The Border Gateway Protocol (BGP) allows internal and external networks or autonomous systems (AS's) to exchange routes. BGP is a path-vector routing protocol, which simply means that its job is to exchange routing information between autonomous systems to find the most efficient path for your data to the Internet. Each BGP router connects to its neighbor (or neighbors) with neighbor statements. Once it forms that neighbor relationship, the BGP routers exchange routes.
This How To discusses the BGP Best Path Selection Algorithm.
With the full Internet BGP routing table being upward of 500K routes and with a BGP router having the potential to be receiving multiple copies of that routing table from multiple providers, it has to have some way to compare those multiple BGP routing tables and select only the best route to go into the IP routing table on the router. It uses the BGP Best Path Selection Algorithm to do this.
You should note that Cisco BGP routers have weight as the first criteria in the table where other brands of routers do not.
Unless there are no options in place to influence the result, the BGP Best Path selects the best path based on the shortest Autonomous System (AS), one or more networks under a single administrative control. However, many times options like weight, local preference, and MED are put in place by network administrators.
Let’s look at the selection criteria, in order, that BGP uses to select the best routes to install into the IP Routing table:
This is a Cisco-defined attribute that is assigned locally to your router and does not get carried through to the router updates. If there are multiple paths to a particular IP address (which is very common), then BGP looks for the path with the highest weight. There are several ways to set the weight parameter, such as the neighbor command, the as-path access list, or route maps.
#2 Local Preference
This is an indicator to the AS as to which path has local preference, with the highest preference being preferred. The default is 100. For example: bgp default local-preference 150
#3 Network or Aggregate
This criterion prefers the path that was locally originated via a network or aggregate. The aggregation of specific routes into one route is very efficient and saves space on your network. For more information on aggregate, please see the Cisco article
#4 Shortest AS_PATH
BGP uses this one only when there is a “tie” comparing weight, local preference, and locally originated vs. aggregate addresses.
#5 Lowest origin type
This deals with protocols such as Interior Gateway Protocol (IGP) being a lower preference than Exterior Gateway Protocol (EGP).
#6 Lowest multi-exit discriminator (MED)
This is also known as the external metric of a route. A lower MED value is preferred over a higher value.
#7 eBGP over iBGP
Similar to #5, BGP AS Path prefers eBGP over iBGP.
#8 Lowest IGP metric
This criterion prefers the path with the lowest IGP metric to the BGP next hop.
#9 Multiple paths
This determines if multiple paths require installation in the routing table.
#10 External paths
When both paths are external, it prefers the path that was received first (the oldest one).
#11 Lowest router ID
This prefers the route that comes from the BGP router with the lowest router ID.
#12 Minimum cluster list
If the originator or router ID is the same for multiple paths, it prefers the path with the minimum cluster list length.
#13 Lowest neighbor address
This prefers the path that comes from the lowest neighbor address.
There are some commands that you will use over and over with the BGP metrics. They are show ip bgp and show ip bgp summary. Here is a truncated example of output from the show ip bgp command:
Cell_Router# show ip bgp
BGP table version is 14, local router ID is 18.104.22.168
Status codes: s suppressed, d damped, h history, * valid, > best, i -internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i22.214.171.124 126.96.36.199 0 200 0 200 i
> 188.8.131.52 184.108.40.206 0 300 0 300 i
*>i220.127.116.11/16 18.104.22.168 200 0 200 400 i
>i22.214.171.124 126.96.36.199 0 100 0 i
*>i188.8.131.52 184.108.40.206 0 100 0 i
*> 220.127.116.11 0.0.0.0 0 32768 i
The routes with the * beside them were selected as the best path by the BGP Best Path Selection Algorithm.
We hope this helps as a quick reference for BGP path selection.