Please refer to the topology on the following link :
Explaination says that :
RTB indicates a neighbor that has direct connection, which is 126.96.36.199.
RTA indicates an external neighbor that does not have direct connection.
I didn't understand it. There are only two routers in the topology. Then one router has direct connection but not the other router..What's this ??
On RTA, what is the significance of using neighbor 188.8.131.52 instead of using neighbor 184.108.40.206 ?
Afterall both, 220.127.116.11 and 18.104.22.168 are referring to the same non-cisco router RTB..
Is it because of the fact that 22.214.171.124 means bgp router-id on RTB or what ??
That is a difficult to interpret picture. The second eBGP peer is actually behind RTB at IP 126.96.36.199.
The deal with BGP and multi hop is this. eBGP (a peering relationship with another AS) sets the IP TTL to 1 by default. So when this traverses a router, it is decreased to 0 and dropped. This is the same IP Header Field that TTL that is used with traceroute. So as a eBGP packet traverses a router, it is dropped. To remedy this, you use the ebgp-multihop paramater on a neighbor statement, after you define the neighbor. If you don't specify a value, it will default to an initial TTL of 255. I would say that this doesn't need to be that high, but that depends on how many hops the peers are apart. iBGP allows multihop by default. I have not looked at an iBGP packet to see what the initial TTL is, but I'd guess 255 or 127.
Looking at your example further, I see what you are talking about. I wouldn't worry too much about that, just remember that multihop affects the IP TTL Field. That is a confusing example. The only thing I could be guessing is packets in one direction (RTA to RTB) are going through the router logic in RTB, where the return packets aren't. In my last post, I said I thought the second peer was behind RTB, but I'm not sure what they are trying to show. Again, just understand you need to use multihop when you send eBGP traffic through something that will decrement the TTL and drop the packet (i.e. a router).
One more note on this, then I'll give pause for your response. You mentioned that RTB is a "non-cisco" router. I actually interpret third-party in this example to mean another party or network that is outside of your control, most likely the ISP. That is why the policy could be such that there is no direct connectivity of BGP Peers.
I think "third-party", here refers to any router that is not of CISCO brand, that is of any other brand say D-Link Router.
So I understood the picture as One is Cisco Router and Other is (say) D-Link Router.
Then, what confused me was that D-Link Router (RTB) can have Direct Connection with Cisco Router(RTA), But Cisco Router (RTA) can't have Direct Connection with D-Link Router (RTB)..
I didn't get this stuff ..
The concept of ebgp peers needing to be directly connected is per the RFC, not Cisco. So that (or the workaround of multihop) is not reserved to Cisco devices. Whether there's a non-Cisco router in the mix wouldn't actually change anything here.
I have to be very careful talking about BGP (not my strong suit). I just happen to understand eBGP multi-hop from my studies in CCIE Sec. So wouldn't necessarily consider myself an expert in that category, but thanks for the compliment. Scott, on the other hand, is an expert on all things routing (including BGP).
I also have been studying BGP a bit these days and wanted to share a bit of my knowledge.
Rule of thumb.
1. For iBGP peering, there is no restirction to the number of hops in the peering condition.
2. However, when it comes to the eBGP, one of the condition to estabilsh peering is that the peering router( you can consider the IP address of the neighbor) _MUST_ be on the same directly connected network. i.e, two routers in eBGP neighbor relation should have one interface connected on the same IP subnet. if not, the eBGP routers would not even generate an "open' message at all. neibhgor 188.8.131.52 ebgp-multihop is used to overcome this condition. once this command is used in the cnfiguration, the eBGP router understand that the peer does not directly connected.