Multicasts from HSRP

Answered Question
Sep 15th, 2008
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I am seeing a ton of multicasts on my server interface. These multicasts come from HSRP peers on the same vlan.


My question is, is it possible to filter out these multicasts on the ports connected to servers, becasue they dont really care to hear them and its extra processing on the NICs.


I enabled "ip igmp snooping" on the switch but i can still see the multicasts.


Is there anything I can do, or what is the best practice?

Correct Answer by rsgamage1 about 8 years 9 months ago

Hi,


IGMP snooping does not work with HSRP, simply because it cannot work with reserved addresses(e.g.224.0.0.2/01-00-5e-00-00-02)


Ref:http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00800b0871.shtml


HTH

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Giuseppe Larosa Mon, 09/15/2008 - 11:31
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Hello Sparky,

you should see two HSRP hellos directed to IP 224.0.0.2 every 3 seconds with default settings

the 224.0.0.2 is the all routers on segment well known address.

The server NICs should simply discard these frames without processing them because they didn't join this multicast group they are not routers.

If they were broadcast they would be processed.

If the frequency is two every 3 seconds you should be fine.


Hope to help

Giuseppe


cisconoobie Mon, 09/15/2008 - 11:42
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I'm using aggressive HSRP timers.


standby 1 timers msec 200 msec 750


These multicasts come in very fast.


Is there any way to filter them properly through IGMP?

Giuseppe Larosa Mon, 09/15/2008 - 11:46
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Hello Sparky,

you can try to enable multicast routing on the same interfaces.


IGMP snooping to work correctly needs a router that sends IGMP queries: likely the servers will not send IGMP reports for group 224.0.0.2 and this should solve the issue


Hope to help

Giuseppe


cisconoobie Mon, 09/15/2008 - 11:54
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So for the HSRP interfaces, I should enable:


ip pim sparse-dense-mode ?

Giuseppe Larosa Tue, 09/16/2008 - 05:29
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Hello Sparky,

yes and before


ip multicast-routing in global config



Hope to help

Giuseppe


cisconoobie Tue, 09/16/2008 - 05:48
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I am still seeing a lot of 224.0.0.2 multicasts on the servers for that vlan that HSRP is running on.


I am using HSRP version1 because version2 is not supported on the 3750G.


Dunno what to do next.

cisconoobie Tue, 09/16/2008 - 13:44
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Ouch.. ok..a version2 is not supported by my switch. I will need to use default timers.


Thanks..

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