Multicast redundancy: Phantom RP

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Past week two weeks a colleague and also a student asked me about Phantom RP and how it works, all was related with a discussion we have around VXLAN Part 2 post and about supported Multicast configurations for VXLAN in NX-OS.

First of all, and in order to avoid further confusions around it, I would resume current supported methods for VXLAN underlay on Cisco NXOS/ASR devices:

Being clarified that, we can continue with the original purpose of this post.
So, based in our previous post we have configured our Nexus 5K / 7K underlay to run multicast in to support Flood and Learn configuration, by that time we choose Bidir PIM since is the only supported method in N5K. So let's get some background about bidir and how can we make it redundant (can we?)

BiDir PIM

PIM Bi Directional mode enable multicast group to route traffic over a single shared tree rooted at the RP, instead of using different unidirectional or sources tree. Since RP is the root  (his IP address :) ) is good to not to place it on a router but on an unused IP on the network reachable from PIM domain (this will be seen later in PhantomRP configuration).
Explicit join messages are used to establish group membership, Traffic from sources is unconditionally sent up the shared tree toward the RP and passed down the tree toward the receivers on each branch of the tree (note: traffic is not sent unidirectional to RP)

Bidir-PIM shares mechanisms of PIM-SM like unconditional forwarding of the source traffic toward the RP but without the registering process for sources (https://tools.ietf.org/html/rfc7761#section-4.2). Based on that forwarding can take place based on (*,G) entries, removing the need of any source specific state and, therefore, expanding scaling capabilities. This image extracted from Cisco white paper are good to see the differences in upstream process towards the RP in SM vs BiDir:


"PIM-SM cannot forward traffic in the upstream direction of a tree, because it only accepts traffic from one Reverse Path Forwarding (RPF) interface. This interface (for the shared tree) points toward the RP, therefore allowing only downstream traffic flow. In this case, upstream traffic is first encapsulated into unicast register messages, which are passed from the designated router (DR) of the source toward the RP. In a second step, the RP joins an SPT that is rooted at the source. Therefore, in PIM-SM, traffic from sources traveling toward the RP does not flow upstream in the shared tree, but downstream along the SPT of the source until it reaches the RP. From the RP, traffic flows along the shared tree toward all receivers."

Need of redundancy? Let's do it

We mention that our shared tree is rooted at RP address, so in order to give him redudancy we need a way to duplicate this or use a virtual IP. For bidir pim no traffic is targeted at RP (no control plane functions) so our solution is easier, instead of actually assign same IP in a sort of anycast we can just advertise it thru our IGP, the only issue foreseen is that the actual shared tree should be only one at a given time (we dont want that our RPF interface changes everytime) so in oirder to avoid that we can leverage the path decision to a more specific match in the RIB (by advertising same subnet with largest mask by some of the redundant points).
Well, that was so much talk I think that a code/config snippet worths more than a millon words:


Primary


Secondary (hmm.. if you don't see any difference here is a hint: look at the mask)




Now it's done, you can run your set of favourite verification commands to see if this is working:



Also you can shutdown the active interface (lo1) and see how does this change and our redundancy is working.

For CCIE / CCDE students:
- What is the convergence time of RP in case  of a failure on primary?
- Can we give sub-second convergence?
- In flood and learn configuration for VxLAN what would you recommend ASM or bidir PIM?
- In case of choosing ASM how is your redundancy going to be solved?
- Why are we using "ip ospf network point to point" ?

More on Multicast ASM/SSM/Bidir comparisson: http://lostintransit.se/2015/08/09/many-to-many-multicast-pim-bidir/



7k , bidir , CCDE , CCIE , CCIE-DC , CCIEDC , convergence , multicast , n5k , n7k , n9k , pim , redundancy , VXLAN