300-510 · Question #126
Refer to the exhibit. A network engineer has divided AS into confederations. Due to repeated ASN, when the 10.0 0.0/8 prefix from R1 arrives to R2, BGP automatically rejects it. What should the engine
The correct answer is B. Configure the command allowas-in on R2.. BGP's loop prevention mechanism rejects any route whose AS-Path contains the receiving router's own AS number, as this indicates the route has already traversed that AS. In a confederation, member sub-ASes may appear in the AS-Path when routes traverse the confederation, causing
Question
Refer to the exhibit. A network engineer has divided AS into confederations. Due to repeated ASN, when the 10.0 0.0/8 prefix from R1 arrives to R2, BGP automatically rejects it. What should the engineer do to fix the problem so that BGP allows that prefix on R2?
Exhibit
Options
- AConfigure the command as-override on R1.
- BConfigure the command allowas-in on R2.
- CConfigure the command allowas-in on all the PE routers.
- DConfigure the command as-override on R2.
How the community answered
(27 responses)- A4% (1)
- B81% (22)
- C4% (1)
- D11% (3)
Explanation
BGP's loop prevention mechanism rejects any route whose AS-Path contains the receiving router's own AS number, as this indicates the route has already traversed that AS. In a confederation, member sub-ASes may appear in the AS-Path when routes traverse the confederation, causing a router to see its own AS number and reject the prefix. The 'allowas-in [number]' command configured on R2 instructs BGP to accept routes even if the local AS appears in the AS-Path (up to the specified number of occurrences). Option A (as-override on R1) replaces the customer AS with the provider's AS and is used in MPLS VPN PE-CE scenarios, not confederation designs. Option C is overly broad and unnecessary. Option D (as-override on R2) is applied outbound on a sender, not to fix inbound route acceptance.
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