From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Mar 29 2008 - 09:21:44 ART
Hi Sadiq,
The DR is on the same subnet as the source and is the first hop router
in the multicast SPT. Any device is a potential source for any
multicast group, and so an SPT tree is not built until multicast traffic
begins to flow. In order to have the traffic reach the RP before an SPT
tree actually exists, the DR encapsulates the multicast traffic it
receives from the source with a unicast header and sends it direct to
the RP. If the RP wants the traffic (i.e. it has receivers) it will
request that a SPT tree be built back. You only want one DR for a
source subnet.
Designated forwarders, send native multicast traffic onto other subnets
going away from the source (or RP). You only want one DF on a subnet
for a given (S,G), so if there are multiple paths from the source, and
traffic for the group is being sent onto the subnet from each, you need
to perform an election. Normally in a simple multicast tree you have a
single DR, but multiple DFs.
The information at the link is correct, but does not appear to tally
with your explanation. Have another look at the following two quotes:
"In a redundant configuration where multiple routers connect to the same
LAN segment, only one router forwards the multicast traffic from the
source to the receivers on the outgoing interfaces. In this kind of
topology, only the Protocol Independent Multicast designated forwarder
(PIM-DF) forwards the data in the common VLAN, but the non-PIM-DF
receives the forwarded multicast traffic. "
" In PIM sparse mode, a first-hop router that is the designated router
(DR) for the interface may need to encapsulate the source traffic in a
PIM register message and unicast it to the rendezvous point (RP). "
Paul
Sadiq Yakasai wrote:
> The MCast DR router is not responsible for forwarding MCast traffic
> outward from the LAN segment though. The DR's role is strictly related
> to the membership of the MCast devices on that segment.
>
> The MCast Designated Forwarder is responsible for forwarding MCast
> traffic out of a LAN segment with redundant exit points.
>
> http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/6.x/configuration/guide/multi.html
>
> Sadiq
>
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