350-401 · Question #812
Lab Simulation 1 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access
To enable communication between Remote Site 1 and Remote Site 2 (both in AS123) through Headquarters (R2 in AS456), R2 must be configured with the as-override command for its neighbor R3, which will modify the BGP AS_PATH attribute to bypass the default BGP loop prevention mechan
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Explanation
To enable communication between Remote Site 1 and Remote Site 2 (both in AS123) through Headquarters (R2 in AS456), R2 must be configured with the as-override command for its neighbor R3, which will modify the BGP AS_PATH attribute to bypass the default BGP loop prevention mechanism.
Approach. The correct interaction is to access the console for R2 and apply the following configuration:
configure terminal
router bgp 456
neighbor 192.168.1.3 as-override
end
write memory
After applying this command, BGP will reconverge, and R3 will receive the 1.1.1.1/32 route from R1, and R1 will receive the 3.3.3.3/32 route from R3, thereby enabling full reachability between the two remote sites.
Common mistakes.
- common_mistake. 1. Attempting to configure R1 or R3.
- Using
neighbor allowas-inon R2 or R3. - Configuring
next-hop-selfon R2. - Redistributing routes on R2.
- reason. 1. Configuring R1 or R3: The lab guidelines explicitly state, 'No configuration changes are permitted on R1 or R3.' Any attempt to do so would result in failing the lab item.
- Using
neighbor allowas-in: This command is typically configured on the receiving router (R3 in this case) to accept routes that contain its own AS in the AS_PATH. However, the problem statement mandates that only R2 can be configured, makingallowas-inon R3 an invalid option. If configured on R2 for R1 or R3, it wouldn't resolve the issue of R2 suppressing advertisements to R3 based on AS_PATH. - Configuring
next-hop-selfon R2: This command ensures that the advertising router's IP address is used as the next hop for routes advertised to iBGP peers. While often necessary in iBGP setups, it does not address the eBGP AS_PATH loop prevention mechanism at play here. - Redistributing routes on R2: While R2 could theoretically redistribute the routes into another routing protocol or directly into BGP, this is an inefficient and generally discouraged practice for solving BGP AS_PATH issues. It also doesn't directly address the BGP loop prevention mechanism and can introduce other complexities or sub-optimal routing paths. The
as-overridecommand is the clean and standard BGP solution for this specific problem.
Concept tested. The core technical concepts being tested are BGP loop prevention mechanisms, specifically the role of the AS_PATH attribute in eBGP, and the functionality of the as-override command to allow transit routing through an intermediate AS between two segments of the same remote AS. This scenario highlights the need to understand how BGP handles route advertisements when an external AS acts as a hub for multiple spokes belonging to the same AS.
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