352-011 · Question #85
You work as a network designer for a company that is replacing their Frame Relay WAN with an MPLS VPN service, where the PE-to-CE routing protocol is BGP. The company has 3000 routes in their distribu
A and B are correct because they both solve the core problem - the SP's 1000-prefix limit - without losing reachability to any destination. Why A works: Advertising a single default route (0.0.0.0/0) from the distribution routers into the MPLS network consumes only 1 prefix per V
Question
You work as a network designer for a company that is replacing their Frame Relay WAN with an MPLS VPN service, where the PE-to-CE routing protocol is BGP. The company has 3000 routes in their distribution routers, and they would like to advertise their access routers through the MPLS network. Their service provider, however, only supports 1000 prefixes per VRF. Which two design solutions can be applied to ensure that your access routers will be able to reach all devices in your network? (Choose two.)
Options
- AConfigure the distribution routers to send a default route to the MPLS network
- BConfigure null routes and aggregate routes for the prefixes in your network on the distribution
- CSummarize the routes on MPLS WAN interfaces of the distribution routers
- DUse prefix lists on the distribution routers to control which routes are sent to MPLS network
- EConfigure the access routers to send a default route to the MPLS network
Explanation
A and B are correct because they both solve the core problem - the SP's 1000-prefix limit - without losing reachability to any destination.
Why A works: Advertising a single default route (0.0.0.0/0) from the distribution routers into the MPLS network consumes only 1 prefix per VRF, well under the 1000-prefix cap. Remote sites learn a default route and forward unknown traffic back through the distribution routers, which have the full 3000-route table locally.
Why B works: Aggregating (summarizing) the 3000 routes into supernets reduces the prefix count below 1000, and the companion null routes prevent routing loops that aggregation can introduce (without a null route, a packet for an unallocated subnet inside a summary could loop).
Why the others fail: C sounds similar to B but omits the null route safety mechanism, making it an incomplete/risky solution. D (prefix lists) would filter routes outright - remote sites simply couldn't reach the blocked prefixes, breaking the "reach all devices" requirement. E is backwards - access routers don't hold the full routing table, so their default route into MPLS would be meaningless for this problem.
Memory tip: Think "Shrink or Default" - you either shrink the prefix count (aggregation with null routes = B) or replace all prefixes with a single default (A). Any option that filters without replacing, or that adds routes in the wrong direction, fails the reachability test.
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