Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 295
Role of the DR and the BDR

Role of the DR and the BDR

•  The DR issues Host Membership queries for the networks attached to it for which it is the DR.
•  An MOSPF that is not the DR ignores reports.
•  Prevents unnecessary replication of packets.
•  The BDR performs the same functions as the DR except that it does not transmit queries until the DR goes away.
•  Set non-MOSPF routers with a priority of 0 to ensure they do not become the DR or BDR.

It is the designated router (DR) that issues the Host Membership Queries for the networks attached to it. An MOSPF router ignores reports for those networks not elected the DR. Any responses (IGMP reports) received from the networks builds entries based on groups, in the database. A group address (member) in this database will be deleted when the DR does not receive a report from that member. In a multicast environment, it is very important that the DR is a multicast-enabled router.

Having the DR become the querier prevents unnecessary replication of packets. This prevents multicast datagrams from being replicated as they are delivered to local-group members. This allows for different entries in the local-group database for the DRs in the autonomous system, which means that each router in the autonomous system has a different local-group database. However, the MOSPF link-state database and the datagram shortest-path trees are identical in each router belonging to the autonomous system.

The backup designated router (BDR) performs the same functions as the DR. It does not send out a query, but it processes the IGMP reports (host responses) so that it will contain a complete picture of the DR. In case the DR fails, the BDR can take over. One word of caution: You never want an non-MOSPF router to become the DR or BDR. To disable these routers from becoming the DR or BDR, set their priority to 0. Without the DR or BDR being an MOSPF router, MOSPF cannot operate properly.

When an IGMP report is received, the DR (all other routers except for the BDR discard this message) does some simple error checking (making sure the address is not in the local use range of 224.0.0.0– 244.0.0.255). If there is not an entry in the local-group database, it creates one (using the format of IP group address, attached network number) and sets the age entry to 0. In the local-group database, there is only one entry per multicast address, even if multiple hosts report membership. The DR may transmit a new group-membership LSA. Group-membership LSAs are only flooded to those neighbors that have indicated (through their database description packets) that they are multicast ready. This is accomplished by setting the MC (Multicast Capable) bit in the OSPF Options field of all Hello packets, Database Description packets, and all link-state advertisements.


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