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New Member

IGMP question

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

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: IGMP question

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

6 REPLIES
Cisco Employee

Re: IGMP question

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Silver

Re: IGMP question

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

Cisco Employee

Re: IGMP question

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Silver

Re: IGMP question

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

New Member

Re: IGMP question

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

Hall of Fame Super Silver

Re: IGMP question

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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