PIM – Assert Mechanism

As in last post, I discussed about Multicast DR and querier. In this post I would like to discuss about PIM assert messages.  When it is important ?

Coming to this subject – In shared LAN networks, There may be more than one routers for redundancy and they can have reachability to RP. But as the redundancy is important same time need to know, how the PIM messages will be handled ?  The topology would be similar to below figure.

        

In this topology, the Multicast group hosts will receive duplicate packets from multicast Routers. To overcome this issue PIM have assert messages mechanism. This will help to decide PIM designated forwarder.

In this example,

1. Router 1 is the RP, forwards multicast traffic to entire network.
2. Routers 2 and 3 are redundant routers in a enterprise location. These routers forward the multicast traffic to LAN Router.
3. Assume Router 3 transmits first multicast packet. Router 2 receives the same multicast packet in its outgoing interface, In R2 multicast routing table this group is destined to go out from outgoing interface.
4. Router 2 then forwards multicast packet to LAN and Router 3 receives it, which means that Router 3 has also received data on an outgoing interface.
5. Receiving an unexpected packet on an outgoing interface of Routers. Both Routers get alerted to the fact that other PIM-SM neighbors on the LAN are also forwarding traffic to the group.
6. This means group hosts will receive duplicate data.

 This will big issue in big enterprise network, To overcome this problem,

1. Routers generate Assert messages to select a single router to forward traffic. Here Downstream routers can see the Assert messages so that they know which one was elected and where to send subsequent Join messages afterwards.
2. In our example, LAN Router sent Join messages to both gateway Router 2 and Router 3. That means as Local router received query from 2 routers, It replied to both. This means in return it will duplicate packets.
3. After the election of PIM forwarder using Assert messages, all Join messages will go to either Router 2 or Router 3, depending on which becomes the designated forwarder.

Election using Assert Messages

1. The router generating an Assert message with the lowest Administrative distance is elected  as the forwarder.  If all the routers are running the same unicast protocol.
2. Then router assert message with the best unicast routing metric will be elected. For example, if all the routers are using RIP, the router with the smallest hop count is elected. If the metrics are equal.
3. The router with the highest IP address is elected.

4.      After selection of PIM designated forwarded, The other router will prune its interface on physical media. So that elected members should send join messages to elected Router.

Hope this explanation will help you to understand the use of PIM assert messages in network. It is used to select a PIM forwarder to avoid duplicate multicast packets generation in network.  

Hope this post is informative for you. 

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