300-510 · Question #221
Refer to the exhibit. Company X is multihomed to a service provider. With the default settings, the link between PE2 and CE1 is the primary link, but company X has asked for the link between PE4 and…
The correct answer is B. Configure the AS path prepend attribute to increase the number of hops between CE2 and PE4. Local preference is an iBGP-only attribute and is not propagated across eBGP sessions. Applying a local preference route map on PE2 outbound to CE1 (an eBGP neighbor) has no effect because local preference is stripped at eBGP boundaries-CE1 will never receive or act on it…
Question
Refer to the exhibit. Company X is multihomed to a service provider. With the default settings, the link between PE2 and CE1 is the primary link, but company X has asked for the link between PE4 and CE2 to operate as the primary instead. A network engineer used a route map with local- preference to manipulate the routes from PE2 to CE1 to be less desirable and applied the route map on PE2 outbound to CE1. However, the path from PE2 to CE1 is still preferred. Which action must the engineer take so that the desired path is chosen?
Exhibit
Options
- AApply local-preference on PE2 outbound to CE2 with a value less than 100.
- BConfigure the AS path prepend attribute to increase the number of hops between CE2 and PE4.
- CConfigure the weight attribute on PE2 in the outbound direction to CE2 with a value less than
- DApply MED in the outbound direction on PE2 to CE1 with a cost lower than the path from PE4 to
How the community answered
(68 responses)- A13% (9)
- B63% (43)
- C18% (12)
- D6% (4)
Explanation
Local preference is an iBGP-only attribute and is not propagated across eBGP sessions. Applying a local preference route map on PE2 outbound to CE1 (an eBGP neighbor) has no effect because local preference is stripped at eBGP boundaries-CE1 will never receive or act on it, which is why the PE2-CE1 path remains preferred. To influence path selection across AS boundaries, AS path prepend must be used. By prepending additional AS hops on PE2's outbound advertisements to CE1, the PE2-CE1 path appears longer (less desirable) to the customer AS, causing the customer's BGP decision process to prefer the shorter AS path via PE4-CE2, achieving the desired primary link behavior.
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