352-001 · Question #148
An IBGP mesh design is being scoped, and in the discussions, one of the design engineers proposes the use of route reflectors. Which limitation is valid when using route reflectors in this design?
The correct answer is C. Multipath information is difficult to propagate in a route reflector topology. Route reflectors simplify IBGP full-mesh requirements but introduce a known limitation where only the best path is reflected, making multipath propagation across the topology difficult.
Question
An IBGP mesh design is being scoped, and in the discussions, one of the design engineers proposes the use of route reflectors. Which limitation is valid when using route reflectors in this design?
Options
- AThe configuration complexity on the routers will be increased.
- BRoute reflectors will limit the total number of routes in the topology.
- CMultipath information is difficult to propagate in a route reflector topology.
- DRoute reflectors will cause an opportunity to create routing loops.
How the community answered
(61 responses)- A21% (13)
- B7% (4)
- C67% (41)
- D5% (3)
Why each option
Route reflectors simplify IBGP full-mesh requirements but introduce a known limitation where only the best path is reflected, making multipath propagation across the topology difficult.
Route reflectors reduce configuration complexity by eliminating the need for a full IBGP mesh; each peer only needs to peer with the route reflector rather than every other router.
Route reflectors do not impose any limit on the total number of routes carried or reflected in the BGP topology.
A route reflector selects and reflects only the single best BGP path to its clients and non-clients, discarding equal-cost or alternate paths. This means multipath information - needed for load balancing - is not propagated through the topology by default, requiring additional mechanisms such as BGP Add-Path (RFC 7911) to address this limitation.
Although routing loops are a theoretical concern, the BGP originator-id and cluster-list attributes are specifically designed to detect and prevent loops in route reflector topologies, making this a mitigated rather than a valid ongoing limitation.
Concept tested: BGP route reflector multipath propagation limitation
Source: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13754-26.html
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