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scottishnetwork
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Joined: Wed Feb 03, 2016 3:44 pm

Multicast Multiple Subnets on Same Router

Wed Feb 03, 2016 4:58 pm

I've been experimenting with getting multicast operating across two subnets/VLANs on the same router. The hardware configuration is as follows:

Internal Bridge [contains source] <===> MikroTik Router <===> Visitors Bridge

I've enabled IGMP and PIM on the appropriate interfaces. Note that loopback0 (172.16.5.254) is a local interface I've configured as an RP. The problem occurs with or without an RP designated.


/routing pim> interface print 
Flags: X - disabled, I - inactive, D - dynamic, R - designated-router, 
v1 - IGMPv1, v2 - IGMPv2, v3 - IGMPv3 
 #      INTERFACE                            PROTOCOLS                           
 0  Rv2 internal-bridge                      pim                                 
                                             igmp                                
 1  Rv2 visitors-bridge                      pim                                 
                                             igmp                                
 2  Rv2 loopback0                            pim                                 
                                             igmp                                
 3 DR   register                             pim 

When it comes trying everything out, all users on the internal subnet can pick up the multicast traffic on 239.0.0.1 (the test IP). However, nobody on the visitors subnet can see the traffic.

That said, the router see the joins from both sides (see below):

/routing pim> igmp-group print 
Flags: v1 - IGMPv1, v2 - IGMPv2, v3 - IGMPv3, 
I - include, E - exclude, F - forward, D - don't forward 
    INTERFACE                GROUP           SOURCE          TIMEOUT                       
v2E visitors-bridge          239.0.0.1       0.0.0.0         4m              
v2E internal-bridge          239.0.0.1       0.0.0.0         3m54s

The router also knows where the source is (172.16.5.254 in this example).
/routing pim> join print 
Flags: RP - (*,*,RP), WC - (*,G), SG - (S,G), SG_rpt - (S,G,rpt) 
       GROUP           SOURCE          RP             
    SG 239.0.0.1       0.0.0.0         172.16.5.254   
SG_rpt 239.0.0.1       172.16.2.4      172.16.5.254   

To add to this, the MRIB looks good and I've disabled all firewall rules between the two subnets. That said, I can't see any traffic for 239.0.0.1 hitting the firewall in the forward or input chains.

Is there anything obvious I could be missing? The router has recently been updated to version 6.32 but didn't work with previous firmware versions.

If it helps, the source is a Linux box running VLC.
 
robertkjonesjr
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Joined: Tue Jul 03, 2012 1:39 am

Re: Multicast Multiple Subnets on Same Router

Fri Feb 05, 2016 1:22 am

Something to check: verify the ttl of the multicast packets to be sure they are not being dropped at the router.
 
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ZeroByte
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Re: Multicast Multiple Subnets on Same Router

Fri Feb 05, 2016 1:32 am

and of course there's nothing silly like a filter rule in the firewall that blocks the multicast packets, right?
(I wonder if strict mode source route checking would block multicast packets - surely not)

EDIT:
In fact, if your forward chain that allows lan1<>lan2 connectivity is based on IP addresses, then that could very well be the case since the destination IP of the multicast stream isn't the IP range of the second lan....
When given a spoon,
you should not cling to your fork.
The soup will get cold.
 
scottishnetwork
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Joined: Wed Feb 03, 2016 3:44 pm

Re: Multicast Multiple Subnets on Same Router

Sat Feb 06, 2016 1:56 pm

Something to check: verify the ttl of the multicast packets to be sure they are not being dropped at the router.
It appears I forgot to check the basics! VLC by default sends multicast with a TTL of one. Thus why the router was dropping it and not passing the traffic onto the other subnet. Bumping the TTL up got it working right away.

As for the firewall rules suggestion, I've now turned the rules for stopping unicast traffic between the two subnets back on and the multicast comes through fine. With the TTL bumped up a simple allow traffic to 239.0.0.1 shows the packet and byte count going up now (it didn't work before).

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