,

I would suggest the following:

use sh ip mroute and sh ip igmp groups to understand what are the multicast groups in use.

Once you know what are the multicast groups you want to control use QoS.

define a class-map that uses an extended IP ACL.

example : suppose multicast are 225.70.50.x

access-list 121 permit ip any 225.70.50.0 0.0.0.255

class-map moh_traffic

match ip address 121

on the lan interface that receives the traffic define a policy-map that will police to 256 kbps the moh traffic

policy-map limit_moh_traffic

class moh_traffic

>> police 256000 ! other param needed here

class default

int gi0/0

service-policy input limit_moh_traffic

Hope to help

Giuseppe

Joseph W. Doherty · ‎08-19-2008

Are you sure the traffic is multicast? Normally, multicast wouldn't flow between router interfaces anywhere unless multicast routing was enabled.

If you can uniquely identify the traffic, you could shape it, police it (rate limit it) or drop it outright.

Performing any type of shaping or policing is likely to reduce the quality to such an extent, it might be pointless in allowing any, at a reduced data rate, to transit your WAN link.

The easiest way to drop the traffic completely is via an ACL. If you want to try to shape it or police it, either could likely be done via a CBWFQ policy.

[edit]

Giuseppe's post is an example of a CBWFQ policy doing policing.

PS:

Another approach might be to deprioritize the specific MOH traffic so it only uses available bandwidth. If there's enough for it at any time, fine and good. If not enough bandwidth, its quality suffers, as likely to happen with any type of rate control.