GRE tunneling can be used to encapsulate Multicast traffic. This could be useful for;
- An advantage of GRE tunneling relative to IPsec tunneling is that GRE can offer finer-grained QoS because routers have visibility into the IP packet header
- Tunnel Multicast traffic over unicast only networks
GRE tunnel does not maintain state, when only on side is configured it will show up/up. You need to enable keepalive support under the tunnel interface to resolve.
Tunnel source and destination must be statically defined and not routed through the tunnel interface.
The unicast source still needs to be reachable out the tunnel or RPF check will fail. Might need a mroute entry to achieve.
interface tunnel 0 ip address <IP & Mask> tunnel mode gre ip tunnel source
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