discard route

networking

A route for a summary prefix whose next hop is nowhere, so traffic for unreachable parts of the summary is dropped.

A router that advertises a summary installs one, usually pointing to Null0 or a blackhole. Without it, a packet for a dead component matches the router's default route and is sent back the way it came, looping until its TTL runs out.

RFC 4632's second aggregation rule says a router that generates an aggregate must discard packets that match the aggregate but none of its more-specific routes: the next hop should be the null destination, to prevent forwarding loops. RFC 1519 called it a sink route in 1993. Longest match makes it work, since the components the router still reaches are longer prefixes and win, while the discard entry beats the default. RFC 4632 names the default as the special case a network must not follow for its own aggregates.

OSPF builds it in: RFC 2328 inserts a discard entry for each active area address range before the lookup, and when that entry is the best match the packet is dropped and an ICMP destination unreachable is returned. Elsewhere ICMP depends on the route type. On Linux a blackhole route discards silently while unreachable answers host unreachable; in FRRouting, Null0 or blackhole drops and reject answers with ICMP.

The price is visibility: RFC 4632 notes that traceroute then points at the network advertising the aggregate, not at the one that stopped advertising the more-specific prefix. FRRouting's documentation also shows a static route to Null0 at distance 255, a floating blackhole that only takes over when the real next hops are unreachable.

Also known as: null route, null0 route, blackhole route, sink route, discard entry, null0

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