Question about routing multicast

Unanswered Question
Apr 26th, 2010


I've got two WAN links (1G each) I'd like some multicast data i.e from some multicast groups,  to primarily use WAN1 as their primary route and WAN2 as their backup, whilst at the same time all other traffic uses WAN2 as the primary and WAN1 as the backup.

My question is what technology would allow me to do that?



  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Mon, 04/26/2010 - 04:30

Hello Dan,

unicast routing can be influenced using modified costs so that WAN2 is preferred over WAN1

you can influence multicast routing using ip mroute command for the sources of multicast traffic pointing or unicast static routes for the sources

Another tool you can use is policy based routing to move unicast traffic over the link you want to use.

In this way multicast routing will follow unicast routing best paths and you can move unicast traffic flows with PBR.

Hope to help


dan_track Mon, 04/26/2010 - 08:31

Thanks for that.

If I use pbr to influence unicast routing, wouldn't that mean that unicast would be flowing down the WAN1, which should only be having multicast data on it? Can you please give me an example on this?



Giuseppe Larosa Mon, 04/26/2010 - 09:28

Hello Dan,

PBR does not change the IP routing table and that is the clean advantage of its use in your scenario.


let's suppose we use WAN1 for multicast and WAN2 for unicast.

We make WAN1 the preferred link by giving it both sides the lowest cost.

int WAN1

ip router ospf 10 area 0

ip ospf cost 10

ip pim sparse-dense-mode

int WAN2

ip router ospf 10 area 0

ip ospf cost 20

ip pim sparse-dense-mode

on internal LAN(s) we put PBR

! multicast traffic appears only as destination traffic

access-list 111 remark unicast traffic

access-list 111 deny ip any

access-list 111 permit ip any any

route-map divert_unicast permit 10

match ip address 111

set ip next-hop WAN2-next-hop-ip-address

int LAN

ip policy route-map divert_unicast


the configuration has to be applied on both sides on both routers.

A key point is that PBR does not apply to packets that are locally generated on routers, so OSPF hellos and PIM messages are still sent out WAN1 link.

if WAN2 fails all traffic, unicast and multicast, goes on WAN1

if WAN1 fails all traffic, unicast and multicast,  goes on WAN2

BGP as suggested by Edison can be used to differentiate paths for unicast and multicast and can be used when we want  to keep separate paths.

BGP multi protocol is used for this by making difference between address family ipv4 unicast and ipv4 multicast.

A network command in address family ipv4 multicast means an advertisement of a possible multicast sources in that IP subnet and allows the RPF check to be passed.

a possible link

Hope to help


dan_track Mon, 04/26/2010 - 08:32


Thanks for the link, however, it isn't working as the page is not found. Can you repost please.




This Discussion