While I was doing some BGP labs I came across an interesting topic which is BGP over GRE. Using automatic tunneling techniques along with BGP is the core of MPLS VPNs and I think it is worth seeing the effect of using manual tunnels along with BGP. Let´s have a look at the following topology which illustrates my example:
The core IGP is EIGRP while BGP is used between AS 100 and AS 200 to advertise both loopback IP. Only router R5 and R1 are running BGP. For the BGP peering R1 IP 100.1.0.1 and R2 IP 200.1.0.5 will be used. When looking at a traceroute output from R1 to R5 the number of hops showed is 3 which means that we will have to use either ebgp multihop or ttl security as EBGP TCP packets are sent with a TTL 1 which means that EBGP peers are supposed to be directly connected. In this example I will use ttl security which while having the same effect as ebgp multihop adds some security. Let´s build the EBGP session between R1 and R5:
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