Imagine your router has two different ways to reach the same destination network — one learned via OSPF and another via a static route. Which path should it choose? That's where Administrative Distance (AD) comes in. AD is the trustworthiness rating Cisco routers use to select the best routing source when multiple routing protocols provide routes to the same prefix. For the CCNA 200-301 exam, understanding AD is critical because it's tested under exam objective 3.1 (Compare and contrast routing protocol characteristics) and appears in troubleshooting scenarios where route selection doesn't behave as expected. In real networks, misconfigured AD can cause suboptimal routing or black holes, making it a fundamental concept for any network engineer.
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Think of a company CEO who receives daily reports from different departments about project status. The CEO gets reports from three sources: the Vice President (VP), a trusted senior manager, and a new intern who just started. The VP has been with the company for 20 years and always provides accurate, verified data — the CEO trusts this source the most. The senior manager is reliable but sometimes uses outdated spreadsheets. The intern is eager but often makes mistakes. When the CEO receives conflicting reports about the same project (e.g., one says 'on track' and another says 'delayed'), the CEO must decide which source to believe. The VP's report is taken as truth because of the source's higher trustworthiness. In networking, Administrative Distance works exactly like this trust hierarchy. Each routing protocol (the messenger) has a default trust rating (AD value) — the lower the number, the more trustworthy the source. When a router learns the same destination from multiple protocols (e.g., OSPF AD 110 and EIGRP AD 90), it chooses the route with the lowest AD, just as the CEO chooses the VP's report over the intern's. This trust-based selection happens before any metric comparison; the router doesn't even look at metrics if the AD values differ. The CEO doesn't evaluate the intern's data quality — she simply discards it because the source is less trusted. Similarly, the router installs only the route from the most trusted protocol into the routing table. The other routes are kept in the routing information base (RIB) as backup but never used unless the primary route fails. This mechanism ensures that routing decisions are based on the reliability of the learning method, not just raw path cost.
What is Administrative Distance?
Administrative Distance (AD) is a numerical value assigned to each routing source — including routing protocols and static routes — that indicates the trustworthiness of that source. The scale ranges from 0 to 255, where a lower number means higher trust. AD is used by Cisco routers to select the best route when multiple routing protocols provide routes to the same destination network. It is the first tiebreaker in route selection, evaluated before any metric (such as hop count or bandwidth).
Why AD Exists
In a network, multiple routing protocols might run concurrently (e.g., OSPF and EIGRP). Each protocol builds its own routing table, but the router must install only one route per prefix in the global routing table. AD provides a consistent, predictable way to choose between protocols. Without AD, the router would have to compare metrics across different protocols, which is like comparing apples and oranges — OSPF uses cost (based on bandwidth), while EIGRP uses a composite metric (bandwidth, delay, reliability, load). AD eliminates this problem by prioritizing the source itself.
Default AD Values on Cisco IOS
Cisco IOS assigns default AD values to each routing source. Here are the key defaults you must memorize for the exam:
Connected interface: 0
Static route: 1
EIGRP summary route: 5
External BGP (eBGP): 20
Internal EIGRP: 90
IGRP: 100
OSPF: 110
IS-IS: 115
RIP: 120
EIGRP external route: 170
Internal BGP (iBGP): 200
Unknown: 255
Notice that directly connected interfaces (AD 0) are the most trusted, followed by static routes (AD 1). This makes sense — if you manually configure a route, you probably know what you're doing. Dynamic protocols like OSPF (110) and RIP (120) are less trusted. The unknown value 255 means the route is not usable.
How AD Works Step by Step
A router receives routing updates from multiple sources (e.g., OSPF and EIGRP) advertising the same destination network 10.1.1.0/24.
The router's Routing Information Base (RIB) collects all routes from all protocols.
For each destination prefix, the RIB compares the AD values of the competing routes.
The route with the lowest AD is selected and installed in the routing table (the Forwarding Information Base, FIB).
The router uses the selected route for forwarding packets. The other routes remain in the RIB as backup but are not used unless the best route fails.
If the best route goes down (e.g., OSPF neighbor loss), the router removes it from the routing table and then checks the RIB for the next best route (by AD) for that prefix.
AD vs. Metric
It's crucial to understand the difference: AD selects between routing protocols; metric selects between routes within the same protocol. For example, if OSPF learns two paths to the same network, it uses cost (metric) to choose the best. But if OSPF (AD 110) and EIGRP (AD 90) both know about the same network, AD makes the decision — EIGRP wins regardless of OSPF's cost. The router never compares OSPF cost to EIGRP composite metric.
Verifying AD in IOS
Use the following commands to check AD values:
show ip route: Displays the routing table. Each entry shows the AD and metric. Example output for an OSPF route:
O 10.1.1.0/24 [110/20] via 192.168.1.1, 00:00:10, GigabitEthernet0/0The value in brackets [110/20] is AD/metric.
show ip protocols: Shows the AD value configured for each routing protocol. Example:
Routing Protocol is "ospf 1"
Output delay 1 second out of 500 ms
Default networks flagged in outgoing updates
Default AD is 110show ip route [network]: Shows detailed info about a specific route, including AD.
Changing AD
You can change the AD of a routing protocol or a specific route using the distance command. For example, to change OSPF's AD to 150 globally:
router ospf 1
distance 150To change AD for a specific route learned via EIGRP:
router eigrp 100
distance 130 10.1.1.0 0.0.0.255Static routes also allow per-route AD assignment:
ip route 10.2.2.0 255.255.255.0 192.168.1.2 200This sets the static route's AD to 200, making it less preferred than OSPF (110).
Interaction with Floating Static Routes
A floating static route is a static route with a higher AD than a dynamic protocol. It acts as a backup: if the dynamic route fails, the static route takes over. For example, if OSPF (AD 110) learns a route, you can configure a static route to the same network with AD 120. The static route is not installed while OSPF is up, but if OSPF fails, the static route appears in the routing table. This is a common design for backup links.
Common Pitfalls
Forgetting that AD is checked before metric: On the exam, you might see a question where OSPF has a lower cost than EIGRP, but the answer still says EIGRP wins because of lower AD. Candidates often compare metrics across protocols — don't do that.
Assuming all static routes have AD 1: Static routes can have AD values from 1 to 255. The default is 1, but you can change it.
Confusing AD with metric: AD is the trustworthiness of the source; metric is the cost of the path. They serve different purposes.
Not knowing default values: The exam expects you to know the default AD values for common protocols like OSPF (110), EIGRP (90 internal, 170 external), RIP (120), and static (1).
Summary
Administrative Distance is a fundamental mechanism for route selection between different routing sources. It ensures predictable behavior in multi-protocol environments. For the CCNA exam, memorize the default AD values, understand the difference between AD and metric, and know how to verify and change AD in IOS.
Identify the routing sources
Begin by determining which routing protocols or static routes are providing routes to the destination network. Use the command 'show ip protocols' to see which routing protocols are running and their configured AD values. Also, check for any static routes with 'show running-config | include ip route'. For example, if you see both OSPF and EIGRP running, note their default ADs (110 and 90 respectively). If the same network is advertised by both, the router will prefer EIGRP because of its lower AD. This step sets the stage for understanding which source will be chosen.
Check the routing table
Use 'show ip route [destination-network]' to see which route is currently installed. The output will show the protocol code (e.g., O for OSPF, D for EIGRP) and the AD in brackets. Example: 'D 10.1.1.0/24 [90/2172416] via 192.168.1.1' indicates EIGRP with AD 90. If you expected OSPF but see EIGRP, AD is the reason. This step confirms the actual route selection. If no route appears, the destination might not be reachable, or the route might have been suppressed.
Verify AD values per protocol
Run 'show ip protocols' to display the default or configured AD for each protocol. Look for lines like 'Default AD is 110' for OSPF. If the AD has been changed, it will show the custom value. Also, check for any 'distance' commands in the router configuration. This step helps you confirm whether the AD is default or modified. For static routes, use 'show ip route static' to see the AD assigned to each static route (default 1 unless changed).
Understand route selection logic
The router's RIB (Routing Information Base) collects all routes from all sources. For each prefix, it selects the route with the lowest AD. If multiple routes have the same AD (e.g., two OSPF routes), metric is used as tiebreaker. The selected route is installed in the FIB (Forwarding Information Base) for packet forwarding. If the best route fails, the RIB promotes the next best AD route. This logic is deterministic: lower AD always wins, regardless of metric. Candidates often forget that AD is checked before metric.
Modify AD if needed
If you need to change route preference, use the 'distance' command under the routing protocol configuration. For example, to make OSPF preferred over EIGRP, you could lower OSPF's AD to 80 or raise EIGRP's AD to 120. For static routes, set AD at the end of the 'ip route' command. Example: 'ip route 10.2.2.0 255.255.255.0 192.168.1.2 150' creates a floating static route. After modification, verify with 'show ip route' and 'show ip protocols' to ensure the change took effect.
Troubleshoot unexpected route selection
If traffic is taking an unexpected path, start by checking the routing table for the destination. Use 'show ip route [destination]' to see which route is installed. Then, 'show ip protocols' to check AD values. If you see a route from a protocol you didn't expect, compare ADs. For example, if you configured a static route with default AD 1 but OSPF (AD 110) is installed, the static route might have a typo or be pointing to a next-hop that is down. Use 'show ip route static' to verify static routes are present. Also, check for floating static routes that might have higher AD than intended.
In enterprise networks, Administrative Distance is a critical tool for designing robust and predictable routing. One common scenario is the use of a floating static route for backup connectivity. Imagine a branch office connected to the corporate headquarters via a primary MPLS link running OSPF (AD 110). The network engineer configures a backup DSL link with a static default route pointing to the internet, but sets the AD to 120. This ensures that under normal conditions, the OSPF-learned route is used. If the MPLS link fails, OSPF withdraws the route, and the static route with AD 120 is installed, providing backup connectivity. Without AD, the router would use the static route (AD 1) over the dynamic OSPF route, defeating the purpose of dynamic failover.
Another scenario involves merging two companies with different routing protocols. Company A uses EIGRP (AD 90), and Company B uses OSPF (AD 110). After the merger, they need to exchange routes. A network engineer might configure redistribution between the two protocols. However, if both protocols learn the same prefix, EIGRP will be preferred by default (AD 90 vs 110). If the engineer wants OSPF routes to be preferred, they can adjust the AD of OSPF to 85 or increase EIGRP's AD to 120. This fine-tuning prevents suboptimal routing without complex filtering.
A third scenario involves security zones. A firewall might redistribute a default route into an OSPF domain with a high AD (e.g., 200) so that it only becomes active if all other routes fail. This is a form of 'last resort' routing. Misconfiguration can cause routing loops or black holes. For example, if an engineer accidentally sets a static route's AD to 1 when it should be a floating static, the router will prefer the static route over dynamic routes, potentially sending traffic to a dead next-hop.
Performance considerations: AD comparison is CPU-light and happens in the control plane. However, in very large networks with many routing protocols, the RIB can become complex. Best practice is to minimize the number of protocols and use route filtering to avoid unnecessary AD comparisons. Also, when changing AD, document the change because it can confuse other engineers who expect default values.
The CCNA 200-301 exam tests Administrative Distance under exam objective 3.1: 'Compare and contrast routing protocol characteristics'. You must know the default AD values for connected, static, OSPF, EIGRP (internal and external), and RIP. Expect scenario-based questions where you are given a routing table excerpt and asked which route will be used. The trap is often that candidates compare metrics across protocols. For example, a question might show an OSPF route with cost 10 and an EIGRP route with metric 2172416, and ask which is installed. Many candidates think lower cost wins, but the correct answer is EIGRP because AD 90 < 110. Another common trap is forgetting that EIGRP has two AD values: internal (90) and external (170). An external EIGRP route (AD 170) is less preferred than OSPF (110).
Specific values to memorize: Connected (0), Static (1), eBGP (20), EIGRP internal (90), IGRP (100), OSPF (110), IS-IS (115), RIP (120), EIGRP external (170), iBGP (200). The exam may also ask about floating static routes: you need to know that you can set a static route's AD to a value higher than a dynamic protocol to create a backup.
Command outputs to recognize: In 'show ip route', the AD appears in brackets before the metric. For example, 'O 10.1.1.0/24 [110/20]' means OSPF AD 110, metric 20. If a question shows a routing table with multiple entries for the same network, look for the one with the lowest AD. If ADs are equal, then compare metrics.
Elimination strategy for scenario questions: First, identify all routes to the destination. Second, compare AD values — the lowest AD route is installed. If ADs are equal, compare metrics. If metrics are equal, the route with the highest administrative weight (or load balancing) may be used. Always ignore metrics when ADs differ. Also, remember that connected routes (AD 0) always win over any dynamic protocol.
Common wrong answers: 'The route with the lowest metric is chosen' (ignores AD), 'All routes are installed and load balanced' (only equal AD and equal metric routes are load balanced), 'The route learned first is chosen' (Cisco routers do not use first-learned logic).
Administrative Distance (AD) is a trustworthiness rating from 0 (most trusted) to 255 (untrusted).
Default AD values: Connected 0, Static 1, eBGP 20, EIGRP internal 90, OSPF 110, RIP 120, EIGRP external 170, iBGP 200.
AD is compared before metric; the lowest AD route is installed in the routing table regardless of metric.
Floating static routes use a higher AD (e.g., 120) to act as a backup to a dynamic protocol.
Use 'show ip route' to see installed routes and their AD; use 'show ip protocols' to see configured AD per protocol.
To change AD globally for a protocol, use the 'distance' command under router configuration mode.
EIGRP has two default ADs: internal (90) and external (170); external EIGRP is less preferred than OSPF (110).
These come up on the exam all the time. Here's how to tell them apart.
Administrative Distance (AD)
Trustworthiness of the routing source
Compared before metric
Range 0-255, lower is better
Used to select between different routing protocols
Global per-protocol or per-route
Metric (e.g., OSPF Cost)
Cost of a specific path
Compared only when ADs are equal
Range varies by protocol (e.g., OSPF 1-65535)
Used to select between routes within the same protocol
Per-route, based on interface characteristics
Mistake
A route with a lower metric is always preferred over a route with a higher metric, even across different protocols.
Correct
AD is compared first. A route from a protocol with lower AD is chosen regardless of metric. Metrics are only compared within the same protocol or when ADs are equal.
Candidates often confuse the role of AD and metric because both are numbers associated with routes.
Mistake
All static routes have an AD of 1 by default and cannot be changed.
Correct
Static routes default to AD 1, but you can set any AD from 1 to 255 in the 'ip route' command. This allows floating static routes.
Many study materials emphasize the default AD of 1 without mentioning the configurable option.
Mistake
EIGRP external routes have the same AD as EIGRP internal routes.
Correct
EIGRP internal routes have AD 90; EIGRP external routes have AD 170. External routes are less preferred than OSPF (110) and RIP (120).
The term 'EIGRP' is often used generically, so candidates forget there are two AD values.
Mistake
If a router learns the same route from two different protocols, both routes are installed and load balanced.
Correct
Only the route with the lowest AD is installed. Load balancing only occurs when multiple routes have the same AD and same metric (from the same protocol).
Candidates confuse load balancing with route selection; they think multiple routes are always used.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Administrative distance (AD) measures the trustworthiness of the routing source (e.g., OSPF vs EIGRP), while metric measures the cost of a specific path within a protocol (e.g., OSPF cost based on bandwidth). AD is compared first; only when ADs are equal does metric matter. For example, if OSPF (AD 110) and EIGRP (AD 90) both know a route, EIGRP wins regardless of metric. But if two OSPF routes exist, the one with lower cost (metric) wins. For the exam, memorize default ADs and understand that AD is the first tiebreaker.
Yes, you can change the AD of a routing protocol globally using the 'distance' command under router configuration mode. For example, 'router ospf 1' then 'distance 80' changes OSPF's AD to 80. You can also set AD for specific routes using access-lists. For static routes, you set AD as an optional parameter: 'ip route 10.1.1.0 255.255.255.0 192.168.1.1 150'. Changing AD is useful for route preference tuning, such as making a static route a backup (floating static). However, be cautious: changing AD can cause unexpected routing behavior if not documented.
A floating static route is a static route with a higher AD than a dynamic protocol, so it acts as a backup. For example, if you have OSPF (AD 110) learning a route, you configure a static route to the same network with AD 120. Under normal conditions, OSPF's route (AD 110) is installed. If OSPF fails, the static route (AD 120) appears in the routing table. This provides redundancy without interfering with the primary dynamic route. The term 'floating' refers to the route not being active until the primary fails. On the exam, know that you set the AD of the static route higher than the dynamic protocol's AD.
Key defaults for CCNA: Connected interface: 0, Static route: 1, eBGP: 20, EIGRP (internal): 90, IGRP: 100, OSPF: 110, IS-IS: 115, RIP: 120, EIGRP (external): 170, iBGP: 200. Unknown routes: 255 (not usable). Memorize these values. The exam often tests these directly or in scenario questions. Note that EIGRP has two values: internal (90) and external (170). External EIGRP is less preferred than OSPF and RIP.
Use the 'show ip route' command. Each route entry shows the AD in brackets before the metric. For example, 'O 10.1.1.0/24 [110/20]' means OSPF (O) with AD 110 and metric 20. To see AD for a specific route, use 'show ip route 10.1.1.0'. To see the AD configured for each routing protocol, use 'show ip protocols'. This command displays the default or configured AD for each protocol. For static routes, 'show ip route static' shows all static routes with their AD.
If the ADs are equal, the router then compares metrics. The route with the lower metric is installed. If metrics are also equal, the router may load balance across both routes (if they are from the same protocol and have equal cost). However, if the routes are from different protocols, they cannot be in the same routing table entry for load balancing; only one will be installed. The router uses a tie-breaking rule (e.g., highest reliability, lowest hop count, etc.) but typically the first learned route may be installed (though this is not deterministic). To avoid ambiguity, ensure ADs differ if you want a specific protocol to be preferred.
Indirectly, yes. If AD is misconfigured, it can cause suboptimal routing that might lead to loops. For example, if you set a static route with AD 1 pointing to a router that has a default route back to you, you create a routing loop for packets destined to that network. Also, if you redistribute routes between protocols and AD values cause a route to be preferred that points to a less optimal path, it can cause temporary loops. However, AD itself is a selection mechanism; loops usually result from incorrect route information or redistribution. Proper design and use of AD can prevent loops by ensuring backup routes are only used when primary routes fail.
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