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Secondary IP Addresses and Multicast

mikepinto
Level 1
Level 1

Are there any issues I should be concerned about with receiving multicast on interfaces with secondary IP addresses?

Thanks,

Mike

18 Replies 18

Richard Burts
Hall of Fame
Hall of Fame

Mike

I am not aware of any issues with receiving multicast on interfaces with secondary addresses. The processing of multicast and access to multicast is dependent on the broadcast domain. So whether the interface has a single unicast address or has more than one unicast address (through the secondary address) should not impact multicast.

Is there some particular reason for asking? Is there some specific thing that you are trying to address?

HTH

Rick

HTH

Rick

Rick,

Thanks for the reply. It doesn't work on secondary addresses in the configuration I have. I was looking to see if it might be a code issue or can't be done. The secondary IP Address never gets put into being a PIM interface.

Mike

Mike

I am a bit confused. Are you saying that if you configure an interface with ip pim that it does not become a PIM interface if it has a secondary?

Or are you saying that when you have configured the interface with PIM that it identifies the interface using the primary address and not the secondary address?

Perhaps it would help if you would post the interface configuration and what you expect it to do that it is not doing.

HTH

Rick

HTH

Rick

The secondary IP address does not become a PIM interface. I am connecting from a 6504 to 2 seperate routers. The 2 routers are on seperate subnets, but in same vlan. So under the interface VLAN X, the config has a primary ip address going to 'Router 1' and a secondary IP address going to 'Router 2'. The RP and source of the multicast is off of 'Router 2'. This configuration does not work. Multicast traffic does not come down the link of 'Router 2'.

Mike

Would you post the configuration of the interface on Router 2 and of the VLAN interface? It would also help if you would post the output of show ip pim interface.

HTH

Rick

HTH

Rick

Rick,

I am attaching the vlan interface config and sh ip pim int. 'Router 1' and 'Router 2' are not mine. Please keep in mind that if I swap the IPs (primary <-> secondary) it works.

Thanks,

Mike

Mike

Thanks for the additional information. Your comment that if you swap primary <-> secondary that it works is interesting and helpful. It is best practice when configuring secondary addresses on router interfaces that routers that connect to each other should both use the same subnet as the primary address. In your current config they are mismatched and when you swap them then they match (for router 2) for the PIM upstream.

Most of the routing protocols need their neighbors to be in the same subnet that they are in. Apparently PIM also has that requirement.

HTH

Rick

HTH

Rick

Rick,

Thanks for the response. I guess then there is no way to provide redundancy in this case without manual configuration changes.

Mike

Hi Mike

The routers are connected in the same vlan but with differnet subnets. Have you thought of moving the secondary address to a new vlan, so it will be the primary address on that. Then it will work.

/Mikael

Mikael,

We can only have the one VLAN.

Mike

Mike

A big part of the issue is the implementation that has the connection to router 1 in one subnet and the connection to router 2 in a different subnet but has them both in the same VLAN. Is there a reason why it needs to be that way? Would it be possible to have a separate VLAN per subnet?

HTH

Rick

HTH

Rick

Rick,

The VLAN has to be the same. We have no control over that.

Mike

You are experiencing a typical problem with secundary addresses. Running multiple subnets on the same VLAN is a bad idea and generates only problems (DHCP, IGP neighbors,etc..), i can say from experience, but i know: it is so easy to add extra ip addresses, no ?

What you could try. Do you have another port that you can configure as router port ?

I know it is messy, but you could try the following:

current situation

int x/x

ip add

ip add secundary

ip pim sparse-mode

suggestion:

int x/x

no switchport

ip add

îp pim sparse-mode

int x/x+1

no switchport

ip add

ip pim sparse-mode

and you connect BOTH interfaces to a switch where you put both interfaces in the same vlan for example.

First, i understand the design is flawed, but it can't be changed. I agree with your suggestion, but it can't be done in our case because the links to 'Router 1' and 'Router 2' are trunks.

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