300-415 Question #190: Real Exam Question with Answer & Explanation

The correct answer is A: 100 10666. For a prefix advertised via OMP with an overlay AS, the AS-PATH on a connected BGP router typically includes its local AS and the OMP overlay AS.

Architecture

Question

Refer to the exhibit showing a network diagram with Overlay AS 10666, BGP AS21, BGP AS100, BGP AS20, BGP AS104, R100, VE100, VE104, R104, and various IP prefixes and links. An engineer configured OMP with an overlay-as of 10666. What is the AS-PATH for prefix 104.104.104.104/32 on R100?

Options

A100 10666
B100 20 104
C100 10666 20 104
D100 10666 104

Explanation

For a prefix advertised via OMP with an overlay AS, the AS-PATH on a connected BGP router typically includes its local AS and the OMP overlay AS.

Common mistakes.

B. This path '100 20 104' represents a pure BGP path without including the OMP overlay AS (10666), which is incorrect for a route traversing the SD-WAN fabric.
C. This path '100 10666 20 104' incorrectly includes AS 20 as part of the primary path for 104.104.104.104/32 to R100, and often OMP simplifies the external AS numbers when advertising routes.
D. This path '100 10666 104' incorrectly includes AS 104 as a distinct AS in the path after the OMP AS, which isn't how OMP typically modifies the AS-path when originating a route from a BGP domain into the overlay.

Concept tested. Cisco SD-WAN OMP AS-PATH modification

Reference. https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/sdwan-xe-gs-book/m-sdwan-config.html#C_ID_0EC830D9_00

Topics

#OMP#AS-PATH#BGP Redistribution#SD-WAN Overlay AS

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