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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