Exam 350-501 topic 1 question 112 discussion

D is correct. There is distribute list configured for neighbor. Where do you see any configured access-lists? AS-PATH filtering configures by "neighbor <IP address> filter-list <acl-number> <in | out>"

^$ matches an empty AS PATH so it will match all prefixes from the local AS https://networklessons.com/bgp/bgp-regular-expressions-examples

A is correct: The BGP configuration in the exhibit includes the use of an AS-path access list, which influences which routes are advertised to the specified neighbor (10.10.10.1). Here's a breakdown: The access list ip as-path access-list 1 permit ^$ allows routes with an empty AS path, which means it only permits locally originated routes. The command neighbor 10.10.10.1 distribute-list 1 out applies this access list to outbound updates to the neighbor 10.10.10.1, meaning only routes matching the criteria of access list 1 will be advertised. Based on this configuration: Local routes (those originating in the current AS) are permitted. Routes from other ASNs (which have non-empty AS paths) are denied. Thus, the correct answer is A. Local routes are permitted, and routes from other ASNs are denied.

^$ matches an empty AS PATH so it will match all prefixes from the local AS @Wrecking_ball

neighbor 10.10.10.1 distribute-list 1 out : You’re applying an AS-path access list (numbered 1) as an outbound filter for the neighbor. The access list permits routes with an empty AS-path (i.e., originating from your AS). ip as-path access-list 1 permit ^$ : Defines an AS-path access-list. In this case, access-list number 1 permits AS paths that match the regular expression "^$", which effectively matches routes originating from the local AS (AS 65001). This access-list is used by the distribute-list to control outbound route advertisements.

D is correct

D- is correct

Autonomous system path filters can be configured with the ip as-path access-list and neighbor filter-list commands. The access-list (IP standard) and access-list (IP extended) commands can be used to configure standard and extended access lists for the filtering of advertisements. The route-map (IP) command can be used to filter advertisements. Route maps may be configured with autonomous system filters, prefix filters, access lists and distribute lists. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/command/irg-cr-book/bgp-m1.html#wp2056491650

based on IOS regex, option A

D is the correct answer , labbed it out and distribute-list doesnt work with as-path matching, only access-list or prefixes.

No , my bad is not A

A is correct Router1(config)#ip as-path access-list 15 permit ^$ Router1(config)#router bgp 65500 Router1(config-router)#neighbor 192.168.1.5 remote-as 65520 Router1(config-router)#neighbor 192.168.1.5 filter-list 15 out In this case, the filter permits only routes that have an empty AS Path, meaning that the routes must have originated locally within this AS. This filter suppresses any external routing information when forwarding its routing table. So the external networks don't know about any downstream networks that can be reached through this router.