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show ip mroute 239.1.1.5 command

Answered Question
Feb 19th, 2014
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Hello All,


I have a multicast configuration that is now working.  I am trying to gain further understanding on this subject.

When I do a "show ip mroute 239.1.1.5" there is a line in the report that I have not yet been able to find information on what it means as follows:

"Incoming interface: VLAN5, RPF nbr 10.1.3.1*".   The nbr part is what I do not know.


I have included the report in an attachment.


Thanks.


Regards,


Jeff

Attachment: 
Correct Answer by Jon Marshall about 3 years 6 months ago

Jeff


The RPF neighbor is the L3 router/switch that is one hop closer to the source of the multicast stream.


If the RPF neighbor in the (, G) entry was 0.0.0.0 then that would mean the source was directly connected in vlan 5.


In your example the RPF neighbor is 10.1.3.1 which means there is a L3 device with an interface using that IP that is nearer the source of the mutlicast stream.


The RPF (Reverse Path Forwarding) check is done for all multicast packets on a L3 device ie. if the interface the mutlicast packet was received on is the same interface that the IP routing table says is used to route to the source IP then the RPF check is passed and the mutlicast packets can be forwarded to any other interfaces that have devices who want the stream.


If the packet arrives on an interface and according to the IP routing table the route to the source IP is not via that interface then the packet is dropped and not forwarded.


This is how PIM avoids routing loops for multicast traffic.


Edit - the above concerning the ip mroute entries is relevant to PIM dense mode as this is what you are running.


Jon

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Jeff Harned Wed, 02/19/2014 - 13:44
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Hi Jon


Well, once again you have helped me out!!  In my current configuration I have two source ports; one with an IP address of

10.1.2.2 (VLAN 5) and the other 10.1.3.1  (VLAN 6).   VLAN 5 is associated with multicast groups 239.1.1.3 and 239.1.1.5.

VLAN 6 is associated with groups 239.1.1.4 and 239.1.1.6.  So it makes sense with your explanation.


Thanks.


Regards,


Jeff

Jon Marshall Wed, 02/19/2014 - 13:55
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Jeff


No problem and glad to hear it is all working well.


Jon

Correct Answer
Jon Marshall Wed, 02/19/2014 - 12:42
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Jeff


The RPF neighbor is the L3 router/switch that is one hop closer to the source of the multicast stream.


If the RPF neighbor in the (, G) entry was 0.0.0.0 then that would mean the source was directly connected in vlan 5.


In your example the RPF neighbor is 10.1.3.1 which means there is a L3 device with an interface using that IP that is nearer the source of the mutlicast stream.


The RPF (Reverse Path Forwarding) check is done for all multicast packets on a L3 device ie. if the interface the mutlicast packet was received on is the same interface that the IP routing table says is used to route to the source IP then the RPF check is passed and the mutlicast packets can be forwarded to any other interfaces that have devices who want the stream.


If the packet arrives on an interface and according to the IP routing table the route to the source IP is not via that interface then the packet is dropped and not forwarded.


This is how PIM avoids routing loops for multicast traffic.


Edit - the above concerning the ip mroute entries is relevant to PIM dense mode as this is what you are running.


Jon

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