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Administrative Distance — Route Preference

How a router picks between routing sources for the same prefix: administrative distance (Cisco/Arista) vs route preference (Junos/Nokia/Huawei), where it sits after longest-prefix match, the canonical Cisco defaults, a multi-vendor comparison table, floating statics, and redistribution gotchas.

When two routing sources offer a route to the same destination, which one does the router install? Administrative Distance (AD) is the tie-breaker: a per-source "trustworthiness" rank — lower is more preferred. It's a favorite interview question because the defaults are arbitrary, vendor-specific, and full of traps (eBGP beating your IGP, iBGP losing to it).

AD is local to one router, compared only between sources offering the same prefix/mask, and it comes into play after longest-prefix match. Within a single protocol, the metric breaks the tie — AD only chooses between protocols.

Where AD sits in route selection

1. Longest-prefix match first. A more specific route always wins — a RIP /32 beats an OSPF /24 for that host, regardless of AD. AD never overrides prefix length.
2. Then AD decides between sources for the same prefix (e.g. OSPF vs eBGP both offering 10.0.0.0/24).
3. Then the metric (cost, hop count, etc.) decides within that winning protocol.

The winner goes into the RIB/routing table; the losers stay in their protocol database as backups. On Cisco/Arista it's called AD; on Juniper, Nokia, and Huawei the same idea is route preference.

Cisco — the canonical defaults

Two things interviewers press on: eBGP (20) is preferred over every IGP — an eBGP-learned prefix beats the same prefix from OSPF/IS-IS; and iBGP (200) is worse than every IGP. Also note Cisco uses 110 for both internal and external OSPF — it does not split them by AD (other vendors do).

Multi-vendor comparison

Same concept, different defaults. Values are the vendor's default preference (lower wins):

The big cross-vendor differences to remember:

• Cisco/Arista prefer eBGP (20) over IGPs; Juniper/Nokia give BGP 170 (both e/iBGP, so IGPs win); Huawei sets BGP to 255 by default — every IGP and even static beats it until you lower it.
• Cisco doesn't split OSPF by AD (110 for O and O E2); Juniper/Nokia/Huawei do (internal 10, external 150).
• Static is very trusted on Cisco/Arista (1) but sits above the IGPs on Huawei (60) — so on Huawei a static can lose to OSPF for the same prefix.
• EIGRP is Cisco-originated (Arista also implements it); the others have no equivalent.

Using & changing it

• Floating static route: configure a static with a high AD (e.g. Cisco ip route ... 250) so it only installs when the dynamic route disappears — a cheap backup path.
• Per-protocol / per-route override: Cisco distance (globally or per-source), Junos/Nokia/Huawei preference. Change it to steer which source wins during redistribution or migration.
• Redistribution loops: when a prefix re-enters via two protocols, AD (plus route tags/filters) decides the winner — get it wrong and you get suboptimal routing or a loop.
• AD 255 = "do not install." On Cisco a route with AD 255 is discarded, not used.