Which statements about BGP policy-based routing are true? (Choose two.)

Which statements about BGP policy-based routing are true? (Choose two.)

  • BGP policy-based routing is performed on a router’s inbound interface.
  • A BGP administrator can use policy-based routing to alter the final destination of the packet.
  • BGP policy-based routing will select the next-hop of the packet based on its source address.
  • BGP policy-based routing can be used to alter the path selection for a packet in a downstream AS.
Explanation:
BGP policy-based routing is performed on a router’s inbound interface. BGP policy-based routing will select the next-hop of the packet based on its source address. It does this through the use of route maps.

Below is a partial output of the show run command executed on a router that has a BGP configuration that uses a route map to alter the local preference of a route (172.16.0.0/16) to 90 if it is advertised from the BGP neighbor at 10.5.5.1:

300-410 Part 05 Q14 066

300-410 Part 05 Q14 066

The above output indicates that the local preference for the route to 172.16.0.0/16 is 90 ONLY if it comes from 10.5.5.1, but not if the same route is advertised from 10.5.5.35.

Route maps can be used to influence a part of the routing table without affecting the rest of the table. Consider an example where a router had two routes to every network in the table, and it prefers Neighbor A as the next hop for all routes. If you desired to change the next hop for one of the routes to Neighbor B without affecting the others, you could use route maps to take two different approaches, altering different attributes, which would arrive at the same result. The approaches would be:
Apply a route map to Neighbor B incoming that would set the local preference to 200 (default is 100) for the route. Local preference values determine the path used to exit the AS. A higher value is preferred.
Apply a route map to Neighbor A such that it advertises the route with a MED of 200. Med values determine the preferred path into the AS. A lower value is preferred. The default is 0.

Either of these approaches would result in the next hop for the network hanging to Neighbor B without affecting the others,

Policy-based routing does not alter the destination address of the packet. It can only alter the path to that final destination.

The BGP routing policy in one AS cannot determine the BGP routing policy in another AS.

Objective:
Layer 3 Technologies
Sub-Objective:
Identify suboptimal routing

Leave a comment