Which two statements about BGP facilitate the prevention of routing loops between two autonomous systems? (Choose two.)

Border Gateway Protocol (BGP) utilizes the ASPATH attribute as its primary loop prevention mechanism between different Autonomous Systems (EBGP). When an EBGP router advertises a route to an external neighbor, it prepends its own Autonomous System (AS) number to the ASPATH attribute. This creates a sequential record of all autonomous systems the route update has traversed. Upon receiving a route update, an EBGP router examines the ASPATH. If the router detects its own AS number already present within the ASPATH list, it immediately drops the route. This behavior indicates that the routing update has looped back to an AS it previously transited, thereby effectively preventing inter-AS routing loops across the network topology.

A: EBGP routers prepend, not append, their AS number to the ASPATH. Prepending adds the AS to the beginning of the sequence, accurately reflecting the reverse path.

B: Accepting routes containing their own AS number would actively cause routing loops. EBGP routers are strictly designed to reject, not accept, these routes to maintain loop-free topologies.

1. Juniper Networks Official Documentation: Junos OS Routing Protocols Library, "Understanding BGP Path Attributes". Specifically, the section on ASPATH details that the local AS is prepended to the AS path when advertising to an EBGP peer, and routes are rejected if the local AS is found in the received ASPATH. (Document ID: Junos OS Release 21.4).

2. IETF Standard / Academic Reference: Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

Section 5.1.2 (ASPATH): Defines the prepending mechanism when originating or propagating routes to EBGP peers.

Section 9.1.2.2 (Breaking Routing Loops): Explicitly states, "If the ASPATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the Phase 2 decision function." (DOI: 10.17487/RFC4271).