From: Anekwe, Abdul (Abdul.Anekwe@sig.com)
Date: Wed Jul 18 2007 - 18:00:18 ART
This is an interesting one.
Here is the setup
CoreA<----fa0/0-SwitchA f0/1 - Vlan100----fa0/1SwitchB fa0/0 -->Core B
** Routing **
Switch A & Switch B are EIGRP stub routers (default options) where
Switch B routes via Core B, and Switch A routes via Core A. Vlan100
running between the switches. SwitchA & SwitchB are also neighbors
routers on that Vlan
** Multicast **
Vlan100 also has a lot of multicast subscribers on both SwitchA &
SwitchB. SwitchB is the PIM DR for a multicast group that is requested
by hosts on Vlan100. PIM Sparse Mode only for this group.
** Problem **
SwitchB looses its uplink routed port to the CoreB. HSRP swings over,
SwitchA is now HSRP master for Vlan100, and it also has the routed path
to the core. No problem there. But SwitchB is still the PIM DR for the
group. So multicast is forwarded to the group only by SwitchB. And
since SwitchA is not the DR for the group, it no prunes the group from
its shared tree. However, SwitchB has no route back to the RP
(subsequently source) because its only uplink port to the core is down.
Hence the clients don't get the data.
** Our Work Around **
We added a another routed link between SwitchA & SwitchB. This another
new point to point link will have SwitchA be the DR for this same group
on the interface connecting it to SwitchB. So multicast data can get
flooded to SwitchB if in event SwitchB's uplink port to CoreB goes down
and vis versa.
We know this works...but is there a better way?
What do you think?
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