Why use GRE? Is IPSec enough?

[Rod25]

Hi Guys,
I am working on a VoIP network with DSL routers. It was recommended to use GRE with IPSec. I cannot find information as to why GRE is recommended. I know that if you want to forward multicast traffic then GRE forwards it but the new version of IPSEC does this also?

Any ideas would be appreciated.

Thanks
Rod

 

[Jynxx]

Not certain about any new versions of IPSEC, however we use GRE tunnels to do full dynamic routing (OSPF) within the IPSEC tunnels.

I could be wrong but I do not believe that you can do dynamic routing any other way over the IPSEC tunnels.