The BGP best-path selection algorithm refers to the logic used by BGP to select the best path when multiple routes to the same network are available. BGP uses a set of Path Attributes (PA) to identify the preferred route. In the first part of this blog post, I will discuss the following attributes for the best-path selection:
- Local preference
- Local originated
- Shortest AS_Path
Let’s begin by examining the following network topology:
Each router represents a different Autonomous System (AS) and R5 has a loopback interface which will be advertised to the other routers through BGP. Here is the external BGP (eBGP) configuration on every router:
Note that the 126.96.36.199/32 network corresponding to the R5 loopback (Lo5) interface has been advertised into BGP through the network command.
R1 can follow two possible routes to reach the 188.8.131.52/32 network: one route using R2 as the next-hop and other route using R3 as next-hop. If we examine the output of the show ip bgp command on R1 we can see that R1 has learned the two routes to the 184.108.40.206/32 networks and the preferred path selected by BGP is through R2:
The BGP path-selection algorithm has chosen the route with the least AS hop count. The path learned through R2 has an AS path length of 2 (AS200 and AS500), while the path learned through R3 has an AS path length of 3 (AS300, AS400 and AS500). The AS hop count is represented by the AS_Path atribute. The output of the show ip route bgp command on R1 confirms that the route through R2 has been installed in the routing table, which corresponds to the route with the shortest AS Path:
Now imagine that, for some reason, we want to force R1 to select the longer route through R3. One option is to configure the Weight attribute on the routes learned through R3. The BGP weight is the first attribute examined when selecting the best path. The default value for this attribute is 32768 for locally originated routes and 0 for routes learned through a BGP peer. Let’s change the weight associated with the BGP adjacency with R3:
We need to perform a soft reset of the BGP neighbor relationships (using the clear ip bgp * in command) in order to see the efect of the weight attribute:
Now the preferred route is through R3, even though it is a longer route in terms of AS path length. It can be confirmed by examining R1’s routing table:
The weight attribute is a Cisco defined attribute and only works with Cisco devices. If we are working in a multi-vendor environment, we can use the local preference attribute for the same purpose. The local preference is the second attribute examined in the BGP path-selection algorithm when the weight value cannot be used as a tie breaker. The weight attribute is locally defined and it is not advertised to other peers. In contrast, the local preference attribute is a well-known discretionary attribute and is advertised thoughout an AS. A higher local preference value is preferred over a lower value and it is locally configured in a AS to influence the selection of the next-hop for the outbound traffic (this attribute is not advertised between eBGP peers).
Let’s configure a local preference value of 500 on R1 using a route-map as follows:
After the configuration of the local preference attribute, we can verify that now R1 prefers the route through R3 to reach the network 220.127.116.11/32:
Now imagine that a router has learned two routes to the same network and both have the same local preference. Which is the next attribute examined by the BGP path selection algorithm? BGP will always prefer a route that has been locally originated over a route learned from a BGP peer. A route can be advertised locally though the network command, by aggregation or redistribution. To discuss this point, let’s advertised into BGP the 10.0.12.0/30 network between R1 and R2:
If we examine the BGP table in R1, we can see that R1 has learned two routes to the 10.0.12.0/30 network: one route using R2 as next hop and the preferred locally originated route represented by a next hop of 0.0.0.0. Notice the difference in the AS path between the two routes. The route through R2 has an AS path of (200, i) and the local route an AS path of (i). The i origin code represents a route locally originated using a network command. The origin codes will be discussed in my next post. Also notice that Cisco by default assigns a weight of 32768 to local routes, which ensures that the local routes will always be the preferred routes:
In summary, if a router learns two or more routes to the same destination, BGP will select the route with the higher weight attribute. If all routes have the same weight or if the weight attribute is not configured, BGP will select the route with the higher local preference. In the next decision point, a locally originated route is preferred versus a route received by a BGP peer. Finally, if there is a tie in all the BGP attributes discussed so far, the route with the shortest AS path will be selected as the preferred route.
The Acummulated Interior Gateway Protocol (AIGP) is another attribute that can also be configured to influence the selection of the best route, but it is beyond the scope of this post. In Part II, we will discuss other important attributes that are considered by the BGP path-selection algorithm (Origin type, Lowest MED, eBGP over iBGP and Lower RID).