Administrative Distance (AD) – Detailed Explanation

Definition of Administrative Distance

Administrative Distance (AD) is a numeric value that ranks the trustworthiness of routing sources. It helps routers select the most reliable route when multiple routing sources are available.

Purpose of AD in Routing Decisions

AD acts as a tiebreaker between different routing protocols offering paths to the same destination. The route with the lowest AD is selected for the routing table.

Default AD Values for Common Routing Protocols

Route Source	Administrative Distance
Connected	0
Static Route	1
eBGP	20
EIGRP (internal)	90
OSPF	110
RIP	120
EIGRP (external)	170
iBGP	200

How AD Affects Route Selection

If a router receives multiple routes for the same destination, it chooses the one with the lowest AD. If multiple routes have the same AD, the protocol's metric is used next.

Example

Destination 192.168.1.0/24 is learned from:

OSPF (AD 110, metric 100)
RIP (AD 120, metric 2)

OSPF is selected because of its lower AD.

Difference Between AD and Metric

AD: Compares trust across different protocols (lower is better)
Metric: Compares routes within the same protocol

Example

Two OSPF routes with same AD (110):

Route 1: metric 100
Route 2: metric 50

Route 2 is selected due to lower metric.

Modifying AD (e.g., Static Routes)

Router(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.1 150

This makes the static route less preferred than OSPF (AD 110).

Floating Static Routes

Used as backup paths when the main dynamic route fails.

Router(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.2 130

Verifying AD in Routing Tables

Router# show ip route


C  192.168.1.0/24 is directly connected, GigabitEthernet0/0
S  10.0.0.0/24 [1/0] via 192.168.2.1
O  172.16.0.0/16 [110/65] via 192.168.3.1, 00:02:34, Serial0/0/0
D  192.168.4.0/24 [90/2172416] via 192.168.5.1, 00:01:23, FastEthernet0/0

Troubleshooting Conflicting Routes

Router# show ip protocols


Router(config)# router ospf 1
Router(config-router)# no distance 130

AD in Multi-Protocol Environments


Router(config)# router eigrp 100
Router(config-router)# distance eigrp 120 170

Impact of Misconfigured AD

Can result in suboptimal routing, loops, traffic drops, or premature failover. Always test and document AD changes.

Temporary vs. Permanent AD Changes

Temporary: Lost after reboot (unless saved)
Permanent: Use write memory to retain

Best Practices

Document changes
Use floating static routes appropriately
Test in lab or during maintenance
Verify changes with show ip route
Keep AD values consistent across routers

Vendor-Specific AD Behavior

Vendor	Routing Preference Type	OSPF Default	Static Default
Cisco	Administrative Distance	110	1
Juniper	Route Preference	10 (internal), 150 (external)	5
Huawei	Preference	10	60

Juniper Example

set protocols ospf preference 50

Practical Example


Router(config)# router eigrp 100
Router(config-router)# distance eigrp 120 170

Router(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.2 130

Router# show ip route

Administrative Distance (AD) Quiz

1. What does Administrative Distance (AD) represent in routing?

A. The bandwidth of a route B. The metric within a routing protocol C. The trustworthiness or preference of a route source D. The delay to reach the destination

Correct answer is C. AD indicates how trustworthy or preferred a route source is when choosing routes.

2. Which routing source has the lowest default Administrative Distance?

A. Directly connected routes B. Static routes C. OSPF routes D. RIP routes

Correct answer is A. Directly connected routes have an AD of 0, the highest trust.

3. What is the default Administrative Distance for OSPF?

A. 90 B. 120 C. 1 D. 110

Correct answer is D. OSPF’s default AD is 110.

4. Which command modifies the Administrative Distance of a static route in Cisco IOS?

A. ip route 192.168.1.0 255.255.255.0 10.0.0.1 B. ip route 192.168.1.0 255.255.255.0 10.0.0.1 150 C. router ospf 1 distance 150 D. ip nat inside source static 150

Correct answer is B. The last number in the ip route command sets the AD.

5. What is a "floating static route"?

A. A static route with a higher AD than dynamic routes used as backup B. A static route with the lowest AD C. A static route learned via OSPF D. A dynamic route with manual override

Correct answer is A. Floating static routes act as backups with higher AD values.

6. Which routing protocol has a default AD of 90 for internal routes?

A. OSPF B. RIP C. eBGP D. EIGRP (internal)

Correct answer is D. Internal EIGRP routes have an AD of 90.

7. What happens if two routes to the same destination have the same AD?

A. The router chooses the route with the higher AD B. The router uses the routing protocol’s metric to select the best route C. The router loads both routes equally D. The router ignores both routes

Correct answer is B. When AD is equal, the metric is used to break ties.

8. How can modifying Administrative Distance affect routing?

A. It changes the metric within the protocol B. It increases bandwidth C. It changes which route the router prefers between multiple protocols D. It disables routing for that protocol

Correct answer is C. AD sets the preference between different protocols' routes.

9. Which command shows the routing table with AD values on a Cisco router?

A. show ip route B. show running-config C. show interfaces D. show protocols

Correct answer is A. "show ip route" displays routing table entries with AD and metrics.

10. What is the risk of misconfiguring Administrative Distance?

A. Routing loops and suboptimal routes B. Network interface failure C. Loss of VLAN configuration D. Blackholing traffic and unintended failover

Correct answer is D. Misconfigured AD can cause traffic blackholing and improper failover behavior.

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