What are two uses of the AS-path attribute? (Choose two.)
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A. B. C. D.AD
The AS-path attribute is an important attribute in BGP (Border Gateway Protocol), which is a protocol used to exchange routing information between different autonomous systems (AS) on the Internet. The AS-path attribute is used to track the path that a BGP route takes as it passes through different autonomous systems.
The two uses of the AS-path attribute are:
A. It is used in BGP route selection: When BGP routers receive multiple routes to the same destination from different autonomous systems, they use the AS-path attribute to select the best path. The AS-path attribute is used to determine the shortest path to the destination, as well as to identify any potential routing loops.
B. It is used to avoid routing loops between autonomous systems: The AS-path attribute is also used to prevent routing loops between autonomous systems. If a BGP router receives a route with its own AS number in the AS-path attribute, it knows that the route has already passed through its own AS, and it will not forward the route back to that AS again. This helps to avoid the creation of routing loops, which can cause routing instability and network congestion.
C. It is important to note that the AS-path attribute is not used by IGPs (Interior Gateway Protocols) like OSPF or IS-IS to avoid loops within an autonomous system. IGPs use other mechanisms like the Shortest Path First (SPF) algorithm to ensure loop-free routing within an AS.
D. The AS-path attribute is also not used to ensure that a packet traverses the least number of routers. This is the role of routing metrics like hop count or delay, which are used by IGPs to determine the shortest path to a destination within an AS.
In summary, the two uses of the AS-path attribute are to help in BGP route selection and to prevent routing loops between autonomous systems.