Switching

Hi all.

I’ve read that IGMP proxy allows an IGMP Snooping L2 switch to send fewer IGMP Report messages to the IGMP L3 router. But how is that so?

  Router

       |

       |

L2 Switch

  |        |

  |        |

 R1   R2

1: Every time R1 or R2 joins a group by sending an IGMP Report, the L2 Switch will still have to send an IGMP Report to the Router.

2: Every time the Router sends a General Query, the L2 Switch still has to respond with IGMP Reports for both R1 and R2’s Groups.

3: Every time R1 or R2 leaves a group by sending an IGMP Leave, the L2 Switch will still have to send an IGMP Leave to the Router.

Where is the benefit of configuring IGMP Proxy on the L2 Switch? How does it reduce the IGMP Reports sent to the Router?

Many thanks,

Deepak

 HI Deepak,

Greeting,

IGMP is a network layer (Layer 3) protocol used to establish membership in a Multicast group and can register a router to receive specific Multicast traffic. (Refer to RFC 1112 and RFC 2236 for information on IGMP versions 1 and 2.)

Multicast aware switches are slowly making their way into the network cores for businesses and universities that have heavy traffic to move through their networks. Multicast filtering is achieved by dynamic group control management. By default, all Multicast traffic should be blocked until requested by a Multicast group member. (Default behavior depends on switch manufacturer.) The master of the IGMP filter lists is the router or switch that is configured to act as the IGMP Querier. The responsibility of the Querier is to send out IGMP group membership queries on a timed interval, to retrieve IGMP membership reports from active members, and to allow updating of the group membership tables.

A Layer 2 switch supporting IGMP Snooping can passively snoop on IGMP Query, Report, and Leave (IGMP version 2) packets transferred between IP Multicast routers/switches and IP Multicast hosts to determine the IP Multicast group membership. IGMP snooping checks IGMP packets passing through the network, picks out the group registration, and configures Multicasting accordingly.

Without IGMP Querying/Snooping, Multicast traffic is treated in the same manner as a Broadcast transmission, which forwards packets to all ports on the network. With IGMP Querying/Snooping, Multicast traffic is only forwarded to ports that are members of that Multicast group. IGMP Snooping generates no additional network traffic, which significantly reduces the Multicast traffic passing through your switch.

If your network distribution core does not support IGMP Querying/Snooping, the AVN streams will still function as designed but your network may be subjected to high traffic loads and condensed collision domain due to the broadcasting action used by the older switch or hub. If this is the case, you may wish to isolate the streaming nodes within the network so that the streams may be viewed without crossing the normal network traffic along its path.

Otherwise, for a general performance improvement, you may consider upgrading your network core to a switch that is Multicast aware.

Please mark "Accepted Solution" if this helps you solve your query. Kudos are always appreciated

Thanks 

Suraj 

Hi Suraj.

Can you give 1 or 2 examples of how configuring IGMP Proxy on the L2 switch reduces the amount of IGMP Reports it sends to the IGMP Router?

Thanks,

Deepak

@djadhav you initially wrote:

Hi all.

I’ve read that IGMP proxy allows an IGMP Snooping L2 switch to send fewer IGMP Report messages to the IGMP L3 router. But how is that so?

  Router

       |

       |

L2 Switch

  |        |

  |        |

 R1   R2

1: Every time R1 or R2 joins a group by sending an IGMP Report, the L2 Switch will still have to send an IGMP Report to the Router.

=> not necessarily.  If R1 and R2 both join the same group, then only sends one join is sent to the L3 Multicast Router.  Based upon this I think this answers your other questions.  Instead of R1/R2, it is really C1/C2 (clients), no?  BTW, IGMP itself operates at L2.  A L3 IP Multicast device, has IGMP support built in once enabled for L3.  No need to explicitly enable or set it.

2: Every time the Router sends a General Query, the L2 Switch still has to respond with IGMP Reports for both R1 and R2’s Groups.

=> Yes but only one, vs potentially many.

3: Every time R1 or R2 leaves a group by sending an IGMP Leave, the L2 Switch will still have to send an IGMP Leave to the Router.

=> only when the last member of a group leaves

Where is the benefit of configuring IGMP Proxy on the L2 Switch? How does it reduce the IGMP Reports sent to the Router?

Many thanks,

Deepak

Hi Deepak,

IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic to control the delivery of IP multicasts. Network switches with IGMP snooping listen in on the IGMP conversation between hosts and routers and maintain a map of which links need which IP multicast transmission. Multicasts may be filtered from the links which do not need them, conserving bandwidth on those links.

Purpose
A switch will, by default, flood multicast traffic to all the ports in a broadcast domain (or the VLAN equivalent). Multicast can cause unnecessary load on host devices by requiring them to process packets they have not solicited. When purposefully exploited, this can form the basis of a denial-of-service attack. IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group they have not explicitly joined. It provides switches with a mechanism to prune multicast traffic from links that do not contain a multicast listener (an IGMP client).
Essentially, IGMP snooping is a layer 2 optimization for layer 3 IGMP. IGMP snooping takes place internally on switches and is not a protocol feature.

IGMP querier
For IGMP, and thus IGMP snooping, to function, a multicast router must exist on the network and generate IGMP queries. Without a querier, IGMP membership reporting may be incomplete and the tables associating member ports and multicast groups are potentially incomplete, and snooping will not work reliably. Some IGMP snooping implementations include full querier capability.
IGMPv2 and IGMPv3 contain provisions for selecting a querier when multiple are available. The querier with the lowest IP address is given the role.
IGMP general queries from the querier must be unconditionally forwarded by all switches involved in IGMP snooping.
Proxy reporting
IGMP snooping with proxy reporting or report suppression actively filters IGMP packets to reduce the load on the multicast router. Joins and leaves heading upstream to the router are filtered so that only the minimal quantity of information is sent. The switch is trying to ensure the router only has a single report for the group, regardless of how many active listeners there are. If there are two active listeners in a group and the first one leaves, then the switch determines that the router does not need this information since it does not affect the status of the group from the router's point of view. The next time there is a routine query from the router the switch will forward the reply from the remaining host. In the presence of proxy reporting, the router will generally only know about the most recently joined member of the group.


Have look at the below link.
https://www.juniper.net/documentation/en_US/junos/topics/concept/igmp-snooping-qfx-series-overview.html

Please mark "Accepted Solution" if this helps you solve your query. Kudos are always appreciated

Thanks
Suraj