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Multicast routing in IP Fabric 3.5
Updated: September 2, 2021

Multicast routing in IP Fabric 3.5

Multicast routing and related implementations can be a real pain. If you want to make a network engineer nervous (and it doesn’t matter whether managing enterprise or service provider networks), just whisper few evil words “PIM”, “distribution tree”, “IGMP”. And if it’s not enough and you want to continue with fatality combo, follow that up with “RPF”.  

Joking aside, how can IP Fabric help you with multicast?

Helicopter view – where is your multicast

A good strategic overview is necessary to win a battle, here represented as a list of routers where a multicast routing table is present, with a number of entries for each router.

Routers with enabled multicast routing

Source locations

Thanks to our intelligent network model, we can detect not only sources with IP addresses from directly connected networks, but also not directly connected addresses (for example used for Anycast SSM).

Multicast sources and their location

Joined clients

What would be multicast sources without multicast clients? IGMP group table will show us all interfaces with connected clients, including the subscription to specific multicast groups.

Distribution tree – ingredient #1 – PIM

Multicast flows through distribution trees, which are build by PIM protocol with different flavors. For verification, you can use a table.

PIM neighbors

Better yet, you can visualize protocol relationships in the diagrams.

PIM neighbor relationships visualized in a diagram

Distribution tree – ingredient #2 – Multicast routing tables

If you want to check how multicast traffic flows, you have to go to the multicast routing table and find the correct entry and copy packet from an incoming interface to all outgoing interfaces. On every single router. You know the drill.

Multicast routing tables

Multicast distribution tree visualization

Or you can let IP Fabric do all the hard work for you. You can use the path lookup feature to simulate multicast traffic flow and visualize the whole distribution tree for a specific group. And it even works for shared trees and multiple sources in the same group.

Multicast routing distribution tree in path lookup visualization

Distribution tree – ingredient #3 – Reverse Path Forwarding

You know that desperate feeling. You configured everything right, sources are sending traffic, receivers are joined to groups, trees are built, but it still doesn’t work. Meet RPF checks in multicast PIM routing, which solve one of the biggest nightmares with operating multicast networks - troubleshooting multicast routing and RPF.

If your trees are not correctly aligned with unicast routing (multicast loop prevention) then say goodbye to your multicast traffic. But routers tell you where the problem is, you just need to be lucky or pedantically look everywhere. Or have a platform with multicast intent verification capability, such as IP Fabric!

Multicast RPF in intent verification check

RPF issue can also be troubleshot directly from path lookup by applying intent verification, so you can see on which devices and interfaces RPF check fails.

Multicast RPF intent verification applied to path lookup distribution tree

Multicast time machine

"Hello. This is IT. Have you tried turning it off and on again?" With IP Fabric helping users with their IPTV or conference problems is much more deterministic. Why? Because you can go back in time and compare what was working before and compare with the state of the network right now. You can see missing sources, receivers or whole parts of the tree.

Path lookup multicast distribution tree comparison between two snapshots in time

Where are the switches?

So far, we have covered all of the multicast routing. In the next release, we will focus on switched networks with IGMP snooping. Stay tuned.

If you have found this article helpful, please follow our company’s LinkedIn or Blog, where more content will be emerging. If you would like to test our solution to see for yourself how IP Fabric can help you manage your network more effectively, please contact us through


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