Multicast routing: From the source… Part 3

We build on our discussion from the previous blogs

The test topolgy

How R3 handles Server2 source multicast trafic?

R3 is the only UP router

It’s mroute table show a correct source and incoming interface but a Null outgoing interface list

The mrib shows

Similarily R3 mfib table shows that the incorming interface is accepting

But all packets are being dropped

No SW forwarding was into play

A debug on mfib shows that these packets were dropped because of no forwarding interface

Back to the mfib table

No SW forwarding

The “Other” counter indicates a total of 35 drops, 13 due to RPF

A debug ip pim shows nothing…

Let’s enable a downward PIM neighbor

PIMingly The neighbor is UP and added to the (*, which now in the forwarding state

A debug ip mrouting indicates that upon the reception of the trafic the mroute entry is created

Check the status of the mroute

We check the mfib table

The source is pruned and the “Other” counter is incrementing

From the beginning only 2 packets were SW forwarded

We clear mfib counters

Remark the NS flag for negate Signalling

Redo test

The flow is in mroute

R2 sends a prune request which supposes that it has received a multicast packet from the source

This is confirmed at R3 mfib

What “debug ip mfib pak” says?

We send exactly 3 packets from the source

First of all the signalling was needed to determine the outgoing intreface

Then TS (no idea!), FS (fast switching) and PS (process switching) accepted forwarding

The PS indicates no fowarding interfaces, dropping

The mfib table confirms that one packet were fowarded (FS)

The source stops

And the mfib at R3 is flagged with NS

Let’s now add a static igmp on R2 interface facing the receivers

At R3 the mroute shows a flagged T route and correctly populated incoming and outgoing lists

The mfib table has changed also

Now the incoming interface is marked A for accept

And the outgoing interface as F for forward

All packet were SW forwarded and no drops were registered

The debug shows the packet are being forwarded because the forwarding interface is known now

In dense mode PIM is not into much play

Routers on the path still signal its willingness to get the trafic or not (Prune)


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