Multicast Registering

[davidhuynh5]

My (S,G) entry is always stuck in "Registering".

What does Registering mean and how can I fix this problem? My primary RP is 10.4.1.81 and backup RP is 10.0.1.7.

Thanks in advance.

sa26-lan-r1#sh ip mroute 239.246.0.28      
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, E - Extranet,
       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
       V - RD & Vector, v - Vector
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 239.246.0.28), 01:07:38/stopped, RP 10.4.1.81, flags: SPF
  Incoming interface: Vlan11, RPF nbr 10.0.22.254, RPF-MFD
  Outgoing interface list: Null

(10.20.32.109, 239.246.0.28), 01:07:38/00:03:28, flags: FT

  Incoming interface: Vlan10, RPF nbr 0.0.0.0, Registering

, Partial-SC

  Outgoing interface list:

    Vlan11, Forward/Sparse-Dense, 01:06:41/00:02:51, H

[Peter Paluch · Accepted solution]

Hi David,

Registering means that your router is receiving the multicast packets from a source, it is encapsulating them into PIM Register messages (hence the name) and is sending them as unicast packets to the RP. The RP shall decapsulate them and send the original packets down the shared tree as native multicast.

If your RP receives a Register message, it should send a PIM Join message towards the source of the multicast stream that should eventually reach your router. In that moment, you should also start routing the multicast stream natively, along with sending the stream as Register messages. As soon as the native multicast hits the RP, thanks to the tree branch created by the PIM Join messages, the RP will send a Register-Stop message back to you, and you should stop sending the Register messages, leaving only the native multicast stream to flow.

So if your group is stuck in Registering state, there is a problem that essentially prevents you from receiving the Register-Stop message. The reasons include:

• The Register stream not reaching the RP (ACLs, plain reachability issues - the Register stream is a unicast stream)
• The RP not accepting the Register messages from you (check the source address of the Register stream!)
  
• The RP not knowing the backward route either to you or to the source of the stream (again, be careful about the souce of the Register packets)
  
• Problem in propagating the PIM Join message from the RP towards the source of the multicast stream

Best regards,

Peter

[Mahesh Gohil]

Hello David,

S,G entry created by RP so can you check for rpf reachability from RP.

I mean Can you try command

sh ip mroute 

on RP and check for RPF neighbor field. Should not
be empty and check for that ip in routing table. It shoul d be reachable from RP

Hope this helps

Regards

Mahesh

[davidhuynh5]

Hi Peter,

Thank you for the clear explanation; you are a very good writer. I have a question regarding this statement "The RP not accepting the Register messages from you (check the source address of the Register stream!)" Can you elaborate on this and what command can I run to check the source address of the Register stream?

[Peter Paluch]

Hi David,

Okay, let's see... First of all, the RP may be configured using the

ip pim accept-register

command that allows it to filter PIM Register messages according to an ACL or a

 route-map

If this command is used on the RP then it is necessary to verify whether your router is among the list of allowed sources of PIM Register messages. See the following URL for the description of the command:

http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.html#wp1039548

Regarding the source address used by your router in the PIM Register messages - I am not sure if there is such command available to verify the current source address. I assume that because the PIM Register packet is originated by your router, it will use the egress interface's IP address as the source. However, you can configure your router to always use a particular source IP address when sending PIM Register messages using the

ip pim register-source

command, see the description here:

http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.html#wp1041397

Obviously, using a loopback address also advertised in your IGP routing protocol is the most appropriate way of using this command.

Best regards,

Peter