Troubleshooting BGP Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Border Gateway Protocol Overview

1. Border Gateway Protocol is a Path Vector protocol
2. BGP is primarily used to exchange NLRI between routing domains or autonomous systems
3. There are 4 (four) BGP processes that run when BGP is enabled in Cisco IOS-based devices:

1. The BGP Open process
2. The BGP I/O process
3. The BGP Scanner process
4. The BGP Router process

1. The BGP Open process is used for peer establishment
2. The BGP I/O process handles the reading, writing, and execution of BGP messages
3. The BGP Scanner process periodically scans the BGP RIB
4. The BGP Router process sends/receives routes, establishes peers, and interacts with the RIB
5. There are three major components of the BGP Router process. These components are:

1. The BGP Routing Information Base (RIB)
2. The IP RIB for BGP-learned Prefixes
3. The IP Switching Component for BGP-learned Prefixes

1. In Cisco IOS software, BGP information is stored in one of two data structures or tables
2. These two data structures or tables are the Neighbor Table and the BGP Table
3. The Neighbor Table contains a list of all the configured neighbors of the local BGP speaker
4. The BGP Table or BGP Routing Information Base (RIB) contains all BGP routes
5. All Border Gateway Protocol messages all share a common header which is 19-bytes long
6. There are only four BGP messages available. These four messages are:

1. The OPEN Message
2. The UPDATE Message
3. The NOTIFICATION Message
4. The KEEPALIVE Message

1. The different states BGP will go through before a neighbor relationship is established are:

1. The Idle State
2. The Connect State
3. The Active State
4. The OpenSent State
5. The OpenConfirm State
6. The Established State

1. Border Gateway Protocol path attributes fall into four separate categories, which are:

1. Well-known mandatory
2. Well-known discretionary
3. Optional transitive
4. Optional non-transitive

1. There are BGP two attributes that are used to influence the inbound path:

1. The MULTI_EXIT_DISC attribute
2. The AS_PATH attribute

1. There are BGP two attributes that are used to influence the outbound path:

1. The LOCAL_PREF Attribute
2. The WEIGHT Attribute

Troubleshooting Neighbor Relationships

1. The most common reason for failing to establish peer relationships are misconfigurations
2. Common misconfigurations include:

1. Using the Incorrect Autonomous System Number for the Peer
2. Using the Incorrect IP Address for the Peer
3. Incorrect Authentication Parameters
4. No IP Connectivity between Indirectly Connected Peers

1. In addition to misconfigurations, the following can prevent adjacencies from establishing:

1. Access Control Lists Filtering BGP Packets
2. Layer 1 and Layer 2 Issues
3. Device Resource Consumption

Troubleshooting Route Advertisement

1. Route advertisement issues with BGP are commonly due to device misconfigurations
2. The network command must match the exact route BGP is to advertise
3. The aggregate-address command requires a specific route for aggregate advertisement
4. By default, the aggregate-address command does not suppress specific prefix
5. The BGP Scanner process checks the BGP Table for updates every 60 seconds
6. Following policy modification, clear the BGP session for changes to apply immediately

Troubleshooting Route Redistribution Issues

1. By default, BGP will use the IGP metric when routes are redistributed into BGP
2. When redistributing OSPF into BGP, only internal and Type 1 externals are redistributed
3. By default, only external BGP routes are redistributed into IGPs
4. The bgp redistribute-internal command is required to redistributed iBGP routes