PlantUML DiagramRoute Selection with Multiple Sources
A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
| Protocol | Default AD | Notes |
|---|---|---|
| Connected | 0 | Never loses |
| Static (to next-hop) | 1 | Admins trust their own config |
| eBGP | 20 | Exterior, more trusted than IGP? Yes, per design |
| EIGRP (internal) | 90 | Cisco-proprietary, very trusted |
| OSPF | 110 | Industry standard |
| IS-IS | 115 | Common in service provider |
| RIP | 120 | Legacy, least trusted IGP |
| EIGRP (external) | 170 | Redistributed |
| iBGP | 200 | Internal BGP |
| Unusable | 255 | Prevents installation |
Administrative distance is the tie-breaker when two different sources provide a route to the same prefix. Lower wins, always. No calculation, no protocol-specific logic — just a number. It's the router's trust rating for each source.
The AD values aren't arbitrary — they reflect a philosophy. Connected routes are absolute truth (AD 0). Static routes are configured by a human who presumably knows what they're doing (AD 1). eBGP is trusted because it reflects real routing decisions made by the broader internet (AD 20). Internal routing protocols get various values based on historical precedent and vendor politics — EIGRP is more trusted than OSPF partly because Cisco defined it.
Only the lowest-AD route is installed in the FIB (the forwarding table). But the higher-AD routes stay in the RIB as backups — if the static route is removed, the EIGRP route immediately takes over. This is the failover mechanism.
! Change OSPF's administrative distance globallyrouter ospf 1distance 85! Different AD for specific routesrouter ospf 1distance ospf external 120 inter-area 115 intra-area 110! Static route with custom AD (floating static)ip route 0.0.0.0 0.0.0.0 203.0.113.1 200! Per-prefix AD via route-mapip access-list standard PREFER_OSPFpermit 10.5.0.0 0.0.255.255route-map AD_CHANGE permit 10match ip address PREFER_OSPFset distance 50
The global `distance 85` command lowers OSPF's AD so it beats EIGRP (90). This is used in migration scenarios — you're moving from EIGRP to OSPF, and you want the new protocol to take over before you remove the old one. Lowering OSPF below EIGRP's AD forces the switch.
Changing AD is powerful and dangerous. Lowering OSPF's AD below 90 in a network where EIGRP is still running creates an immediate cutover — routes shift to OSPF paths that may not be ready. Plan AD changes carefully, and change back once the migration is complete.
In a lab with two routers running both OSPF and EIGRP, watch `show ip route` as you shut down one protocol. The route source changes instantly — that's the backup AD route taking over. Then change OSPF's AD to 80 and watch OSPF win even when EIGRP is up.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.