That's the reason BGP is classified as an exterior gateway protocol and it's the only one on its class. Other protocols are interior gateway protocol where the neighbor needs to be a directly attached device. You can circumvent this limitation with GRE tunnels.
You also need to keep in mind that while you can connect to a neighbor several hops away, the transit network must have knowledge of the networks being advertised in BGP unless you are doing MPLS or GRE tunneling. Without a tunneling solution, the BGP routes would be blackholed.
BGP, unlike EIGRP or OSPF, operates on the application layer of the OSI model. That means it uses a transport layer protocol -- TCP port 179, actually -- to send and receive protocol data from a statically defined neighbor. Accordingly, that neighbor does not need to be directly connected -- as long as a route exists to the neighbor's IP address. That route can be provided by a dynamic routing protocol (an IGP) or through static routes.
Just imagine the case in which an FTP client and an FTP server establish a session between them through the use of a connection-oriented protocol, like TCP. Does the FTP client and the FTP server have to be directly connected? Of course not. But the two ends do have to be reachable to each other.
This is different from EIGRP or OSPF, which operate on the network layer (IP protocol ports 88 and 89) and multicast Hello packets to dynamically discover neighbors on directly connected subnets (layer 2 adjacency) . They would have to sit on the same subnet, unless you simulate a direct connection through the use of a GRE tunnel.
Thanks guys. Funny someone mentined MPLS. My problem is that our WAN is built using three differnet technolgies. We contract with a vendor for MPLS which we use to run our voice traffic. We built our own short-range wireless network to get 100MB WAN speeds for our data traffic. Lastly we have DSL/cable connections with GRE tunnels as backup circuits.
My problem is that management wants to develope a system whereby any WAN technology can backup and other. For example, wireless goes down use Internet/GRE for data. MPLS goes down use wireless for voice. Wirelss and MPLS goes down use Internet/GRE for everything. And these are only a few of the many combinations!
Making things harder not all sites have all WAN technologies and the layout is not a nice clean hub-spoke setup. I have some sites that are piggy-backed on other sites using wireless and others that are piggybacked using MPLS. Basically a classic cluster f*@k of a network and their is no appetite to change it because the high speed of the wireless is a must and the QoS and long distance coverage of the MPLS is also a must.
I use RIP over the Internet/GRE tunnels and since the MPLS routers are not directly connected I use icmp (ip sla) tracking to detect MPLS outages and bring up floating static and RIP backup routes.
My thinking was to replace the RIP/ip sla (complicated, hard to expand setup) with one routing protocol that could handle the MPLS.
Looks like BGP would be my only option but it seems akward to use this as an internal protocol and the blackhole problem someone mentioned might also be an issue.
Maybe I should just concentrate on trying to further refine my current setup.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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