What are some common BGP origin code and AS path scenarios and how would you handle them?

2 AS path manipulation

BGP AS path is a list of AS numbers that a route has passed through from the source to the destination. BGP uses AS path as one of the criteria to select the best path to a prefix. BGP prefers routes with shorter AS path lengths, so a route with fewer AS numbers is more preferable than a route with more AS numbers. You can manipulate the AS path of a route by using the as-path attribute in a route map or a policy statement. For example, you can prepend your own AS number multiple times to make a route less attractive to other AS, or you can remove some AS numbers to make a route more attractive.

3 AS path filtering

BGP AS path filtering is a technique to control which routes are accepted or advertised based on the AS path attribute. You can use AS path filters to implement routing policies, such as preferring certain paths over others, avoiding routing loops, or preventing transit traffic. You can create AS path filters by using regular expressions that match the desired or undesired AS numbers or patterns in the AS path. For example, you can use ^$ to match routes that originate from your own AS, or ^1234_ to match routes that start with AS 1234.

5 BGP best path selection

BGP best path selection is the process of choosing the best route to a destination among multiple available routes. BGP uses a set of rules or criteria to compare different routes and select the best one. The first criterion is the weight attribute, which is a Cisco-specific attribute that is local to the router and not propagated to other routers. The second criterion is the local preference attribute, which is a global attribute that indicates the preference of a route within an AS. The third criterion is the locally originated routes, which are routes that are originated by the router itself or by an IGP within the AS. The fourth criterion is the AS path attribute, which is a global attribute that shows the number of AS hops to the destination. The fifth criterion is the origin code attribute, which is a global attribute that indicates how the route was learned. BGP continues to compare other attributes until it finds a unique best path or a tie.

6 BGP troubleshooting tips

Troubleshooting BGP can be difficult due to its complexity and variety of configurations. To help diagnose and fix BGP issues, there are several tips and tools available. The show ip bgp command can be used to show the BGP table and route attributes. Additionally, the show ip bgp summary command displays the status and statistics of BGP neighbors, while the show ip bgp neighbors command shows detailed information and configuration of a specific BGP neighbor. Debugging can be enabled with the debug ip bgp command, while the clear ip bgp command can reset the session and clear the table. Finally, ping and traceroute commands can test connectivity and paths to a destination, while route maps, policy statements, and filter lists can configure and apply BGP policies or filters.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?