So in the previous few videos, I talked about one dynamic way of routers
to dynamically learn about who the rendezvous point is for any given group.
That was the Cisco proprietary auto RP feature.
Now I'm going to introduce our last topic in a series of videos, which
is the PIM bootstrap router.
And let's do some comparisons between the two.
So there are some similarities between the PIM BSR and the auto RP features.
By the way, BSR stands for bootstrap router.
So both of them provide dynamic discovery of PIM rendezvous points.
In both protocols, we have candidate RP's announcing their capability
to be the RP for the PIM BSR.
We actually have a router called the bootstrap router that's a role that
a router plays who transmits messages that allow other routers to learn
of potential RP.
So this is kind of like the role that the mapping agent played in auto
RP. But there are some very big differences between them.
Number one, PIM BSR is an IETF standard.
As a matter of fact, if you go all the way back to the original PIM specification,
RFC 2117, that was the very first RFC for PIM sparse mode.
Even in there, it talked about a bootstrap router and the process for
doing that. So this has been around for a while.
But it really came into its own when PIM version 2 came out.
So the current RFC for this is RFC 5059.
And like we said, auto RP Cisco proprietary.
So PIM BSR really was developed for PIM version 2.
Chances are probably not going to run across too many devices these days
running PIM version 1, so that's not going to be a problem.
But only works for PIM version 2, whereas auto RP works for either version
of PIM, if that's a concern for you.
And PIM BSR, one of the things you lose is the ability to define the scope.
Remember with auto RP, when you were configuring your router as an RP
candidate, one of the things you had to put in in the command was the
scope. And that gave you the ability to control the TTL of those RP announced
messages that the candidate RP's were sending.
So in that way, you could say, all right, well, I only want this guy to
be the RP so far into the network by limiting how far away his messages
can go. With PIM BSR, that's not an ability that you can control.
Few more differences.
One of the big benefits of the BSR is not only is it an IETF standard,
but you can do away with dense mode.
Remember how with PIM sparse dense mode, which is what we needed for auto
RP, we had all these risks of multicast streams possibly being flooded
in dense mode if there was no rendezvous point to handle them.
And there were certain ways we could get around that like the sync RP
and stuff. But here, you don't have to worry about that.
Every single interface can be in sparse mode.
Don't have to worry about the possible risk of flooding of multi multicast
traffic. Also, in auto RP, the device that was responsible for sending
the messages to everybody saying, okay, here's your list of rendezvous
points was called the mapping agent.
And we said, you could have for redundancy two or three or four mapping
agents as long as the messages they were sending out were identical.
Well, in PIM BSR, there's only one BSR.
And that is elected from candidate BSRs.
So you still have your redundancy there, but there's only one bootstrap
message being sent out from the BSR router itself.
Another difference with auto RP, auto RP had some reserved addresses.
We see them here, 2240139 and 2240140.
The PIM bootstrap router protocol uses the exact same multicast address
as normal PIM uses, 2240013.
And there are some situations when using this feature that unicast messages
go out as well. And I'll talk about that.