Information Technology Reference
In-Depth Information
The multicast state information for R4 is shown in Example 11-6.
Example 11-6
Multicast State for the Shared Tree and Source Tree on R4
R4#show ip mroute 224.1.1.1
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 MSDP Advertisement,
U - URD, I - Received Source Specific Host Report, Z - Multicast Tunnel
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 224.1.1.1), 00:01:17/stopped, RP 10.1.1.2, flags: SF
Incoming interface: Serial4/0, RPF nbr 10.2.1.10
Outgoing interface list:
Serial2/0, Forward/Sparse, 00:01:17/00:03:25
(10.5.1.5, 224.1.1.1), 00:00:16/00:03:16, flags:
Incoming interface: Serial3/0, RPF nbr 10.2.1.5
Outgoing interface list:
Serial2/0, Forward/Sparse, 00:00:16/00:03:25
Example 11-6 contains an important detail in the incoming interface for the (*,G) and (S,G)
for the 224.1.1.1 group. The (*,G) has an incoming interface of Serial4/0, whereas the
(S,G) has an incoming interface of Serial3/0. This means that there is a more optimal path
for the MDT for this particular source through a path different than down the shared tree.
A process known as SPT switchover is used to transition traffic from the shared tree to the
more optimal source tree. This divergence triggers the SPT switchover as soon as traffic
rates reach a certain threshold. The default threshold is a single packet.
Because the optimal path for traffic destined for the receiver is not through the RP, the R4
is prompted to build an SPT directly to the source and prune this particular source from the
shared tree. A special prune called an RP-bit prune is used to prune a single source from
the shared tree or to prune from the (*,G) tree on an (S,G) basis. This special prune ensures
that the entire shared tree is not pruned, because this would prevent the receiver from
receiving any traffic sent down the shared tree from a new source. Figure 11-16 shows the
SPT switchover process.
After the SPT switchover, the RP no longer forwards traffic from this particular source. The
RP then prunes the SPT from the RP to the source. The new state information in Example 11-7
shows that the traffic has moved onto the SPT. You can see the indication that traffic is
flowing down the SPT instead of the shared tree by comparing the flags for the (S,G) in
Examples 11-6 and 11-7. The addition of the T flag means that the SPT switchover has
occurred.
