PIM DR & IGMP querier in IGMPv1/v2

If there are multiple routers on a LAN, a designated router (DR) must be elected to avoid duplicating multicast traffic for connected hosts. PIM routers follow an election process to select a DR. The PIM router with the highest IP address becomes the DR.

The DR is responsible for the following tasks:

• Sending PIM register and PIM Join and Prune messages toward the rendezvous point (RP) to inform it about host group membership.

• Sending IGMP host-query messages.

• Sending host-query messages by default every 60 seconds in order to keep the IGMP overhead on hosts and networks very low.

Routers That Run IGMPv2

IGMPv2 improves the query messaging capabilities of IGMPv1.

The query and membership report messages in IGMPv2 are identical to the IGMPv1 messages with two exceptions:

• IGMPv2 query messages are broken into two categories: general queries (identical to IGMPv1 queries) and group-specific queries.

• IGMPv1 membership reports and IGMPv2 membership reports have different IGMP type codes.

IGMPv2 also enhances IGMP by providing support for the following capabilities:

• Querier election process―Provides the capability for IGMPv2 routers to elect the IGMP querier without having to rely on the multicast routing protocol to perform the process.

• Maximum Response Time field―A new field in query messages permits the IGMP querier to specify the maximum query-response time. This field permits the tuning of the query-response process to control response burstiness and to fine-tune leave latencies.

• Group-Specific Query messages―Permits the IGMP querier to perform the query operation on a specific group instead of all groups.

• Leave-Group messages―Provides hosts with a method of notifying routers on the network that they wish to leave the group.

Unlike IGMPv1, in which the DR and the IGMP querier are typically the same router, in IGMPv2 the two functions are decoupled. The DR and the IGMP querier are selected based on different criteria and may be different routers on the same subnet. The DR is the router with the highest IP address on the subnet, whereas the IGMP querier is the router with the lowest IP address.

Query messages are used to elect the IGMP querier as follows:

1. When IGMPv2 routers start, they each multicast a general query message to the all-systems group address of 224.0.0.1 with their interface address in the source IP address field of the message.

2. When an IGMPv2 router receives a general query message, the router compares the source IP address in the message with its own interface address. The router with the lowest IP address on the subnet is elected the IGMP querier.

3. All routers (excluding the querier) start the query timer, which is reset whenever a general query message is received from the IGMP querier. If the query timer expires, it is assumed that the IGMP querier has gone down, and the election process is performed again to elect a new IGMP querier.

By default, the timer is two times the query interval.