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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Multicast - PIM Dense Mode and PIM Sparse Mode

In this section we will discuss about PIM, What is PIM?  What is PIM Dense mode and PIM Sparse mode?

Let’s start with Multicast PIM...

PIM – Protocol Independent Multicast, It a routing protocol for Multicast Traffic. As you enable Multicast with “ ip multicast-routing” in global configuration mode. The Router will be enabled for multicast. IGP routing protocols exchange network information of one router’s network with others. Same multicast Protocol will exchange multicast information of Routers with each other. 

                For better understanding, you can keep in mind. It works bit similar to IGP. Multicast protocols will share multicast groups information with each other.

PIM is a protocol which will enable multicast routing in Router. AS you enable Router interface for Multicast it will try to form neighbour ship on Multicast address 224.0.0.13.

                Multicast Routing is totally dependent on IGP routing. IGP routing will help multicast PIM to find best way to reach source. PIM will create a routing table which is known as mroute table (Multicast Routing table) which will consist multicast entries.

PIM is a collection of multicast routing protocol, Two major PIM protocols are -

PIM DENSE MODE and PIM SPARSE MODE

PIM DENSE MODE:

PIM Dense mode is more similar to Source tree. In dense mode Source Tree will be start from source of network who is generating multicast traffic and other Multicast host Routers will be acting as branches of Tree which are trying to access the multicast stream. In PIM dense mode Router will initially flood multicast packet across network. This process will help in creation of (S, G) entry in all routers. This process is repeated every three minutes in PIM dense mode.  In this process those routers which don’t need the multicast stream that router will prune back the traffic flow.

Check both the diagrams for better understanding....

                 

PIM Dense mode flooding example -

In this example, multicast traffic being sent by the source is flooded throughout the entire network.

As each router receives the multicast traffic via its RPF interface (the interface in the direction of the source), it forwards the multicast traffic to all of its PIM-Dense neighbors.

Note that this results in some traffic arriving via a non-RPF interface such as the case of the two routers in the center of the drawing.  (Packets arriving via the non-RPF interface are discarded.) These non-RPF flows are normal for the initial flooding of data and will be corrected by the normal PIM-DM pruning mechanism.

Some key points about PIM Dense Mode - 

• PIM Dense mode is also known as PUSH MODE.
• PIM dense is recommended for small networks.
• PIM dense mode is very easy to configure on Routers.
• PIM dense mode is not scalable.

For summary – PIM dense mode is flooded across the network, then Routers which don’t have request for that particular group will prune it. The PIM dense tree is formed. Same process is repeated every three minutes. This process is not efficient in large enterprises network. So PIM dense mode is used in small networks, to avoid more configurations and easy management.

Dense mode command:

Interface mode – ip pim dense mode.

PIM SPARSE MODE –

PIM Sparse mode similar to shared Tree. PIM sparse mode is having a central point of contact for all Multicast host routers. As the Routers will receive the request from host to join particular multicast group, They will check the multicast routing table which is received from PIM and in case of the router is not aware, The request will be forwarded to RP (Rendezvous Point). RP is a designated router in network selected by network administrator as per requirement/Topology. This router will be responsible for providing information about every multicast group to host. All multicast source will register with RP. RP will aware about all multicast group and how to reach to multicast source.

                As RP will receive request from host, It will update source information and path to host router. Same time host router will use IGP information and find the best path and reach to source.

                In sparse mode, RP plays very important role. We can configure RP in many ways. For Cisco routers we can configure Auto-RP, other way are Static RP in each router and BSR – Industry standard (bootstrap router). We will discuss about BSR in upcoming posts.

Now we need to understand. In sparse mode RP is playing major role. Here the model is like shared tree. You can designate a Central Router as RP and every Multicast host traffic will pass thru RP.

Go thru diagram for more understanding....

In sparse mode, every router will have (*,G) entry, As the multicast traffic will be active in router. Then it will add (S, G) entry.

Some key point of PIM Sparse Mode - 

• Sparse mode is recommended for structured Multicast network Design
• We can say, it is very useful in large networks. We can deploy redundant RP to manage our network efficiently.
• Sparse mode requires planning for deployment. It is having some more configurations as compared to dense mode.
• PIM sparse mode is pull mode.
• Sparse mode is scalable.

Sparse mode command –

Interface mode – ip pim sparse-mode

                                             I explained in very simple language, which will clear your concepts about PIM and PIM Dense mode and PIM Sparse mode easily.

Hope this information is useful to you..

If you like this post and for any queries, Plz comment with your mail id...