Multicast Multiple Path Question

[gongya001]

Based on this diagram, Which path does R8 select to reach 239.5.5.5 on R5 ?

thanks !!

[Joseph W. Doherty]

Using PIM? If so, DM or SM? If SM, where RP and configured for switch to shorted path? If PIM, what routing protocol being used, and cost of links?

[gongya001]

I used Dense Mode and ospf for IGP. All the cost is default.

R8#ping 239.5.5.5
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.5.5.5, timeout is 2 seconds:

Reply to request 0 from 5.5.5.5, 59 ms
Reply to request 0 from 5.5.5.5, 60 ms

Which interface does R8 use as source to reach this Multicast group ?

thanks !!

[Joseph W. Doherty · Accepted solution]

If I remember correctly, it should use the same path as unicast routing would. But, if routing has multiple equal cost egress interfaces, it should use low or high (I forget which) interface IP, to select an interface.

[gongya001]

thanks a lot !!

[gongya001]

https://www.cisco.com/c/en/us/td/docs/ios/12_4t/ip_mcast/configuration/guide/mctlsplt.html#wp1061381 explains the highest PIM neighbor behavior. But I can't use this to explain my scenario.

[gongya001]

Looks it works based on the highest PIM neighbor behavior.
I mtrace from R8 lo0 to notice it does follow he highest PIM neighbor behavior.
R8#mtrace 8.8.8.8 10.10.1.5 239.5.5.5
Type escape sequence to abort.
Mtrace from 8.8.8.8 to 10.10.1.5 via group 239.5.5.5
From source (?) to destination (?)
Querying full reverse path...
0 10.10.1.5
-1 10.10.4.4 ==> 10.10.4.4 PIM [8.8.8.8/32]
-2 10.10.4.1 ==> 10.10.8.1 PIM [8.8.8.8/32]
-3 10.10.8.10 ==> 10.10.9.10 PIM [8.8.8.8/32]
-4 10.10.9.8 ==> 0.0.0.0 None No route

when I ping 239.5.5.5 from R8
I notice they are load balanced.
R8#ping 239.5.5.5
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.5.5.5, timeout is 2 seconds:

Reply to request 0 from 5.5.5.5, 2 ms
Reply to request 0 from 5.5.5.5, 2 ms
R5#debug ip icmp
ICMP packet debugging is on
R5#
*Feb 22 20:10:53.921: ICMP: echo reply sent, src 5.5.5.5, dst 10.10.9.8, topology BASE, dscp 0 topoid 0
R5#
*Feb 22 20:11:19.199: ICMP: echo reply sent, src 5.5.5.5, dst 10.10.9.8, topology BASE, dscp 0 topoid 0
*Feb 22 20:11:19.210: ICMP: echo reply sent, src 5.5.5.5, dst 10.10.7.8, topology BASE, dscp 0 topoid 0
R5#
*Feb 22 20:11:45.876: ICMP: echo reply sent, src 5.5.5.5, dst 10.10.7.8, topology BASE, dscp 0 topoid 0
*Feb 22 20:11:45.876: ICMP: echo reply sent, src 5.5.5.5, dst 10.10.9.8, topology BASE, dscp 0 topoid 0
R5#