Longest Prefix Match

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Plain-English explanation

When multiple routes match a destination IP, the router picks the most specific match — the route with the longest prefix length (most network bits). This is longest prefix match (LPM) and it drives every forwarding decision.

Example: routes for 10.0.0.0/8 and 10.1.0.0/16. Destination 10.1.1.50 matches both — /16 wins because it is longer (more specific).

Deep dive

Matching rule: Compare destination IP to each route's network + mask. Collect all matches. Choose highest prefix length. Tie-breakers: lower AD, then metric (protocol-specific).

| Destination | Routes in table | Winner | |-------------|-----------------|--------| | 10.1.1.50 | 10.0.0.0/8, 10.1.0.0/16 | 10.1.0.0/16 | | 192.168.1.100 | 192.168.0.0/16, 192.168.1.0/24, default | 192.168.1.0/24 | | 8.8.8.8 | default only | 0.0.0.0/0 |

Default route is shortest match (/0) — catches only what nothing else matches.

Summarization interaction: Upstream routers may have one summary route while downstream has specifics — LPM ensures traffic follows the detailed path where present.

Step-by-step — exam scenario

Routing table on R1:

S    10.0.0.0/8 via 1.1.1.1
O    10.1.0.0/16 via 2.2.2.2
C    10.1.2.0/24 is directly connected
S*   0.0.0.0/0 via 3.3.3.3

Where does 10.1.2.75 go? Matches /8, /16, /24 — connected /24 wins (longest).

Where does 10.2.0.1 go? Matches /8 only → via 1.1.1.1.

Where does 172.16.1.1 go? Only default → 3.3.3.3.

Commands to know

Verify specific lookup

show ip route 10.1.2.75 show ip route longer-prefixes 10.1.0.0

Troubleshooting

| Issue | LPM angle | |-------|-----------| | Traffic wrong path | More specific route elsewhere in table | | Summary blackhole | Specific route missing downstream | | Default catches internal traffic | Missing specific routes — design error |

When adding routes, ask: "Is there already a more specific match?"

Exam relevance

Exam trap

A /8 and /24 both match — always pick /24. Prefix number larger = longer match = more specific.

AD vs LPM

LPM picks candidates first among routes in the table at same AD level. If two routes same prefix length, lower AD wins.

Practice checklist

Given five routes, determine forwarding for ten destinations
Explain why default route is last resort logically
Design two overlapping static routes and predict behavior
Use show ip route [ip] to confirm LPM on lab router
Connect LPM to subnetting skills from Week 2

Destination 10.1.5.10 matches routes 10.0.0.0/8 and 10.1.0.0/16. Which is used?

Which prefix length is the most specific?

Learning objectives