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Routing & Switching · Intermediate

Null0 Interface

A virtual Cisco interface that discards traffic routed to it, used for summary and blackhole routes.

Definition

Routes pointing to Null0 drop matching packets without generating CPU-intensive processing, which prevents loops when a summary covers unused subnets. EIGRP and OSPF automatically create a Null0 discard route for summaries, and operators use Null0 for remote-triggered blackhole filtering. Junos uses the discard or reject next-hop for the same purpose.

Example

ip route 10.0.0.0 255.0.0.0 Null0 anchors a BGP aggregate.

Exam tip

The automatic Null0 route for a summary prevents a routing loop when a default route also exists; know why it is installed.

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