A packet arrives and nothing in the routing table matches its destination. What does the router do?
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Say this first
It drops the packet and sends an ICMP Destination Unreachable (type 3) back to the source, rate limited. Many platforms use code 0 (network unreachable); RFC 1812 tells routers to prefer code 1 (host unreachable), so you may see either. With a default route there is always a match, so this only happens when the router has no gateway of last resort.
Go deeper
What you see
show ip route says Gateway of last resort is not set. A Windows host shows
Reply from 10.0.12.1: Destination net unreachable. (or host unreachable, depending on the code the
router sends) with the router's address. A ping from another IOS
router shows U.U.U: each U is an unreachable reply and each dot is one the router suppressed by rate
limiting.
Related cases that look similar
| Situation | Result |
|---|---|
| No route at all | drop, ICMP unreachable (type 3, code 0 or 1 by platform) |
| Route exists, destination is on a connected subnet, ARP gets no answer | drop, ICMP host unreachable (code 1) |
| Route points to Null0 | drop, ICMP unreachable unless no ip unreachables is set on Null0 |
| TTL reaches 0 | drop, ICMP time exceeded (type 11) |
Rate limiting and hardening
IOS rate limits ICMP unreachables by default (ip icmp rate-limit unreachable). Many operators also put
no ip unreachables on Null0 and on internet-facing interfaces, so a flood of packets to discarded
destinations does not turn into a flood of ICMP from the router's CPU.
They will ask next
Why does a traceroute sometimes show a star at the last hop of a router?
The last hop answers with ICMP port unreachable, and IOS rate limits unreachables. A burst of three probes often gets one reply suppressed, so you see something like
2 msec * 2 msec.
Traps
- Saying the router floods the packet or sends it to every neighbor. Routers drop what they cannot route.