Multicast PIM questions

[vv0bbLeS]

Hello all,

Diving into the PIM "routing protocol" and have a few questions:

1. PIM neighborships - if PIM does not send out any routing updates to its neighbors, what is the point of a PIM neighborship? (In my packet captures, all I ever see are PIM "Hello" messages and that's it).
2. Is PIM what puts the dynamic routes in the mroute table? (according to the "method" you chose of course, e.g. dense-mode, sparse-mode, etc.)

[Giuseppe Larosa]

Hello @vv0bbLeS ,

1) PIM neighborships are needed and they are mantained by PIM Hello packets. A multicast enabled router needs to know on what interfaces are connected live PIM neighbors because for example in PIM sparse mode the PIM join message has to be sent on the path towards the RP address (at first ) and the source addess of a (S,G) pair. As for other protocols like IGPs the PIM neighborships has a key role . Sending  a PIM join out of an interface where no PIM router is present would be useless.

2) Is PIM what puts the dynamic routes in the mroute table?

PIM does not advertise IP prefixes for multicast purposes like it happens with MP BGP address family ipv4 multicast. 

Rather the ip multicast routing table is populated by seen traffic for all alive (S,G) pairs or (*,G) pairs depending on the PIM mode.

To be accepted traffic must pass the RPF check for the source of the (S,G) or for the RP address for (*,G) when applicable.

The PIM protocol expresses the current interests of each PIM router depending on downstream receivers and other PIM routers as PIM joins for PIM sparse mode or as PIM prune ( "I'm not interested on this Group G") for PIM dense mode.

All these interests have to be renewed and mantained over time by sending again a PIM join or a PIM prune towards the upstream device ( nearest to the source)

To be noted the RPF check is performed on the actual source S of the (S,G) pair.

The group G is examined only if there is some form of filtering on the receiving interface or if the network is configured for a mix of PIM modes like PIM sparse mode and PIM SSM or PIM sparse mode and PIM Bidir .

For example you can have a range of groups for PIM SSM  default is 232.0.0.0/8 

A range of groups for PIM Bidir and so on.

So the router acts on a (S,G) examining the source for the RPF check and the group G to see how to handle the pair.

Hope to help

Giuseppe

[Joseph W. Doherty]

#1

. . . what is the point of a PIM neighborship?

That's explained in @Giuseppe Larosa 's reply.

(In my packet captures, all I ever see are PIM "Hello" messages and that's it).

Two possible reasons for that.

First, Packet Tracer often is missing features, and/or they don't function correctly.

Second, as you haven't explained what you've tried using multicast, cannot say what PIM should be sending as its packets.

#2

Somewhat overlapping with @Giuseppe Larosa . . .

PIM doesn't need place dynamic routes into its mroute tables, but it does record active multicast flows information, though.  PIM uses unicast routing information for routing information.

Possibly, calling this table a mroute table, for PIM, is somewhat inaccurate, as it's unlike other supporting multicast protocols, like DVMRP which actually do its own routing.  "mroute" table, as a name for it, seemed suitable.  I.e. don't assume whatever something is named, is always the best possible name, especially as technology evolves.

An example of naming, perhaps not being ideal, in the past, on these forums, one poster was somewhat indignant that I referred to HSRP as not really being a virtual router (which some platforms now support) as it's just a virtual address.  What the person noted was that the "R", in the acronym was for "router"!  That, of course, is correct.  But at the time Cisco named it, it was, pretty well, representative of its common functional usage.  Later, Cisco provided GLBP, which doesn't use "router" which is also true in the generic acronym name FHRP, for this kind of protocols.