You have configured a BGP network with several routers in the same autonomous system (AS). There are three possible routes from router A to router B in the network. The following conditions exist:All three routes have the same weightAll three routes were originated locally through the use of the network commandThe bgp default local-preference 50 command is executed for all three routesAll three routes have different lists of AS through which they travelWhich of the following parameters is used to select the best path among the three routes?

Last Updated on July 31, 2021 by Admin 1

You have configured a BGP network with several routers in the same autonomous system (AS). There are three possible routes from router A to router B in the network. The following conditions exist:All three routes have the same weightAll three routes were originated locally through the use of the network commandThe bgp default local-preference 50 command is executed for all three routesAll three routes have different lists of AS through which they travelWhich of the following parameters is used to select the best path among the three routes?

  • Weight
  • MED
  • LOCAL_PREF
  • AS_Path                                                                                               
Explanation:
The AS_Path parameter is used to select the best path among the three routes. To select the best path from router A to router B, BGP analyses various BGP attributes that are set 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 selectedBecause the weight attribute is same for all three routes, BGP checks the value of the LOCAL_PREF attribute. However, this attribute is also same for the three routes because the bgp default local-preference 50 command was executed for the three routes, which sets the value of the LOCAL_PREF attribute to 50 for those routes.BGP then checks whether any of the routes were locally originated using either the network or aggregate commands. As stated in the scenario, all three routes were locally originated with the network command during BGP configuration. Consequently, BGP analyzes the value of the AS_PATH attribute. This attribute refers to a list of AS numbers that are traversed by a particular route. The route with the shortest AS_PATH is selected as the best path.The weight attribute is not used to select the best path in this case. The weight attribute for all three routes is the same. If this attribute were different for the three routes, then the route with the highest weight would be considered the best path.The MED attribute is not used to select the best path in this case. The MED, or multi-exit discriminator, specifies the route into an AS that has more than one entry points. A route with the lowest MED is selected as the best path. However, in this case, the MED attribute is not considered because the AS_PATH attribute is different for the three routes. If the AS_PATH attribute for the three routes were the same, then the MED attribute would have been considered.The LOCAL_PREF attribute is not used to select the best path. The LOCAL_PREF attribute is checked if the weight attribute for the routes is same. The LOCAL_PREF attribute refers to the local preference, which specifies the route that has preference to exit the AS for a given destination network. The route with the highest LOCAL_PREF value is selected as the best path. However, the bgp default local-preference 50 command was executed for all three routes. Hence, this attribute is not considered to select the best path between the BGP routers A and B.

Objective:
Layer 3 Technologies
Sub-Objective:
Explain BGP attributes and best-path selection.

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