350-501 Question #259: Real Exam Question with Answer & Explanation

Question

A network engineer is configuring RIP as the routing protocol between multiple PEs and CEs. The engineer must avoid advertising the same routes back to their sources. Which action should be performed on the routers to accomplish this task?

Options

• AConfigure a different route distinguisher for each prefix.
• BDefine the site of origin on each interface.
• CDefine VRFs on each device to separate the traffic.
• DEnable bidirectional forwarding detection on each device.

Explanation

To prevent routing loops and avoid advertising routes back to their sources when redistributing between RIP on CE routers and BGP on PE routers, the Site of Origin (SoO) attribute should be configured on each interface. The SoO attribute acts as a loop prevention mechanism in MPLS VPNs.

Common mistakes.

• A. Route distinguishers (RDs) are used to make VPNv4 routes unique in the global BGP table, but they are not primarily for preventing routing loops when redistributing into an IGP like RIP back to the source.
• C. Defining VRFs (Virtual Routing and Forwarding instances) separates traffic logically, but VRFs alone do not prevent routing loops when redistributing routes between the VRF and the connected CE device without additional mechanisms like SoO.
• D. Bidirectional Forwarding Detection (BFD) is a fast link failure detection protocol that speeds up convergence but does not address routing loop prevention for redistributed routes.

Concept tested. MPLS VPN routing loop prevention (Site of Origin)

Reference. https://www.cisco.com/c/en/us/td/docs/ios/mp_l3vpn/configuration/guide/m3_l3vpn_cfg_guide_xe/m3_redist_igp_mpls_l3vpn.html

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