An ISP customer is assigned the 10.5.0.0/16 aggregate block and wants to advertise it as a single summarized static route toward the ISP, even though only a few /24 subnets within that range are actually in use. To prevent routing loops for the unused address space, the administrator adds: 'ip route 10.5.0.0 255.255.0.0 Null0'. What is the purpose of this command?
- AIt creates a discard route that silently drops packets destined for addresses within the summarized block that don't match a more specific route, preventing loops
- BIt disables OSPF routing for the entire 10.5.0.0/16 range
- CIt configures Null0 as a floating backup route that activates only if the ISP link fails
- DIt forwards all traffic for 10.5.0.0/16 toward the ISP's next-hop router
Show answer & explanationAnswer & explanation
Correct answer: A. It creates a discard route that silently drops packets destined for addresses within the summarized block that don't match a more specific route, preventing loops
A static route to Null0 acts as a discard route. When a summarized aggregate is advertised but not all subnets within it exist, packets for the unused portions could otherwise loop back from upstream routers. Routing them to Null0 causes the local router to silently drop such packets instead of forwarding them in a loop.
Why the other options are wrong
- B. This command has no effect on OSPF process operation; it is purely a static route to a null interface.
- C. Null0 routes are not backups activated on link failure; they act immediately for traffic matching the more specific summarized prefix with no better match.
- D. Null0 is a discard interface; it never forwards traffic toward any next hop.
Null0 Discard Route
A static route pointing to interface Null0 causes the router to silently drop any packets matching that route, commonly used to prevent routing loops when advertising summarized address blocks.
- Command: ip route <network> <mask> Null0
- Longest prefix match still applies: specific in-use subnets are preferred over the summarized Null0 route
- Prevents black-hole loops when an aggregate is advertised but some subnets don't exist
Memory trick: Advertise the whole block, drop the empty corners into the void.