I don't really have any experience of implementing GRE tunnels but from the documentaion it would seem that one of the main uses for this is to pass multicast routing traffic over an environment that wouldn't otherwise support it. My question is, if the environment didn't support multicast but you were using static EIGRP neighbors, or even BGP would there be any need for the GRE tunnels in this instance?
I guess it would depend on your Topology and network requirements. You would see that GRE Tunnels are used in environments where multicast is supported as well. So it's more of knowing what your network demands,
For eg: GRE Tunnels etc are used in OSPf environments where you want to influence routing between areas etc or preferring inter area routes over intra area etc. just an example.or to advertise routes without actually redistributing them into another routing protocol etc. lot of scenarios where you use GRE.
My question is, if the environment didn't support multicast but you were using static EIGRP neighbors, or even BGP would there be any need for the GRE tunnels in this instance?
Yes and No, consider a NBMA network where you have eigrp neighbors unicast rather than multicast In this instance you might not need GRE Tunnel. But if you had a network where you had static routes all over the place, you can still use GRE Tunnels. Lets says you have R1-R2-R3 . all know each other via static routes and no multicast routing protocol used. If you need network behind R1 to talk to network behind R3 without R2 knowing it then you can createa GRE Tunnel.
You might not GRE tunnels if you are using the Unicast delivery like unicast neighbor in EIGRP. You really dont need GRE for BGP. You need GRE for BGP only in case if you have two different networks and you want to form BGP relationship.
GRE is primary being used for IPSEC/VPN technology to enable some of the efiicient data forwarding like multicast to the remote sites. GRE is also being mainly used for L2 VPN connectivity between the sites.
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