You have configured BGP on both rtrA in AS 1 and rtrB in AS 2. There are two routes created using the network command between the two routers. One route traverses through AS 5 and AS 6 from rtrA to rtrB, while the other route traverses AS 7, AS 8, and AS 9 from rtrA to rtrB. Both routes use default values for the Weight and LOCAL_PREF attributes.
Which of the following attributes determines the BEST route between rtrA and rtrB routers?
- Origin type
The AS_PATH attribute is used to determine the best path between the two routes. To select the best path from rtrA to rtrB, BGP analyzes attributes that are set for the two available routes during the configuration of the network. The key BGP attributes and the order in which they are checked are as follows:
Weight – highest weight is selected
LOCAL_PREF – highest LOCAL_PREF is selected
Locally originated routes – local routes are selected
AS_PATH – shortest AS_PATH is selected
Origin type – lowest origin type is selected
Multi-exit Discriminator (MED) – lowest MED is selected
The weight attribute is the first attribute to be checked while selecting the best BGP route. This attribute is relevant only to the local router on which it is set. The value of this attribute can be any number from 0 to 65535. The default values are 32768 for locally originated routes and 0 for other types of routes. Both routes in this case are originated locally and have the default weight values. Therefore, in this case, the weight attribute is not used to determine the best route.
BGP then checks the value of the LOCAL_PREF attribute, which refers to local preference. Local preference is a value indicates the route that is preferred to exit the AS to reach a given network. Routes with higher local preference are selected by BGP. You can set the local preference for a route to any value; however, if you do not, the route uses the default value of 100. Because both routes have the default LOCAL_PREF value, this attribute is not used to determine the best route.
Next BGP checks whether any of the routes are locally originated using the network, redistribute, or aggregate commands. As stated, both routes originated using the network command on the routers. Consequently, BGP analyzes the value of the AS_PATH attribute, which is a list of the AS numbers traversed by a particular route. The route with the shortest AS_PATH is selected as the best path. In this case, the route that consists of AS 5 and 6 is considered the best path because the AS_PATH value for this route is shorter than that for the route traversing AS 7, 8, and 9. The AS_PATH value for the route traversing AS 5 and 6 is [6 5 1], while the AS_PATH for the route traversing AS 7, 8, and 9 is [9 8 7 1].
The other options are incorrect because the corresponding attributes are same for both the routes; hence, those attributes are not considered while BGP determines the best path.
Layer 3 Technologies
Explain BGP attributes and best-path selection