It’s often the more complex IP routing protocols which are the most difficult to diagnose and troubleshoot and BGP (Border Gaetway Protocol) is no exception. Like many such protocol, BGP has a fairly specialised application in that it is used specifically for routing between different routing domains and autonomous systems. You’ll normally find BGP being used in advanced or specialised network environments like Internet Service Providers (ISP) or global corporate networks with advanced routing requirements.
Another situation where you may encounter BGP is when companies have merged, it is ideally suited to bring disparate computer networks together without starting from scratch. During the end of the last century there was a huge amount of these sort of corporate mergers and huge networks needed to be joined together – BGP provided the optimum solution for many of these situations and indeed is still commonly used today. Many a network administrator will have spent hours analysing at the end of residential VPN trying to determine the complexities behind a long established BGP routing tables.
When troubleshooting issues that may be related to BGP it’s important to understand the fundamental characteristics of the protocol. Without knowing these core concepts it can be very difficult to analyse a complex and specialised protocol like BGP:
Neighbour Formation : Like many routing protocols, BGP creates neighbour adjacency between routers before it starts exchanging information. These neighbours though are almost always defined statically rather than dynamically by the protocol. Their formation is normally determined by the setting up of a simple TCP connection, the command for determining a list and status of BGP neighbours is as follows:
show ip bgp neighbor
Most of the important data is found in the first few lines of the output of this show command. The most useful parameter for troubleshooting is the BGP state which will switch from Idle-Active-Open-Established as the formation of the neighbour state takes place. Remember this process can take a little time to complete, especially compared to some modern day routing protocols, so give it time, however if the state ends up as anything other than Established then the formation has not completed successfully.
Other relevant information that is important are the BGP version. There are quite a few different versions of BGP being used in the wild and they will always establish on the lowest common version when establishing a connection. If you see these version constantly changing and switching it is usually indicative of some fundamental network configuration problem.
External BGP : This is usually run between two different but autonomous systems which are defined on networks which must be directly connected. The neighbors are established by specifying the address of the link, for example you could configure by naming the address of a serial links between two routers on the two networks. You may have to use the ebgp-multihop parameter in these situations as often interfaces are not directly connected as specified by the loopback address. In order to ensure that there is a loop-free topology, BGP will ignore any BGP routes which has originated in any autonomous systems (AS).