Administrative Distance: The Full Table and How Routes Compete

Administrative distance is the number a Cisco router uses to rank how much it trusts the source that offered a route. Values run from 0 through 255, and lower means more trusted. The comparison happens only when two different sources hand the router the identical prefix with the identical prefix length, and the winner is installed while the loser is held aside. Connected interfaces are 0, static routes are 1, external BGP is 20, internal EIGRP is 90, OSPF is 110, IS-IS is 115, RIP is 120, external EIGRP is 170, internal BGP is 200, and 255 marks a route as unusable.

Vendor origin

No RFC defines administrative distance. It is a Cisco IOS mechanism, invented to solve a problem the routing protocols themselves cannot solve: each protocol knows how to rank its own paths, but nothing in OSPF tells a router how an OSPF cost of 12 compares with a RIP hop count of 3. Cisco’s answer was to rank the sources instead of trying to convert their metrics. Several other vendors have since adopted the same idea, sometimes under the name route preference and sometimes with different numbers, so the values below are Cisco values and should be recalled as such.

The default values

Route source	Default distance
Directly connected interface (C) and its local host route (L)	0
Static route (S)	1
EIGRP summary route	5
External BGP (B)	20
Internal EIGRP (D)	90
OSPF (O)	110
IS-IS (i)	115
RIP (R)	120
External EIGRP (D EX)	170
Internal BGP (B)	200
Unusable	255

Four pairs in that list account for most of the errors students make. Internal EIGRP is 90 and external EIGRP is 170, so a route redistributed into EIGRP from somewhere else loses to almost everything. External BGP is 20 and internal BGP is 200, which places eBGP above every interior protocol and iBGP below every interior protocol, a deliberate asymmetry. OSPF at 110 and IS-IS at 115 sit close together because they are peers in design. Connected at 0 and static at 1 look adjacent, but the reasoning differs: a connected route is evidence the router gathered itself from an interface that is up and addressed, while a static route is an assertion an administrator typed.

The value 0 belongs to interfaces the router owns. A static route configured with an exit interface instead of a next-hop address still carries a distance of 1, even though older IOS prints it with the phrase is directly connected. That display shape is discussed further in the article on static route configuration.

Route source competition

A router builds its routing table from candidates. Every process that has something to say about a destination submits an entry, and the routing table process decides what actually gets installed. Only the winners are printed, each with its source code letter and its distance in the first half of the bracketed pair, as the fields of show ip route lay out.

Two sources compete only when their prefix and mask match exactly. If EIGRP offers 10.20.0.0/16 and OSPF offers 10.20.5.0/24, there is no contest at all: those are different destinations as far as route selection is concerned, and both entries are installed side by side. The router will use one or the other later, per packet, according to which entry matches the destination most specifically.

When the prefix and mask do match, distance decides, and it decides absolutely. A RIP route at 120 loses to an OSPF route at 110 no matter how good the RIP path looks or how many routers OSPF’s cost implies. Metric never enters the comparison across sources, because an OSPF cost, an EIGRP composite value, and a RIP hop count are computed on scales that have no common unit. Metric is compared only after a single source has won, to choose among that source’s own paths, and equal-cost load balancing therefore happens only within one protocol.

The loser is not discarded. It stays in the offering protocol’s own database and is re-evaluated continuously. The moment the winning source withdraws its prefix, the router installs the runner-up without any manual action and without a reload. That behavior is the entire basis of the floating static route.

Local significance

Administrative distance lives inside one router’s selection process and nowhere else. It is not a field in an OSPF link-state advertisement, not a field in an EIGRP update, not carried in a BGP path attribute, and not exchanged in any hello packet. Change OSPF’s distance on one branch router and every other router in the area, including its direct neighbors, keeps applying whatever value it has configured locally.

That makes a distance change a safe surgical tool and also an easy way to create an asymmetric path by accident, since one device now prefers a different source while its neighbors have not changed.

Configuring a different distance

Static routes accept an optional integer at the end of the command, which replaces the default of 1:

R4(config)# ip route 10.70.0.0 255.255.0.0 172.20.1.2 130

With OSPF already supplying 10.70.0.0/16 at distance 110, that static loses the comparison and stays out of the table. It is configured, evaluated on every selection pass, and installed automatically the instant OSPF stops advertising the prefix. Verification is done against the configuration rather than the table, because a route that lost the comparison is not in the table by design.

Dynamic protocols are adjusted under the routing process instead:

R4(config)# router ospf 1
R4(config-router)# distance 95

That makes every OSPF route on this router more trusted than internal EIGRP. EIGRP splits the setting into internal and external values, and BGP into external, internal, and local. The routing table is rebuilt as soon as the command is accepted.

The value 255

A distance of 255 means the source is not believable, and a route offered at 255 is never installed. Being the only candidate for the prefix does not rescue it. IOS accepts a static route configured with 255 and stores the value exactly as typed, which parks the route in the configuration in a visibly inactive state. Nothing about the number 255 makes a route preferred; the highest value is the least trusted, and 0 is the most trusted.

How the 200-301 exam tests this

• Table recall with near-miss distractors. A multi-select item mixes true values with statements that swap a pair: internal EIGRP given as 170, iBGP as 20, OSPF as 90, static as 0. Learn the values as pairs so the swaps stand out.
• What the concept is for. A definition item offers the correct answer alongside descriptions of a hop count, a link delay, or a value advertised to neighbors. Administrative distance ranks sources, is compared only on matching prefix and mask, and travels nowhere.
• Scope of a change. A stem changes a protocol’s distance on one router and asks what happens elsewhere. Nothing does. Distractors invoke database resynchronization, area boundaries, or hello packets, all of which carry topology and adjacency data rather than trust rankings.
• A configured static that does not appear. A static entered with a trailing number above the interior protocol’s value is doing what it was designed to do when it is missing from the table. The follow-up is to check the running configuration.
• The 255 case. Items ask what an entry at 255 does. It is never installed, no competitor required.

The table itself is pure memorization, and memorization holds up best when it is tested — a run of 200-301 practice questions exposes which pairs you still swap.

Quick reference

• Administrative distance is Cisco-specific, locally significant, and never advertised.
• Lower is more trusted; the range is 0 through 255.
• Connected 0, static 1, EIGRP summary 5, eBGP 20, internal EIGRP 90, OSPF 110, IS-IS 115, RIP 120, external EIGRP 170, iBGP 200, unusable 255.
• Two sources compete only when prefix and mask match exactly.
• Metrics are never compared across protocols; metric applies within one source after it has won.
• A losing candidate is retained and installed automatically if the winner disappears.
• Set it with a trailing integer on ip route, or with distance under a routing process.
• A route at distance 255 is never installed, even unopposed.