Re: IGMP configuration

parisdooz12 · ‎10-17-2011

Hi,

to activate IGMP on an interface, it is necessary to configure PIM on it. It generates PIM messages even if these one are not necessary (which if quite always the case: on an interface is necessary IGMP, or PIM, but normally, not the 2 one).

Is it a way to activate IGMP without activate PIM on the interface?

Thanks.

P.

Reza Sharifi · ‎10-17-2011

Hi,

If you are multicasting over multiple subnets, you need PIM, but if you only have one subnet/vlan then you can just enable IGMP.

HTH

Jon Marshall · ‎10-17-2011

If you only want multicast within the same subnet then rather than enable PIM on the interface, if your switch supports you can enable the IGMP querier function.

The querier function will make the IGMP requests that are needed for IGMP snooping to work properly.

See your switch config docs to see if the querier function is supported.

Jon

parisdooz12 · ‎10-17-2011

Actually, the equipment i spoke is a C6500. On it, I have 3 types of interfaces:

- IP interfaces to connect to the backbone (so PIM is activate on them)

- IP sub-interfaces connected, via a trunk, to a L2 switch

- IP Interface Vlan, connected to L2 switch

On the 2 last types of interfaces, I just want my C6500 act as an IGMP qurier, but not send PIM messages.

Jon Marshall · ‎10-17-2011

The 6500 does indeed support the IGMP querier function. See the config doc for details -

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/snooigmp.html#wp1070778

Note that without PIM enabled the multicast will not be routed across vlans ie. each vlan without PIM will only send multicast messages to clients that register within that vlan.

Jon

parisdooz12 · ‎10-17-2011

PIM is actually indeed activated on the C6500 as I need to route multicast.

But today, PIM is configured on all L3 interfaces, even the one conncted to a L2 switch where only IGMP is necessary.

It not really clear on Cisco document. On some document, they indicate PIM has to be activated on the interface, like here:

http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094821.shtml#dense


interface ethernet0 
ip address  
ip pim sparse-dense-mode  
where Eth0 is a LAN interface where hosts are connected.
In other document, it seems that the simple fact to activate ip multicast is necessary to activate IGMP on the interfaces (except, I guess, the one where PIM is configured):
http://www.comms.engg.susx.ac.uk/fft/networking/SC-BCMSN-Multicast.pdf => p274
So, I I want to explain, my problem is not really linked to IGMP snooping, but to the way to activate IGMP. The pb os that I have not the opportunity to test it easily.

Jon Marshall · ‎10-17-2011

I think there is some confusion here.

If you want to route the multicast traffic then you need to enable PIM. Lets say you have a L3 switch 2 vlans on it - vlan 10 and vlan 11.

You have a L2 switch connected to it with clients in vlan 10. If you want these clients to be able to receive multicast traffic from vlan 11 then you need to enable PIM on both the vlan 10 and vlan 11 L3 interfaces.

PIM by default enables IGMP. But if you don't want to route the multicast traffic and you are not interested in IGMP snooping then why would you need to enable IGMP ?

Jon

parisdooz12 · ‎10-17-2011

IGMP permit to an host to indicate to a router (its gateway) that it want to receive traffic for a multicast group. IGMP permit a router to know multicast group memberships on the L3 interface where IGMP is activated.

IGMP snooping is just an additional feature which permit, on a L2 switch, to send multicast only on ports interested by a specific multicast group.

So let's see 2 cases to take your example:

Case#1: I have vlan 10 & vlan 11 on my C6500, and only multicast client on them (so no multicast traffic from 11 to 10). In this case, I d'ont need PIM on the vl10 & vl11 L3 interfaces, but I need IGMP in any case

Case#2: I have a server in vlan 11 and hosts in vlan 10 (so multicast traffic from 11 to 10). In this case, as multicast is configured on the C6500, multicast routing occurs.

Jon Marshall · ‎10-17-2011

I understand what you are saying, i just don't think you follow what i am saying

Case1# What do you need IGMP for ?

If you do not need to route the traffic then there is no need for the L3 interface to know that clients want to receive the traffic. If you are not interested in filtering multicast at L2 then you have no need for IGMP within that vlan. Multicast traffic will simply be sent to all ports in that vlan ie. treated as broadcast.

So if you are not interested in routing it and you are not interested in filtering the ports at L2 then what exactly do you think IGMP will give you ?

The hosts in that vlan will send an IGMP message when they first boot up but after that nothing. The only thing that will enable IGMP after that is to enable the IGMP querier function for that vlan (if you are not using PIM on the L3 interface).

Case2# Yes you would need PIM enabled on the interface which automatically enables IGMP.

Jon

parisdooz12 · ‎10-18-2011

Perhaps it is a misunderstanding from me, but without IGMP on the interface, I understand that the source will never receive multicast:

- If I use PIM Dense Mode: the router where the host (multicast receiver) is connected will never know that there is a host, so the branch in the tree will be pruned

- In Sparse Mode: quite the same: the router where the host is connected don't know that there is a host for the multicast group, so will never construc the tree up to the RP.

Sorry for my insistance (and perhasp my misunderstanding) and thanks for your patience. I really need to understand well the mecanism.

Jon Marshall · ‎10-18-2011

No problem.

If you use either PIM Sparse or Dense mode then you have to enable PIM on a L3 interface. As soon as you do that IGMP is automatically enabled for you. You cannot run PIM wihout enabling it on the L3 interfaces.

So by configuring PIM on the L3 interface that L3 interface then starts sending out IGMP queries. Any hosts in that vlan will then respond to the IGMP queries. That vlan could be all on the same switch or it could be spread across any number of switches, the same thing happens. All hosts within the vlan see the IGMP query.

If you don't enable PIM within that vlan then -

1) no queries are sent within that vlan

2) no multicast traffic can be routed into that vlan

any multicast traffic generated within tha vlan cannot be sent to any other vlan and will be flooded to all ports within the vlan (other than the one it originated on) ie. it becomes broadcast traffic.

IGMP snooping is a L2 mechanism for filtering multicast traffic within a vlan. It listens to repsonses to IGMP queries. But by default on a switch if you do not have PIM enabled on the L3 vlan interface there is nothing to make the queries. So on most switches there is a function called the IGMP querier which can be enabled. This allows the switch to make IGMP queries within the vlan and so IGMP snooping then has something to listen to and so can filter the multicast.

It is an either or situation ie. you either -

1) want tor route the multicast traffic in which case you run PIM on all the L3 vlan interfaces you want multicast traffic to route between. In this case IGMP is enabled automatically, multicast traffic can be routed and within the vlan IGMP snooping works because IGMP has been enabled. There is no need to enable the IGMP querier function

or

2) you only want to filter multicast traffic within the vlan ie. no need to route. By filter i mean you only want the multicast traffic to go to hosts that are interested in it. In this case because you have not enabled PIM and therefore you have not enabled IGMP, you can use the IGMP querier function within that vlan.

Jon

parisdooz12 · ‎10-18-2011

Ok, so we are agree that without IGMP activated on a L3 interface (through PIM or through IGMP querier), the multicast traffic is not sent on the vlan?

Another observation: I can configure IGMP querier on an L3 interface vlan, but not on a L3 subinterface (gig3/x.y), connected to a L2 switch. In this case, the only way is to activate IGMP seems to configure PIM on the interface (and so, PIM packets are sent through the vlan, even if these packets are not necessary as I have only hosts behind the sub-interface)

Jon Marshall · ‎10-18-2011

Ok, so we are agree that without IGMP activated on a L3 interface (through PIM or through IGMP querier), the multicast traffic is not sent on the vlan?

Multicast traffic is still sent within the vlan but it is treated as broadcast traffic.

I wasn't aware it couldn't be configured on a subinterface on a vlan interface as i have never tried it on a subinterface. So yes for mutlcast to be filtered properly within that vlan you would need to either -

1) statically map the switch ports in that vlan with the multicast address but obviously this is not efficient if you have hosts dynamically joining and leaving the group

or

2) enable PIM on the L3 subinterface. Note with Sparse mode even with PIM enabled the multicast traffic will not be routed to other vlans unless there are clients who request that traffic. With Dense mode it would be flooded then pruned back.

Jon

parisdooz12 · ‎10-18-2011

Ok. Many thanks for your explanations!

P.

Jon Marshall · ‎10-18-2011

No problem, hope it helped.

Jon