Routing

Hello Experts

 

Is there any possibility that RPF check can fail for PIM sparse mode (ASM, SSM)? As per my understanding, in sparse mode, multicast traffic follows the join request so RFP check nevers fails.

 

Appreciated your input

There are (at least) three scenarios where this can happen:

 

(1) During the switchover from RPT (Rendez-Vous Point Tree) to SPT (Shortest Path Tree), there may be packets transiently received through the RPT that gets discarded for RPF-failure due to the fact that the local router already converged to the SPT.

 

(2) Unicast routing instability e.g. a BGP session flaps. This leads to transient drops for RPF-failure.

 

(3) In a multiaccess network like a LAN you can have R1, R2, R3. R1 sends a join to R2, and that brings the traffic to R3, even if R3 is not interested. And R3 may have a better route to the source via a different link. This is drop due to permanent RPF-failure condition.

Hello Ato

 

Thanks for the reply. But I could not get your third point. Could you please elaborate this. It seems very interesting. When R1 sends the join then both R2/R3 will gets the join and will gets the multicast stream and depending upon the DR election, either R2 or R3 will forward the multicast stream to R1.

 

Appreciated 

DR election plays no role in that scenario. R1 sends the join and both R2 and R3 see it. But the PIM join packet has a field in the payload called Upstream Neighbor which is set to R2 LAN address. So R2 forwards.

Hello Ato

 

So if R2 forwards the join request then R2 should receive the multicast traffic , not R3. Is that correct?

May I suggest to download:

http://www.juniper.net/as/en/community/junos/training-certification/day-one/networking-technologies-series/this-week-deploying-bgp-multicast-vpns/

And read chapter 1 in full. The scenario I am referring to is similar to Figure 1.5, but not identical.

Thanks. Now I got your point. But could you please let me know is there any chance that receiver will not get multicast stream in case of sparse mode (where multicast streams follows the join request)

 

Thanks

In a healthy network and in the absence of bugs or misbehavior, one single stream should reach the receiver at the RPF interface in any topology. But there are corner cases where it may be received ALSO in a non-RPF interface (already described in my first answer).

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