300-320 · Question #684
In a scenario in which a backdoor link is used to connect two CE routers using EIGRP between them and the Layer 3 MPLS VPN provider as a PE-CE routing protocol, which BGP attribute can be used to…
The correct answer is C. Cost community. The cost community is a Cisco-proprietary BGP attribute (community value) used to influence BGP best-path selection specifically for scenarios involving EIGRP redistribution into BGP in MPLS VPN environments. When EIGRP routes are redistributed into BGP on a PE router, the cost…
Question
In a scenario in which a backdoor link is used to connect two CE routers using EIGRP between them and the Layer 3 MPLS VPN provider as a PE-CE routing protocol, which BGP attribute can be used to avoid using the backdoor link as the preferred path?
Options
- AAS path
- BOrigin
- CCost community
- DExtent community
How the community answered
(50 responses)- A12% (6)
- B22% (11)
- C62% (31)
- D4% (2)
Explanation
The cost community is a Cisco-proprietary BGP attribute (community value) used to influence BGP best-path selection specifically for scenarios involving EIGRP redistribution into BGP in MPLS VPN environments. When EIGRP routes are redistributed into BGP on a PE router, the cost community carries the EIGRP metric information. The BGP decision process evaluates the cost community before standard attributes like AS path or MED, allowing network engineers to tune path preference so that MPLS VPN paths are chosen over a direct CE-to-CE backdoor link running EIGRP. Without this mechanism, EIGRP's native metric on the backdoor link could inadvertently make it the preferred path.
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