300-320 Question #680: Real Exam Question with Answer & Explanation

The correct answer is B: BGP, along with SoO, can stop the routing loop when there is a backdoor link between the CE. When CE routers at different VPN sites have a backdoor link between them, routing loops can form: a route learned over the MPLS backbone could be re-advertised back into the backbone via the backdoor link. BGP with the Site of Origin (SoO) extended community (B) solves this by ta

Question

Which answer about PE-CE routing design is correct in a Layer 3 MPLS VPN environment?

Options

AEIGRP has the built-in capability to stop a routing loop when there is a backdoor link between the
BBGP, along with SoO, can stop the routing loop when there is a backdoor link between the CE
COSPF does not support a PE-CE design when there is a backdoor link between the CE routers.
DEIGRP does not support a PE-CE design when there is a backdoor link between the CE routers.

Explanation

When CE routers at different VPN sites have a backdoor link between them, routing loops can form: a route learned over the MPLS backbone could be re-advertised back into the backbone via the backdoor link. BGP with the Site of Origin (SoO) extended community (B) solves this by tagging routes with the site that originated them; the PE router drops any route that returns to its originating site. EIGRP (A) does not have a built-in backdoor loop prevention mechanism and requires SoO as well. OSPF (C) can support PE-CE with backdoor links using sham-links, so it is not unsupported. EIGRP (D) can also work with PE-CE and backdoor links when configured with SoO - it is not inherently unsupported.

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