200-301 · Question #611
Refer to the exhibit. Packets received by the router from BGP enter via a serial interface at 209.165.201.10. Each route is present within the routing table. Which interface is used to forward traffic
The correct answer is B. F0/11. Routers use the longest prefix match principle to determine the best route for forwarding traffic, selecting the most specific network address from its routing table.
Question
Refer to the exhibit. Packets received by the router from BGP enter via a serial interface at 209.165.201.10. Each route is present within the routing table. Which interface is used to forward traffic with a destination IP of 10.10.10.24?
Exhibits
Options
- AF0/10
- BF0/11
- CF0/12
- DF0/13
How the community answered
(29 responses)- A14% (4)
- B76% (22)
- C7% (2)
- D3% (1)
Why each option
Routers use the longest prefix match principle to determine the best route for forwarding traffic, selecting the most specific network address from its routing table.
The /27 prefix (10.10.10.0 - 10.10.10.31) is a valid match but is shorter and therefore less specific than the /28 prefix, meaning it would not be chosen by the longest prefix match rule.
When forwarding traffic, a router applies the longest prefix match rule, which means it selects the route with the most specific subnet mask that matches the destination IP address. For the destination IP 10.10.10.24, the /28 network (10.10.10.16 - 10.10.10.31) provides a more specific match than the /27, /26, or /25 networks, directing traffic out F0/11.
The /26 prefix (10.10.10.0 - 10.10.10.63) is a valid match but is shorter and therefore less specific than the /28 prefix, meaning it would not be chosen by the longest prefix match rule.
The /25 prefix (10.10.10.0 - 10.10.10.127) is a valid match but is shorter and therefore less specific than the /28 prefix, meaning it would not be chosen by the longest prefix match rule.
Concept tested: Longest prefix match in IP routing
Source: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/211750-understand-ip-routing-table-lookup.html
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