Easy multicast question:Relationship between PIM DR and IGMP querier

Answered Question
Mar 10th, 2010
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Hi, on a given router(and according to documentation), I see that the IGMP querier IP address will be picked based on the

int vlan x

ip add 10.1.1.1 255.255.255.0


Then the PIM DR is elected based on the highest IP address.


A colleague tells me that I should make both IGMP querier and PIM DR as the same IP address since that may avoid multicast problems.


If that is the case, it seems I could force the DR election using a 'ip pim dr-priority'.


Question:
Anyone can tell me whether this idea of making the IGMP querier and DR as the same IP address is common thing to do and is there real benefit on this as far as you know?

Correct Answer by Giuseppe Larosa about 7 years 4 months ago

Hello Marlon,

generally speaking, there is no real need to force the same device to be  the IGMP querier and the PIM DR on a LAN segment.


there are mechanisms that allow multiple multicast routers to monitor each other:

the IGMP query is received by hosts but also by other multicast routers on subnet.

IGMP reports are received by all multicast  routers on subnet.

Should the current IGMP querier stop to send its queries after an appropriate timeout called igmp querier another device takes the role.


the same happen for PIM where all routers in lan segment exchange PIM messages.


The two different rules have been chosen to divide load on multiple routers


you can see current IGMP group membership also on PIM DR not only on the  IGMP querier just to make an example.

they are the same if everything is working  well.


There are some cases depending on topology, specially with redundant topologies where I've found a benefit from setting  both to the same device.


this happens for example if the two routers have different paths with different metrics to RP ip address.

but it is not a general rule as far as I understand.


Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Wed, 03/10/2010 - 10:57
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Hello Marlon,

generally speaking, there is no real need to force the same device to be  the IGMP querier and the PIM DR on a LAN segment.


there are mechanisms that allow multiple multicast routers to monitor each other:

the IGMP query is received by hosts but also by other multicast routers on subnet.

IGMP reports are received by all multicast  routers on subnet.

Should the current IGMP querier stop to send its queries after an appropriate timeout called igmp querier another device takes the role.


the same happen for PIM where all routers in lan segment exchange PIM messages.


The two different rules have been chosen to divide load on multiple routers


you can see current IGMP group membership also on PIM DR not only on the  IGMP querier just to make an example.

they are the same if everything is working  well.


There are some cases depending on topology, specially with redundant topologies where I've found a benefit from setting  both to the same device.


this happens for example if the two routers have different paths with different metrics to RP ip address.

but it is not a general rule as far as I understand.


Hope to help

Giuseppe

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