Saturday, June 20, 2009


Bidirectional PIM can be used when most receivers of mcast traffic are also senders at the same time. It is an extension to PIM sparse mode that only uses the shared tree for multicast distribution. Packets flow to and from the RP only.

It is relatively easy to configure, although the BIDIR configuration example on the CISCO web site doesn't quite give the full picture, as it only shows configuration on as single router.

BiDir PIM must be enabled on all multicast routers and specified multicast groups need to be configured as BiDir. This can be done using static rp, autop rp or BSR.

I use the simple router topology SW1 ----- R3 ------ R5

On each router i enable BiDir PIM.

conf t
ip pim bidir-enable

The RP (R5 in this case) must specify which bidir groups it services.

ip access-list st 45

ip pim rp-candidate lo0 group-list 45 bidir

ip pim send-rp-announce lo0 scope 16 group-list 45 bidir

For static RP
ip pim rp-address 45 bidir

On SW1 i join the bidir mcast group

conf t
int fa0/0
ip igmp join-group

On R3 i examine the mroute table

Router_3#s ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report,
Z - Multicast Tunnel, z - MDT-data group sender,
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode

(*,, 00:16:10/00:02:22, RP, flags: BC
Bidir-Upstream: Serial2/0, RPF nbr
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:15:52/00:02:34
Serial2/0, Bidir-Upstream/Sparse, 00:16:10/00:00:00

From R5 i verify solution with a ping to multicast group


Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to, timeout is 2 seconds:

Reply to request 0 from, 88 ms
Reply to request 0 from, 228 ms
Reply to request 0 from, 88 ms

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