300-510 · Question #262
Refer to the exhibit. The service provider environment has these configurations: - The OSPF backbone area is configured to advertise loopback prefixes. - The PE routers run the BGP-IPv4 address family
The correct answer is B. PEA(Config)# ip route 192.168.10.0 255.255.255.0 Null0. The problem is that when an individual subscriber (e.g., 192.168.10.8) goes offline, the host route for that subscriber is removed from the routing table. If the aggregate prefix 192.168.10.0/24 was learned only via redistribution of connected or dynamic routes, losing the last s
Question
Refer to the exhibit. The service provider environment has these configurations:
- The OSPF backbone area is configured to advertise loopback prefixes.
- The PE routers run the BGP-IPv4 address family in a BGP-free core
topology.
- The broadband users IP subnet 192.168.10.0/24 is redistributed in BGP
on PEA. The operations team noticed route disruptions when broadband subscriber 192.168.10.8 goes offline. Which configuration must an engineer apply to PEA to resolve this problem?
Exhibit
Options
- APEA(config)# router bgp 65101
- BPEA(Config)# ip route 192.168.10.0 255.255.255.0 Null0
- CPEA(Config)# ip route 192.168.10.0 255.255.255.0 Null0
- DPEA(config)# router ospf 10
How the community answered
(44 responses)- A16% (7)
- B73% (32)
- C2% (1)
- D9% (4)
Explanation
The problem is that when an individual subscriber (e.g., 192.168.10.8) goes offline, the host route for that subscriber is removed from the routing table. If the aggregate prefix 192.168.10.0/24 was learned only via redistribution of connected or dynamic routes, losing the last subscriber could withdraw the entire aggregate from BGP, causing route disruptions for all other subscribers in that subnet. The solution is to add a static route pointing 192.168.10.0/24 to Null0. This creates a permanent, stable aggregate route in the routing table that BGP can always advertise, regardless of whether individual subscriber host routes exist. The Null0 destination acts as a black hole for unroutable traffic within the aggregate, while specific subscriber routes take precedence via longest-prefix match. Options A and C are distractors with incorrect or incomplete configurations. Option D (OSPF) does not address the BGP redistribution stability issue.
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