Multicast - Bootstrap Router (BSR)

As we discussed in last post about BSR in PIM sparse mode, Let’s see more detail about BSR in this section...

BSR is standard protocol, which work with PIM to advertise RP in network. But we need to elect BSR first. We can define BSR as -BSR is Router, Which will collect all RP advertisement from network and make a set of RP address and advertise/share in network.  In Cisco Auto-RP - it is different, Auto-RP will select the best RP and advertise only one best RP in all network.

                Both protocols use the same information which is coming from rp candidate advertisement.

Let me explain this process- First you will select a router. Which you want to designate as BSR

Syntax – ip pim bsr- candidate fastethernet0

Using this command we can designate the router as BSR router in network. Then those routers which are near to source or concern router we want to designate as RP. We need to configure this command.

Syntax- ip pim rp-candidate fastethernet0

At this time, router will start sending candidate RP announcements to BSR Routers. BSR will collect the announcements and make a set of all RP announcements and advertise in network through PIM messages.

                BSR advertisement will go thru RPF check same like PIM. Then all routers will have BSR information in network along with RP information. As the join request will come to router, Router will check the RP address and reach to source. BSR is reducing manual overhead to configure static RP in every router.

                          

             We can configure redundant BSR in network. In that case we need to configure priority for BSR. Highest priority will win and elected as Master BSR. As you enable a router as BSR, it will send bootstrap messages in network.BSR message contains BSR address with priority. As other router will receive the bootstrap messages it will come to know that one more BSR router is in network. It will check the priority associated with message and select our role. Depending on priority master will be elected. Higher wins...

Syntax – ip pim bsr-candidate interface [hash-mask-length [priority]]

Priority – 0 to 255

In Maipu Routers - By default, hash-mask-length is 10 and priority is 0.

In Cisco Routers – by default the priority is 0.

                                    In RP also we can configure priority, In RP lowest priority wins. As we require RP for particular group that time we can configure Access-list and bind it with RP and advertise with priority. In case of lower priority BSR will consider that RP and advertise in network.

Default RP priority in Maipu Router – 192

Default RP priority in Cisco - 0 which specifies 192.

                   

                          If some RP is advertising lowest priority like 190, then it will master RP which BSR will be advertise in network.

                          RP send unicast messages to configured BSR routers continuously. So as the RP change happen in network. It will get notified by BSR immediately.

Hope this post clarified you about BSR and RP election and working flow. We will see configuration and scenario for more understanding in coming posts...

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