Subject: [rbridge] questions on optimizing multicast

From: gsankara at cisco.com (Ganesh CS)
Date: Fri, 12 Aug 2005 18:22:19 +0530

Hi Radia,
I have the following questions on optimizing multicast

1. Currently 224.0.0.1 is used to reach all the routers in the local 
segment and 224.0.0.2 is used to reach all hosts in the local segment. 
With rbridges will the behaviour be retained ? Because I find it 
increasingly difficult to accomplish this using Rbridges. Roughly we may 
have to build '2n' multicast distribution trees for 'n' subnets. Are we 
going to provide equivalent functionality ?

2. Currently when an IGMP is initiated by a node it will reach all 
routers in the same subnet. We looking at something similar i.e. till an 
rbridge is reached kind-of behaviour in case of mixed (rbridge + legacy 
bridge) environments. Right ?

3. Do we account for the actual distance between rbridges in case of 
mixed bridge environments before selecting a designated multicast 
forwarding rbridge for a set of nodes ?

4. On cross-VLAN delivery, there is a issue of multicast scoping vs 
efficient multicast delivery. If at all we require scoping then we 
should go for VLAN based delivery and if scoping is not at all important 
typically for a high volume multicast streams like video streams 
efficient multicast delivery should be the goal. Is VLAN based scoping 
really useful in rbridge scenarios ? What are the other mechanisms for 
scoping that we propose ? One I could see is the ttl based scoping.

5. How do we coexist with IP multicast routing domains ?

-Ganesh

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 12 Aug 2005 10:55:54 -0700

Ganesh CS wrote:
> With rbridges will the behaviour be retained ?

Yes. Note that on an rbridge, like any L2, the default would be that
both map to broadcast ;-)

> Because I find it 

There ought to be exactly two such trees for each vlan. I don't see how
this applies to subnets unless they're mapped to vlans.

> bridge) environments. Right ?

Yes.

> forwarding rbridge for a set of nodes ?

Internally, the rbridge can run any multicast protocol, and use that to
make the decision of which rbridge to forward for a set of nodes.

> scoping that we propose ? One I could see is the ttl based scoping.

Keep in mind that an rbridge campus is a single LAN; AFAIK, there is no
scoping inside a single LAN. I

> 5. How do we coexist with IP multicast routing domains ?

INSIDE rbridges may run multicast routing, but to the outside they look
like an L2. I'm not sure how to map your question to that context...

Joe