IGMP question

Answered Question
Nov 25th, 2008
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Hi,


Base on what rule to elect the designate querier in ICMP v2 network?


e.g.


R1

physical interface: 10.10.1.1

loopback address: 192.168.2.2

router id: 172.16.3.3


R2

physical interface: 10.10.1.4

loopback address: 192.168.2.5

router id: 172.16.3.6




rdgs


Correct Answer by Giuseppe Larosa about 8 years 6 months ago

Hello Anita,

IGMP and PIM make their decisions based on the LAN interface ip address on a segment by segment (Vlan by Vlan) basis: they lack a concept of router-id like OSPF.

So only FE0/0 ip address is to be taken in account.


Actually, after the exchange with Harold and some refresh of details I think it is the opposite:

IGMPv1 follows PIM so will use highest ip address on the lan subnet

IGMPv2 will use the lowest ip address on the LAN subnet so with IGMPv2 and PIM :

one router is the PIM DR and will represent the segment in the multicast routing topology: it will send PIM join messages towards the RP/source of multicast stream.

The other router is in charge of sending IGMP queries every 60 seconds on the segment: notice that IGMP receivers answers are sent to multicast address 224.0.0.2 all routers on subnets so also the PIM DR is aware of what groups have members on the link.

We can say that in this way there is some job sharing on the two PIM routers.


Hope to help

Giuseppe


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Harold Ritter Tue, 11/25/2008 - 07:32
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Anita,


As per RFC2236 section 3, the lower IP address will be elected the querier. So R1 in this case (10.10.1.1 < 10.10.1.4).


Regards

Giuseppe Larosa Tue, 11/25/2008 - 07:41
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Hello Anita,

IGMP version 2 follows the PIM DR election to design the IGMP querier:


the router with the highest interface ip address wins

in your case R2


Edit:


No, it is IGMPv1 that relies on PIM DR election so the lowest ip address is elected querier


IGMPv1 has no querier election process. Instead, it relies on the IP multicast routing protocol

to elect a designated router on the subnet. Because different protocols use different election

mechanisms, it is possible under IGMPv1 to have more than one querier on a subnet.



Hope to help

Giuseppe


Harold Ritter Tue, 11/25/2008 - 13:48
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Giuseppe,


I am a bit confused by the edit notes. I believe you mean that it is indeed the lowest IP address that will be elected as the querier, right?


Just in case, here is an excerpt from RFC 2236 ,Section 3 confirming this:


"With respect to each of its attached networks, a multicast router may assume one of two roles: Querier or Non-Querier. There is normally only one Querier per physical network. All multicast routers start up as a Querier on each attached network. If a multicast router hears a Query message from a router with a lower IP address, it MUST become a Non-Querier on that network."


Regards

Giuseppe Larosa Tue, 11/25/2008 - 13:53
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Hello Harold,

yes the lowest ip address is the querier I checked on Doyle's TCP/IP volume II.


With IGMPv2 the PIM DR and the IGMP querier can be two different routers on the LAN segment.


Best Regards

Giuseppe


anitachoi3 Wed, 11/26/2008 - 22:49
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Hi,


IGMP V1 querier: lowest IP address


IGMP V2 querier: Highest IP address


If, what interfaces?


loopback0

loopback1

FE 0/0

serial 0/1


Thanks


rdgs




Correct Answer
Giuseppe Larosa Wed, 11/26/2008 - 23:20
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Hello Anita,

IGMP and PIM make their decisions based on the LAN interface ip address on a segment by segment (Vlan by Vlan) basis: they lack a concept of router-id like OSPF.

So only FE0/0 ip address is to be taken in account.


Actually, after the exchange with Harold and some refresh of details I think it is the opposite:

IGMPv1 follows PIM so will use highest ip address on the lan subnet

IGMPv2 will use the lowest ip address on the LAN subnet so with IGMPv2 and PIM :

one router is the PIM DR and will represent the segment in the multicast routing topology: it will send PIM join messages towards the RP/source of multicast stream.

The other router is in charge of sending IGMP queries every 60 seconds on the segment: notice that IGMP receivers answers are sent to multicast address 224.0.0.2 all routers on subnets so also the PIM DR is aware of what groups have members on the link.

We can say that in this way there is some job sharing on the two PIM routers.


Hope to help

Giuseppe


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