Thursday, January 26, 2006
Multicast routing over a GRE tunnel. (Question #2)
Consider a network of routers with multicast routing turned on. Somewhere in the network there are a pair of adjacent routers with two equal cost paths between them. You display the config on the routers and see the following -
On Router1
On Router2
That is, there is a GRE tunnel between the two routers and multicast is being run over the tunnel rather than directly on the physical interfaces between the two routers.
Why would such a thing be done between directly connected routers both of whom are multicast capable? Is that tunnel superfluous or is it actually being useful in some manner?
On Router1
interface Tunnel0
ip address 6.1.1.1 255.255.255.0
ip pim sparse-dense-mode
tunnel source Ethernet0/0
tunnel destination 5.1.1.1
!
interface Ethernet0/1
ip address 2.1.1.1 255.255.255.0
!
interface Ethernet0/2
ip address 3.1.1.1 255.255.255.0
On Router2
interface Tunnel0
ip address 5.1.1.1 255.255.255.0
ip pim sparse-dense-mode
tunnel source Ethernet1/4
tunnel destination 6.1.1.1
!
interface Ethernet1/1
ip address 2.1.1.2 255.255.255.0
!
interface Ethernet1/2
ip address 3.1.1.2 255.255.255.0
!
That is, there is a GRE tunnel between the two routers and multicast is being run over the tunnel rather than directly on the physical interfaces between the two routers.
Why would such a thing be done between directly connected routers both of whom are multicast capable? Is that tunnel superfluous or is it actually being useful in some manner?
