Multicast on the LAN

[carl_townshend]

Hi All

We have had reports of some performance issues on our LAN with some IPTV software, the company are saying that the images are pixelated since adding a couple more devices.

They have asked us to enable IGMP snooping.

My question is, how would this improve the issue as the switch forwards the multicast anyway without igmp snooping.

What benefits would this give to performance?

Also some of the TV's are on different switches, same vlan though.

Do I need to enable one of the switches as a querier?

Cheers

[marce1000]

 - Perhaps the benefit lies in the reverse condition in the sense that mulicast traffic will not be send to ports where no iptv device is connected.

 M.

[Dennis Mink]

As far as i know igmp is layer 3 and igmp snooping is dependent on that. So i fail to see how that would improve performamce within a vlan/subnet. Curious to see what other members think about this.

[Giuseppe Larosa]

Hello Carl,

>> My question is, how would this improve the issue as the switch forwards the multicast anyway without igmp snooping.

If IGMP snooping is disabled on your LAN switches, the LAN switches are flooding every multicast stream out every port,  with exception of the source port (from the point of view of each switch) in the same Vlan.

If really IGMP snooping is disabled, the network is full of unnecessary flooding at OSI L2. A single receiver might be impacted by simply having to discard multicast packets it is not interested in receiving.

And yes if you enable IGMP snooping you need to configure one switch (only one, election is not supported) as the IGMP snooping querier in order to have someone to send periodic IGMP Queries and allow the feature IGMP snooping to work by listening to host IGMP reports and creating lists of outgoing layer2 ports for each multicast MAC address.

To be noted in some specific cases, if the receiver is not compliant with multicast standards and does not send out IGMP reports for group it is interested in, after enabling IGMP snooping that host will stop to receive any multicast traffic.

(these special cases can be handled by using static join on switch port)

IGMP snooping + one IGMP querier can provide scalability in a large L2 network by optimizing multicast traffic replication out of L2 access ports by sending traffic of each group only out the ports that requested it.

Note: for the way that the multicast MAC address for IPv4 are built there are actually 32 multicast IPv4 addresses that share the same multicast MAC address.

For example 225.0.0.1 and 225.128.0.1 share the same multicast address at ethernet level as only the last 23 bits are mapped

So the recommendation for optimal control of traffic is to use multicast groups that differ in the last 23 bits of address.

To be noted IGMP snooping does not process link local multicast addresses 224.0.0.X that are still fllooded.

Hope to help

Giuseppe

[carl_townshend]

Hi There

The switches all go back to a common collapsed core, a 6509, where then vlan is tagged too.

Would it be better that this is set to the IGMP querier ?

[Giuseppe Larosa]

Hello Carl,

the C6500 can also perform ip4 multicast routing.

If multicast routing is not needed I agree that the C6500 is the best candidate for the role of IGMP querier on the required Vlan(s)

Hope to help

Giuseppe

[Joseph W. Doherty]

"What benefits would this give to performance?"

Some switches are a bit weak processing multicast. Suppressing the need to send multicast to all VLAN ports might reduce the load on the switch.

Further if you have more than one concurrent IPTV stream, sending undesired additional streams to the same host port could impact how well the host is able to obtain and process the desired stream(s).

[carl_townshend]

Hi Joesph

What IP address can the querier be on? can it be the management vlan of the core switch? and as long as the other switches can route to it that will be OK?

Cheers

[Giuseppe Larosa]

Hello Carl,

you need a Querier in each Vlan because IGMP queries are sent (General Queries) to 224.0.0.1all hosts in subnet multicast address.

This multicast address has link local scope and it cannot be multicast routed.

IGMP is the protocol used between routers/switches and multicast receivers directly connected = in the same IP subnet.

Hope to help

Giuseppe

[carl_townshend]

Hi

Would it not just be easier to configure PIM on the SVI of the core switch?

I have actually done this, and used sparse mode.

all the switches have igmp snooping enabled.

Do I need to do anything else on the switches?

When I look on the switches, they do not seem to be learning an mrouter port, why is this?

cheers

[Giuseppe Larosa]

Hello Carl,

if you have enabled PIM on each SVI, IGMP is enabled by default using version 2.

So you should be fine with your configuration. IGMP snooping on switches should work well.

>>When I look on the switches, they do not seem to be learning an mrouter port, why is this?

They should classify as mrouter ports the ports where IGMP queries or PIM messages are received.

Are the access layer switches uplinks  L2 trunk ports ?

Hope to help

Giuseppe

[carl_townshend]

Hi

We have enabled it on the Firewall L3 interface, it is now all working, you can see the mrouter port being learnt from each switch now on the that vlan.

So by default, if you have a single switch with igmp snooping enabled, if there is no querier or pim router set up, will the switch drop the join requests and just treat them the same as a broadcast and flood to all ports?

[Giuseppe Larosa]

Hello Carl,

good news that you have solved, if the firewall is on the path and it is working at L3, you need to enable PIM on it also, as you did.

If you have enabled IGMP snooping but NO PIM router is  on the Vlan, and no IGMP snooping querier is configured, all multicast traffic will be stopped except link local addresses 224.0.0.X as I have written in my first post in this thread.

Because with no one sending Queries the switch can only see unsolicited IGMP reports from hosts that are not enough to populate the Group to ports mappings.

Hope to help

Giuseppe

[Joseph W. Doherty]

As Giuseppe replied, you're querier needs to be in the network you want it to function for. As it's a L2 feature, you cannot route to that IP.