Multicast Routing - Part V Tunnel

The Multicast tunnel is a feature that can be used to join two multicast routers seperated by a 'non multicast' region.

In the above scenario i create a tunnel between the 2 multicast routers as follows...

R2
interface Tunnel0
ip address 20.20.20.2 255.255.255.252
ip pim sparse-dense-mode
tunnel source Loopback0
tunnel destination 5.5.5.5
ip pim send-rp-announce Loopback0 scope 16
ip pim send-rp-discovery scope 16

R5
interface Tunnel0
ip address 20.20.20.1 255.255.255.252
ip pim sparse-dense-mode
tunnel source Loopback0
tunnel destination 2.2.2.2
end
ip pim send-rp-announce Loopback0 scope 16


The dest pc joins mcast group 225.0.0.1

I then try pinging across the tunnel from the source PC.

Router_2#p 225.0.0.1 repeat 3

Type escape sequence to abort.
Sending 3, 100-byte ICMP Echos to 225.0.0.1, timeout is 2 seconds:
...

Failure!!!


If i enable multicast debugging on R5 the reason becomes clear

debug ip mapcket
Router_5#
*Jan 9 07:03:40.263: IP(0): s=2.2.2.2 (Tunnel0) d=225.0.0.1 id=19, ttl=254, pro
t=1, len=100(100), RPF lookup failed for source or RP
*Jan 9 07:03:40.275: IP(0): s=2.2.2.2 (Tunnel0) d=225.0.0.1 id=19, ttl=253, pro
t=1, len=100(100), RPF lookup failed for source or RP

Whilst the multicast regions are now successfully joined the multicast route now differs from the unicast route. As indicated in the debug the reverse path check is now failing i.e. the path back to the multicast source does NOT match the unicast path to the source.

To resolve this issue i configure a static multicast route on R5 so that all multicast traffic is directed through the tunnel

Router_5(config)#ip mroute 0.0.0.0 0.0.0.0 tu0

I now ping again from R2 and success!!

Router_2#p 225.0.0.1 repeat 3

Type escape sequence to abort.
Sending 3, 100-byte ICMP Echos to 225.0.0.1, timeout is 2 seconds:

Reply to request 0 from 14.0.0.6, 484 ms
Reply to request 0 from 14.0.0.6, 484 ms
Reply to request 1 from 14.0.0.6, 180 ms
Reply to request 1 from 14.0.0.6, 180 ms
Reply to request 2 from 14.0.0.6, 296 ms
Reply to request 2 from 14.0.0.6, 308 ms