IGMP snooping on a ME-3750

Unanswered Question
Aug 11th, 2010

Hi,

I have a very limited understanding of the utilization of IGMP snooping. My understanding is that IGMP snooping is listening IGMP conversation between host and router. When a IGMP report is hear from a host multicast group, the switch creates a multicast entry for that group.

My questions are the following:

1. Does the switch forward the IGMP report from host to all ports associated to the broadcast domain or only router port and how does the switch knows which port is the router port.

2. If IGMP report is only forward to the router port with IGMP snooping, does that means the multicast server cannot be connected on the same VLAN of a single switch as the host.

3. When IGMP is used, is there a command to see which ports are associated to a multicast entry

4. In a case where IGMP snooping is disable, can I assume that IGMP report will be forward to all ports that belongs to the broadcast domain and consequently multicast traffic will be receive on each ports that belongs to the broadcast domain.

5. IGMP snooping main purpose is to reduce bandwidth traffic on port that does not need multicast traffic.

Thanks for your help

Stephane

I have this problem too.
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Mike Pavlovich Wed, 08/11/2010 - 18:38

Answers inline...

Hi,

I have a very limited understanding of the utilization of IGMP snooping. My understanding is that IGMP snooping is listening IGMP conversation between host and router. When a IGMP report is hear from a host multicast group, the switch creates a multicast entry for that group.

Correct


My questions are the following:

  1. Does the switch forward the IGMP report from host to all ports associated to the broadcast domain or only router port and how does the switch knows which port is the router port.


IGMP snooping forwards IGMP reports only out ports where the multicast routers are located. How the switch learns where the multicast routers are is by listening for periodic PIM or DVMRP hellos which are generated by the multicast routers and noting which interfaces they are received on. See the “Multicast router learning mode” in the output below...

3750# show ip igmp snooping
Global IGMP Snooping configuration:
-------------------------------------------
IGMP snooping                : Enabled
IGMPv3 snooping (minimal)    : Enabled
Report suppression           : Enabled
TCN solicit query            : Disabled
TCN flood query count        : 2
Robustness variable          : 2
Last member query count      : 2
Last member query interval   : 1000

Vlan 1:
--------
IGMP snooping                       : Enabled
IGMPv2 immediate leave              : Disabled
Multicast router learning mode      : pim-dvmrp
CGMP interoperability mode          : IGMP_ONLY
Robustness variable                 : 2
Last member query count             : 2
Last member query interval          : 1000

Vlan 16:
--------
--More--                                       

In the following output the switch has received PIM hellos on Gig2/0/12 on both vlan 16 and 40 so the switch knows that there is a multicast router (mrouter) connected to that interface for those 2 vlans. The switch the command was run on is also a multicast router itself for vlan 16 and 40 (PIM is configured on the vlan 16 and 40 interfaces of the local switch) which is why the word “Router” is seen in the output as well after the comma. A copy of IGMP reports received on vlan 16 interfaces of the local switch will be sent out Gig 2/0/12 on vlan 16 to the remote multicast router (mrouter) as well as up to the CPU of the local switch itself. Also, a copy of IGMP reports received on vlan 40 interfaces of the local switch will be sent out Gig 2/0/12 on vlan 40 to the remote multicast router (mrouter) as well as up to the CPU of the local switch itself.

3750# show ip igmp snooping mrouter
Vlan    ports
----    -----
  16    Gi2/0/12(dynamic), Router
  40    Gi2/0/12(dynamic), Router   

Please also note that IGMP snooping performs IGMP report suppression. One multicast router per vlan is ellected as the IGMP querier and sends IGMP queries every 60sec by default for a given multicast group. This is done for each L3 or vlan interface in the multicast routers outgoing interface list (OIF) for the group as see in “show ip mroute” on the multicast router (see below). All the active receivers for that group on the vlan the query is sent out on will respond with an IGMP report.
The periodic query mechanism is required to detect when receivers go away without sending an IGMP leave. This way the IGMP snooping switch knows to remove the interface assoctiated with that receiver from the group list. If an IGMP snooping switch has more then 1 receiver for a given multicast group on a given vlan it will only forward 1 of the IGMP replies for that group and vlan towards the mrouters and suppress the rest. The mrouters only need to receive one reply in order to continue forwarding the multicast stream out that OIF so the other reports are suppressed to save bandwidth.

mrouter#sho ip mroute
IP Multicast Routing Table

(*, 224.1.1.16), 1d00h/stopped, RP 0.0.0.0, flags: D
   Incoming interface: Null, RPF nbr 0.0.0.0
   Outgoing interface list:
     Vlan16, Forward/Sparse-Dense, 1d00h/00:00:00

(10.203.223.39, 224.1.1.16), 00:02:27/00:00:33, flags: PT
   Incoming interface: Vlan16, RPF nbr 0.0.0.0
  
Outgoing interface list:
    
Vlan40, Forward/Sparse-Dense, 1d00h/00:00:00 


2. If IGMP report is only forward to the router port with IGMP snooping, does that means the multicast server cannot be connected on the same VLAN of a single switch as the host.

Multicast servers and clients can be connected to the same vlan and IGMP snooping will still work in that vlan. Even though the IGMP report is not forwarded to a multicast router by the IGMP snooping switch the IGMP snooping switch still creates a local IGMP snooping entry for the interface the IGMP report was received on. When the multicast server and receivers are on the same vlan of the same switch the multicast stream will be received on the interface the server is attached to and forwarded to the other interfaces on the switch in that vlan where an IGMP report was received for that group. The only issue is that you need a multicast querier in the vlan which is typically the job of a multicast router. If there is no multicast router on the vlan then one of the IGMP snooping switches in the vlan must be configured with the IGMP querier feature for IGMP snooping to work.
The periodic query mechanism is required to detect when receivers go away without sending an IGMP leave. This way the IGMP snooping switch knows to remove the interface assoctiated with that receiver from the group list.

The IGMP querier feature is required if you do not have a multicast router on the vlan:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_44_se/configuration/guide/swigmp.html#wp1193337

The job of IGMP querier is typically done my a multicast router by default when it exists on the vlan:

mrouter# sho run int vlan 16
Building configuration...

Current configuration : 139 bytes
!
interface Vlan16
ip address 10.203.223.34 255.255.255.224
ip pim sparse-dense-mode
standby 4 ip 10.203.223.33
standby 4 preempt
end                     
    
mrouter# sho ip igmp interface vlan 16
Vlan16 is up, line protocol is up
  Internet address is 10.203.223.34/27
  IGMP is enabled on interface
  Current IGMP host version is 2
  Current IGMP router version is 2
  IGMP query interval is 60 seconds
 
IGMP configured query interval is 60 seconds
  IGMP querier timeout is 120 seconds
  IGMP configured querier timeout is 120 seconds
  IGMP max query response time is 10 seconds
  Last member query count is 2
  Last member query response interval is 1000 ms
  Inbound IGMP access group is not set
  IGMP activity: 1 joins, 0 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is 10.203.223.35
 
IGMP querying router is 10.203.223.34 (this system)
  Multicast groups joined by this system (number of users):
      224.0.1.40(1)                                       


3. When IGMP is used, is there a command to see which ports are associated to a multicast entry

3750# show ip igmp snooping group count
Total number of multicast groups: 200

3750# show ip igmp snooping group

Vlan      Group                    Type        Version     Port List
-----------------------------------------------------------------------
40        224.1.1.1                igmp        v2          Gi1/0/5, Gi2/0/10,
                                                            Gi2/0/12
40        224.1.1.2                igmp        v2          Gi1/0/5, Gi2/0/10,
                                                            Gi2/0/12
40        224.1.1.3                igmp        v2          Gi1/0/5, Gi2/0/10,
                                                            Gi2/0/12
etc...

4. In a case where IGMP snooping is disable, can I assume that IGMP report will be forward to all ports that belongs to the broadcast domain and consequently multicast traffic will be receive on each ports that belongs to the broadcast domain.

Yes, with IGMP snooping disabled the multicast traffic is treated as a broadcast and flooded in the vlan

5. IGMP snooping main purpose is to reduce bandwidth traffic on port that does not need multicast traffic.

Correct

Steph1963 Tue, 08/17/2010 - 07:36

Hi Mike,

Thanks a lot for this very detailed answered, I have a much better understanding of IGMP now.

Really appreciate your help

Stephane

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