We're going to look at a few well‑known packet captures for IPv6
BIDIR‑PIM here and see if anything is different.
I won't spend a lot of time rehashing things we've already talked about,
instead I'll focus on the pieces that are different for IPv6 BIDIR‑PIM.
Just like in IPv4,
BiDir capability is advertised between neighbors in the PIM Hello,
but unlike IPv4,
the capability is a default option and there's no need to configure it locally.
Instead,
the ability to use a BiDir RP is governed by the local static RP
configuration and whether the BiDir keyword is used.
We can also use an access list to determine what groups will run via
BiDir mode and which will run as traditional sparse mode,
but the RP addresses must be different.
That, of course, makes sense if using phantom RP,
since traditional any‑source multicast needs an actual
router to process source registrations.
Let's move over to PIM Join.
The PIM Join/Prune in IPv6 BIDIR‑PIM isn't at all
different from the packets seen in IPv4.
Since we aren't tracking sources, it's all relative to the phantom RP address.
BIDIR‑PIM uses the designated forwarder election and
RP link to provide loop avoidance, but from a packet perspective,
the PIM Join/Prune is exactly like traditional any‑source multicast.
The requested group is listed and the source
requested is the phantom RP address.
This will be forwarded up toward the RP link and
build the outgoing interface list,
as it would in a normal sparse‑mode deployment.
I'm not going to exhaustively walk through the
designated forwarder election packets,
as I've already done this for IPV4 and that would be boring.
Let's just look at the Offer and Winner packets here.
The first part of any DF election is the Offer.
This is a similar packet to the designated router election,
it's just suggesting that it can be the DF because
of its Unicast metric to the RP.
It's sent to the ALL‑PIM‑ROUTERS group on the segment,
includes the RP address,
as well as the best Unicast metric that the router has to it.
Like IPv4, the DF Winner packet serves two purposes.
The first is to inform every router on the segment that this local
router is assuming the role of designated forwarder.
The other purpose of the packet is as a response to other
routers sending worse Offers to inform that there is a
Winner with a better metric already.
As with the Offer packet,
the PIM options include for which RP the router will be the DF,
as well as the best Unicast metric.
In this module,
we recapped the basics of bidirectional PIM and what's different in IPv6.
We saw IPv6 BIDIR‑PIM on the command line and finished with a
packet capture analysis for the IPv6 PIM Hello,
Join, and a few of the DF election packets.
I hope you've enjoyed this deep dive into the PIM protocol
and learned something along the way.
I've included packet captures, slides, and configurations with the course, so don't let your learning stop here.