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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Multicast - DR and Querying Router

Today I would like to share some important concept about Multicast PIM-Sparse Mode DR and querier elections in PIM and role of them.

As we all know, IGMP has IGMP versions v1, v2 and v3.

In every version, The concept and role is bit different.

First let’s see about roles.

Roles:

Querier – Querying Router tasks is to send IGMP query message to LAN network , ensuring that is any one is still interested to join this multicast stream. As any of host will send a leave message in IGMPv2 , the querier router will send out query addressing to 224.0.0.1 (all multicast host) is anyone else would like to access this stream. If any host sends membership report for particular group. Then It will continue its task, If not It will discontinue.

DR – The DR is responsible for sending Join/Prune messages to the RP. Another role of DR is it sends a source-register message to the rendezvous point, on behalf of a source.

Let’s come to concept –

Many times confusions happens that querier and DR is one router only or what is use of DR?

• In IGMP v1 – Both tasks are performed by one router.
• In IGMP v2 - 
• At start up each router sends a query message to the all 224.0.0.1 from its own interface address. The router with the lowest ip address is elected IGMP querier.

                      AND

• Among all PIM-SM enabled router, Highest IP address will win DR selection, One more criteria can change this election, interface DR priority. But by default priority will be 1 for every interface, you can change priority under interface. Then highest priority will win DR election. Then that particular router will be responsible to send your join messages to RP and ensure about your communication.

This concept mainly applies, in shared LAN segment with more PIM enabled routers. You need to make sure that who should be concern router for DR, to handle join messages.

                                      Otherwise many times, by mistakenly any router in LAN segment enabled with PIM can be DR due to by default selection parameters and will handle your join messages coming from LAN hosts. Which will disturb your Multicast flow, since that another router will not have path to reach RP, So make sure in shared LAN environment you configured proper DR in all PIM enabled router.

The Designated Router in Detail -

When multiple routers are connected to a multi-access network (for example, an Ethernet) one of them must be selected to act as the designated router (DR) for a given period of time. The DR is responsible for sending Join/Prune messages to the RP. To elect the DR, each PIM router on the network examines the received Hello messages and compares its IP address with those of its neighbors. The router with the highest address is the DR. But highest priority can overcome it. 

Configuration :

MP1800(config-if-fastethernet0)#ip pim dr-priority 200 ?   

To check more in Router, Plz use show ip igmp interface (int)(Cisco command) , It will show you querier Router IP address and DR router detail.  In Maipu Routers "show ip igmp interface" will show you querier router IP address and "show ip pim neighbor detail" will show you detail about DR. 

It will show details about querying router and DR to us. 

Hope this information will help you for more understanding about DR and querying router in PIM sparse mode. For more information you can refer RFC 2236.

Hope this post is informative for you.

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Thanks for reading…