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Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) · Cisco
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I thought it was A and because I've always seen a mix of the answer being A and C. Supporters of A would argue that before your advertisement goes out to your neighbor your AS will be put in the AS-Path so having an empty set wont match what the router sees. This was my logic as well and is true, but the catch is when the ACL is evaluated. The router evaluates the route before the local AS it prepends its own AS. Local BGP table | | AS_PATH = "" | Outbound route-map | | matches ^$ | Prepend local AS | | AS_PATH = 45123 | Send to ISP
C is correct.
the regular expression (^$) matches any route that has an empty AS path attribute (that is, no character from start to end). Only locally originated routes have an empty AS path attribute; hence this regular expression is used when matching local routes. This type of filter is used by multihomed customers to send only their address space to their service providers, to prevent them from becoming a transit AS.