Two numbers, two different contests
Each routing source measures paths in its own unit. Open Shortest Path First (OSPF) adds up interface costs, where cost is the reference bandwidth (100 Mb/s by default) divided by the interface bandwidth. BGP compares path attributes. A router has no way to weigh an OSPF cost against a BGP value, so it first asks a simpler question: which source does it trust more for this prefix? That trust ranking is AD, a value from 0 to 255 that stays local to the router and is never advertised.
Only after a source has won does its metric decide between that source’s own paths. Both checks come after prefix length; a more specific route wins before either number is read, as the longest prefix match page shows.
In the blueprint this is v1.1 topics 3.1 and 3.2 and v2.0 topic 3.1, where you read a routing table to find the next hop. Static, default and floating routes are mapped in the routing table and static routes study area.
Default administrative distances
| Route source | Default AD |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| External BGP (eBGP) | 20 |
| OSPF | 110 |
| IS-IS | 115 |
| Internal BGP (iBGP) | 200 |
| Unknown or untrusted | 255 (never installed) |
From Cisco’s What Is Administrative Distance? (updated July 20, 2026; checked October 29, 2026). Cisco’s full table lists more sources.
Reading [AD/metric] in a routing table
An edge router with OSPF toward the core and an eBGP session to a provider.
R1# show ip route...Gateway of last resort is 10.0.13.3 to network 0.0.0.0S* 0.0.0.0/0 [1/0] via 10.0.13.3 10.0.0.0/8 is variably subnetted, 4 subnets, 2 masksC 10.0.12.0/30 is directly connected, GigabitEthernet0/1L 10.0.12.1/32 is directly connected, GigabitEthernet0/1C 10.0.13.0/30 is directly connected, GigabitEthernet0/2L 10.0.13.1/32 is directly connected, GigabitEthernet0/2 172.16.0.0/24 is subnetted, 2 subnetsO 172.16.20.0 [110/3] via 10.0.12.2, 00:14:07, GigabitEthernet0/1B 172.16.30.0 [20/0] via 10.0.13.3, 00:41:55In the O line, 110 is OSPF’s AD and 3 is the cumulative cost. Suppose OSPF also learned 172.16.30.0/24: its copy would stay out of the table, because eBGP at 20 beats OSPF at 110 before either metric is read.
AD and metric, point by point
| Point of comparison | Administrative distance | Metric |
|---|---|---|
| Ranks (differs) | Route sources offering the same prefix | Paths learned by one source for the same prefix |
| Position in [x/y] (differs) | First | Second |
| Lower is better | Yes | Yes |
| Advertised to neighbors (differs) | No, local to the router | Yes, carried and added up in the protocol’s updates |
| Unit (differs) | A trust rank, 0 to 255 | Protocol-specific (OSPF: cost) |
| Changed by (differs) | Per source, or per static route | Interface cost or bandwidth, protocol settings |
≠ marks rows where the two differ.
Frames on AD vs metric
When a frame shows a routing-table line, label the bracket before you read the stem: first number source trust, second number path cost.
Practice frames written by our editors for study, not Cisco items. The 200-301 exam itself mixes multiple choice, drag-and-drop and lab-style items.
Frame 001 of 007 · 0 answered · 0 ok
Keys
1–5 or A–E pick an option (on choose-N frames they toggle; then Check answer) · ← → previous / next frame · ↑ ↓ move in the frame list
- Frame 001 of 007
When multiple routes to the same network exist using different routing protocols, what is used to select the best path?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Each protocol calculates its metric in its own units, such as cost or hop count, so metrics cannot be compared between protocols.
Note on B
The router ID identifies a router inside a protocol such as OSPF; it plays no part in choosing between protocols.
Note on C
Interface priority is an OSPF setting for the DR/BDR election, not a route-selection rule.
Note on D
Correct: administrative distance ranks how trustworthy each route source is, and the lowest AD wins between protocols.
Best answer: D · why
When two routing sources offer the same prefix, the router compares administrative distance: connected 0, static 1, eBGP 20, OSPF 110 and so on. The route with the lower AD goes into the routing table. Metric is used only to compare routes learned by the same protocol, because each protocol measures distance in its own units.
- Frame 002 of 007
Which routing protocol has the best Administrative Distance (AD) for selecting the best path when multiple routes are available?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
A static route has an AD of 1, which is low but still higher than a directly connected network.
Note on B
OSPF has an AD of 110, so connected and static routes are both preferred over it.
Note on C
External BGP has an AD of 20, below OSPF but well above connected and static routes.
Note on D
Correct: a directly connected network has an AD of 0, the most trusted source a router has.
Best answer: D · why
Administrative distance ranks route sources from most to least trusted, and lower wins. The defaults are connected 0, static 1, external BGP 20, OSPF 110, IS-IS 115 and internal BGP 200. An AD of 255 means the route is never used. A directly connected interface always beats any learned route to the same prefix.
- Frame 003 of 007
A network engineer runs the “show ip route” command and observes an entry that reads “O 192.168.2.0/24 [110/200] via 172.20.10.1, 00:00:30, Serial1/0”. What is the administrative distance of this route?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
172.20.10.1 is the next-hop address after via, the neighbor the packet is sent to.
Note on B
200 is the second number in the brackets, the OSPF metric (cost) of this route, not its AD.
Note on C
Correct: in [110/200], the first number is the administrative distance, and 110 is the OSPF default.
Note on D
192.168.2.0 is the destination network, shown together with its /24 prefix length.
Best answer: C · why
A show ip route entry reads: source code (O = OSPF), destination prefix, [administrative distance/metric], next hop, age and outgoing interface. Here 192.168.2.0/24 was learned by OSPF with AD 110 and cost 200, through next hop 172.20.10.1 on Serial1/0, and was last updated 30 seconds ago.
- Frame 004 of 007
In a routing table entry showing ‘O [110/2] via 10.1.1.2, 00:05:36, GigabitEthernet0/0’, what does the value ‘2’ represent?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
OSPF areas do not appear in this entry; you see them in show ip ospf or in the protocol configuration.
Note on B
Correct: the second number in the brackets is the metric, which for OSPF is the cumulative cost to the destination.
Note on C
OSPF does not count hops; its metric is cost, based on interface bandwidth.
Note on D
The administrative distance is the first number in the brackets, 110 here.
Best answer: B · why
In the bracket [110/2], 110 is the administrative distance (the OSPF default) and 2 is the metric. For OSPF the metric is cumulative cost, and each interface’s cost is the reference bandwidth (100 Mb/s by default) divided by the interface bandwidth, so two Gigabit hops add up to cost 2. Hop count is the metric of distance-vector protocols, not OSPF.
- Frame 005 of 007
When a router has multiple routes to the same destination with identical prefix lengths and administrative distances, what does the router use to select the best route?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
The router ID identifies an OSPF router; it is not used to choose between routes in the routing table.
Note on B
Correct: equal prefix and AD mean the routes come from the same protocol, which picks the lowest metric and can load-balance equal metrics.
Note on C
Bandwidth feeds into some metrics, such as OSPF cost, but the decision uses the metric, not bandwidth alone.
Note on D
Route age appears in the routing table, but it is not a tiebreaker for best-path selection.
Best answer: B · why
Route selection runs in order: the longest matching prefix wins, then the lowest administrative distance among equal prefixes, then the lowest metric among routes from the same source. If the metrics tie too, the router can install several equal-cost paths and load-balance across them (OSPF supports ECMP by default).
- Frame 006 of 007
If a router has both a static route and an OSPF route to the same network with identical prefix lengths, which route will be installed in the routing table by default?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Different ADs mean the routes are not equal, so the router installs only the lower-AD route instead of load balancing.
Note on B
Overlapping routes are normal; the router simply picks one source instead of rejecting both.
Note on C
OSPF’s AD of 110 loses to the static route’s AD of 1, so the OSPF route stays out of the table.
Note on D
Correct: a static route has a default AD of 1, lower than OSPF’s 110, so the static route is installed.
Best answer: D · why
For the same prefix, the router installs the route with the lowest administrative distance. A static route (AD 1) beats OSPF (AD 110), so the OSPF route is kept only in the OSPF database. A floating static route reverses this: give the static route an AD above 110, such as 115, and it becomes a backup that appears only if the OSPF route disappears.
- Frame 007 of 007
A router receives two default routes: one from OSPF (AD 110) and one static default route (AD 1). Which default route is installed, and why?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Arrival order does not matter; the router compares administrative distance.
Note on B
Two default routes from different sources are not equal, so only the one with the lower AD is installed.
Note on C
Correct: 0.0.0.0/0 is the same prefix from both sources, so the lower AD wins, and static’s 1 beats OSPF’s 110.
Note on D
Dynamic routes are not preferred by default, and load balancing applies only to equal routes from the same source.
Best answer: C · why
A default route is just the prefix 0.0.0.0/0, and it follows the normal rules. With equal prefixes, the lower administrative distance wins: the static default (AD 1) is installed and the OSPF-learned default (AD 110) is held back. To use the static route only as a backup, configure it as a floating static route with an AD higher than 110.
Administrative distance in brief
Does administrative distance travel in routing updates?
No. AD is a local preference on each router. Two routers can rank the same source differently if one has been reconfigured.
What does an AD of 255 mean?
The source is not trusted at all, and a route with that distance is never installed in the routing table.
Why can’t a router compare an OSPF cost with a BGP metric?
They measure different things in different units. AD exists to settle that contest without converting one into the other.