Troubleshooting BGP problems should be addressed by keeping the OSI model in perspective. For example, if a BGP neighbor relationship is having a problem, physical connectivity to the neighbor should be examined before looking at TCP packets carrying BGP information.
The configuration to operate BGP in Cisco IOS Software is fairly static and simple, but the dynamics behind these simple configuration commands are fairly complex. Therefore, BGP standards as described in RFCs must be understood well before operating BGP in large IP networks. Operators of a BGP network must understand the proper use of Cisco IOS Software configuration commands. Any minor mistake can cause serious problems not only in an operator's own network, but also to peering networks as well. These problems even can cascade into a worldwide BGP problem. For example, bogus static routes can be created for testing purposes in a router that has redistribute static configured in BGP configuration without any filters. This would result in accidentally announcing those bogus static routes to all BGP peers, which would further forward those bogus routes to their BGP peers. This would result in worldwide BGP announcement of fake routes and might wreak havoc in BGP networks. The dynamics behind the static configuration must be understood to troubleshoot problems in BGP.
Commonly, BGP operators face challenges in managing IP traffic flows coming in and going out of their IP networks running BGP. To obtain optimal utilization of network, BGP operators must understand how to use BGP to influence their desired traffic patterns in the network. Typically, tweaking BGP attributes such as LOCAL_PREFERENCE, AS_PATH, MED, and ORIGIN_CODE does this. Therefore, BGP network operators must master these attributes.
Problems might result from the configuration of BGP attributes or how BGP uses these attributes to compute the BGP best path from many paths to forward IP traffic. If proper preference of each attribute is not understood correctly, BGP operators might never influence traffic in their network correctly.
All sorts of problems come in different protocols for a variety of reasons, but a clear and logical approach should be taken to address those problems. This requires solid understanding of the protocols and an awareness of best-practice troubleshooting techniques. This book tries to offer fundamentals of each IP protocol and to provide enhanced troubleshooting techniques as seen in real-world IP networks.
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