Understanding AS Path Attribute in BGP

BC

BGP (Border Gateway Protocol) is a routing protocol used in the Internet to exchange routing information between different Autonomous Systems (AS). The AS path attribute is an important part of BGP, and it provides information about the sequence of AS numbers that a route has passed through.

The two statements that correctly describe how the AS path attribute is used by BGP are:

B. The AS path attribute is updated in EBGP advertisements only and is the primary loop-avoidance mechanism for BGP. C. When local preference is the same, the AS path attribute is a common tiebreaker, where paths with fewer AS hops are preferred.

Let's explore these statements in more detail:

B. The AS path attribute is updated in EBGP advertisements only and is the primary loop-avoidance mechanism for BGP.

When a BGP router receives a route update from an external AS (via EBGP), it adds its own AS number to the AS path attribute and then advertises the route to its internal BGP peers (via IBGP). The AS path attribute is used as the primary loop-avoidance mechanism for BGP, as it ensures that loops are not formed in the BGP network.

For example, let's say there are two routers, R1 and R2, belonging to different ASes, and they are connected to each other via multiple links. If R1 advertises a route to R2, and R2 then advertises the same route back to R1, a loop will be formed. However, if R2 adds its own AS number to the AS path attribute before advertising the route back to R1, the loop is broken.

C. When local preference is the same, the AS path attribute is a common tiebreaker, where paths with fewer AS hops are preferred.

Local preference is an attribute that is used to influence the path selection of BGP routes within the same AS. When multiple routes are available for a particular destination, the router will choose the route with the highest local preference value. If the local preference is the same for multiple routes, the router will use other criteria to choose the best path, such as the AS path attribute.

In general, paths with fewer AS hops are preferred over paths with more AS hops, as they indicate a shorter path to the destination. Therefore, when local preference is the same for multiple routes, the router will choose the path with the fewest number of AS hops.

In conclusion, the AS path attribute is an important part of BGP, and it is used to prevent loops and to influence the path selection of BGP routes. The AS path attribute is updated in EBGP advertisements only, and paths with fewer AS hops are preferred when local preference is the same.