[c-nsp] Multicast gurus?! --Static RP / 'no ip pim dm-fallba ck'

[Tantsura, Jeff]

Hi Bob,

As long as ALL multicast interfaces are configured to be sparse mode only ie

"ip pim sparse-mode" (I assume it is in your case as you don't need dense
mode) no dense mode fallback would occur but by using this feature you'd
still benefit from blocking multicast traffic for groups not specifically
configured:

When a group is not specifically configured (that is, there is no RP for
that group), multicast traffic does not flow across the network. The first
hop router that receives data for such a group creates a (*, G) entry for
that group with an RP address of 0.0.0.0 and drops these packets. There are
no (S, G) entries created on the routers.

When the feature is configured, sparse mode groups operate with an RP
address of 0.0.0.0, which causes the following conditions to apply:

*(S, G) state is maintained unchanged and existing flows are unchanged.

*No PIM join or prune messages for (*, G) or (S, G, RPbit) are sent.

*Received (*, G) or (S, G RPbit) joins or prune messages are ignored.

*No registers are sent and traffic at the first hop is dropped.

*Received registers are answered with register stop.

*Asserts are unchanged.

*The (*, G) outgoing interface (OIF) list is maintained only for the
Internet Group Management Protocol (IGMP) state.

*Multicast Source Discovery Protocol (MSDP) source active (SA) messages for
RP 0.0.0.0 groups are still accepted and forwarded.

*IGMP messages are processed as for any other PIM-SM groups. IGMPv3 (S, G)
memberships are expanded into (*, G) IGMP memberships.

*Any group for which no mode information is available (such as PIM sparse
mode, bidirectional PIM, or Source Specific Multicast [SSM]) are created in
a default mode in which no traffic is forwarded, and no PIM states are
established in the network apart from the network first hop.

Hope this helps,
Jeff

--
Jeff Tantsura  CCIE# 11416
Senior IP Network Engineer

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Bob Arthurs
Sent: 01 August 2006 23:59
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] Multicast gurus?! --Static RP / 'no ip pim dm-fallback'

hello

i wonder if any multicast gurus here can help...

i am wondering if the 'no ip pim dm-fallback' command has any benefit if 
static rps are used in a network (anycast rp and static)? if so, what is the

benefit?!

thankyou very much


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