Subject: [magma] mrouter learning specifics?

From: "Ulises Fabre" <[email-masked]>
Date: Thu, 18 Nov 2004 12:43:42 -0500

Hello

Im wondering on how long should an IGMP snooping switch keep an mrouter
learned on a port? On the IGMP snooping draft page 4 it says

The switch supporting IGMP snooping must maintain a list of

         multicast routers and the ports on which they are attached.

         This list can be constructed in any combination of the following
ways:

         a)  This list should be built by the snooping switch sending

             Multicast Router Solicitation messages as described in IGMP

             Multicast Router Discovery [MRDISC].  It may also snoop

             Multicast Router Advertisement messages sent by and to

             other nodes.

         b)  The arrival port for IGMP Queries (sent by multicast

             routers) where the source address is not 0.0.0.0.

             The 0.0.0.0 address represents a special case where the

             switch is proxying IGMP Queries for faster network

             convergence, but is not itself the Querier.  The switch

             does not use its own IP address (even if it has one),

             because this would cause the Queries to be seen as coming

             from a newly elected Querier. The 0.0.0.0 address is used

But it doesn't recommend an age timeout for the mrouter after it's learned?
Can anyone help me understand what age timeout/how long we should keep the
mrouter in the igmp table?

Can anyone respond to my question or at least your opinions?

Regards

 

Ulises Fabre

Software QA Engineer

Nortel Networks

 

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From: Marshall Eubanks <[email-masked]>
Date: Thu, 18 Nov 2004 13:34:17 -0500

For IGMP Queries, I think that this should be be equal to or longer than the 
Other Querier Present Interval [default = 255 seconds]. Look at 
RFC3376 (see attached text below). Alas, this can and is changed in
networks (I have seen smaller values used), which means
that it will be tough to get it right without having it be
configurable in the snooping switch. (Note that rfc 2236 has exactly
the same text for this interval.)

I see no problem, however, with your timeout  being _longer_ than the 
Other Querier Present Interval. It just
means that you may have state in the switch longer than necessary.
(Maybe other people on the group can see a problem with this, but it's
not obvious to me.) So maybe you should set it to something long,
like 1000 seconds, and leave it at that.

Regards
Marshall Eubanks

6.6.2. Querier Election

   IGMPv3 elects a single querier per subnet using the same querier
   election mechanism as IGMPv2, namely by IP address.  When a router
   receives a query with a lower IP address, it sets the Other-Querier-
   Present timer to Other Querier Present Interval and ceases to send
   queries on the network if it was the previously elected querier.
   After its Other-Querier Present timer expires, it should begin
   sending General Queries.

8.5. Other Querier Present Interval

   The Other Querier Present Interval is the length of time that must
   pass before a multicast router decides that there is no longer
   another multicast router which should be the querier.  This value
   MUST be ((the Robustness Variable) times (the Query Interval)) plus
   (one half of one Query Response Interval).

8.1. Robustness Variable

   The Robustness Variable allows tuning for the expected packet loss on
   a network.  If a network is expected to be lossy, the Robustness
   Variable may be increased.  IGMP is robust to (Robustness Variable -
   1) packet losses.  The Robustness Variable MUST NOT be zero, and
   SHOULD NOT be one.  Default: 2

8.2. Query Interval

   The Query Interval is the interval between General Queries sent by
   the Querier.  Default: 125 seconds.

   By varying the [Query Interval], an administrator may tune the number
   of IGMP messages on the network; larger values cause IGMP Queries to
   be sent less often.

8.3. Query Response Interval

   The Max Response Time used to calculate the Max Resp Code inserted
   into the periodic General Queries.  Default: 100 (10 seconds)

   By varying the [Query Response Interval], an administrator may tune
   the burstiness of IGMP messages on the network; larger values make
   the traffic less bursty, as host responses are spread out over a
   larger interval.  The number of seconds represented by the [Query
   Response Interval] must be less than the [Query Interval].