📘 CCNA 200-301 v1.1

3.1 Interpret the components of routing table

3.1.e Administrative Distance (AD)

1. What is Administrative Distance?

Administrative Distance (AD) is a value that routers use to decide which routing source (protocol or route) is more trustworthy or reliable when there are multiple routes to the same destination network.

• It is a numerical value that represents the trust level of a routing information source.
• Lower AD = More trusted source.
• Higher AD = Less trusted source.

The router always selects the route with the lowest administrative distance if multiple routing sources provide a path to the same destination.

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2. Why is Administrative Distance Needed?

A router can learn routes from different sources:

• Directly connected networks
• Static routes (manually configured)
• Dynamic routing protocols (like OSPF, EIGRP, RIP, etc.)
• Default routes

If two or more routing sources tell the router about the same network, the router must decide which one to believe — that’s where Administrative Distance helps.

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3. How Administrative Distance Works

When a router receives multiple routes to the same destination:

1. The router looks at the AD of each route.
2. The route with the lowest AD is chosen and installed into the routing table.
3. Routes with higher ADs for the same destination are ignored (but kept as backup in case the better route fails).

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4. Default Administrative Distance Values

Cisco routers use default AD values for different routing sources.
Here’s a list you must memorize for the CCNA exam:

Route Source	Administrative Distance (AD)	Description
Connected Interface	0	Most trusted — router is directly connected to the network.
Static Route	1	Manually configured route by the administrator.
EIGRP (Internal)	90	Route learned within the same EIGRP Autonomous System.
OSPF	110	Route learned from OSPF.
RIP	120	Route learned from RIP.
EIGRP (External)	170	Route learned from another EIGRP Autonomous System.
iBGP (Internal BGP)	200	Route learned within the same BGP system.
eBGP (External BGP)	20	Route learned from an external BGP peer.
Unknown Route Source	255	Unusable route — will never be installed in the routing table.

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5. Example in an IT Environment

Imagine a router (R1) that learns about the network 10.1.1.0/24 from two sources:

• From OSPF (AD = 110)
• From a static route (AD = 1)

The router compares both routes:

• Static route AD = 1
• OSPF route AD = 110

✅ The router will choose the static route because 1 < 110 (it’s more trustworthy).

If the static route becomes unavailable (for example, interface goes down), the router will then use the OSPF route (backup route).

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6. Customizing Administrative Distance

You can manually change the AD of routes to influence which route is preferred.

Example:

Router(config)# ip route 10.1.1.0 255.255.255.0 192.168.1.1 5

Here, the number 5 at the end sets the Administrative Distance for this static route to 5 (instead of the default 1).

This can be used to create:

• Primary and backup routes manually.
• Policy-based preferences between routing protocols.

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7. Relationship Between AD and Routing Table

The routing table only keeps the best route for each destination network — based on:

1. Lowest Administrative Distance (which source is trusted)
2. If AD is the same → lowest metric (path cost) is chosen.

Example:

• Two OSPF routes to the same destination — both have AD 110, so the router then checks OSPF metric to decide which one to keep.

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8. Key Notes for the Exam

✅ Memorize Default AD Values — especially for Connected, Static, RIP, OSPF, EIGRP, and BGP.
✅ Lower AD = Higher Trust.
✅ AD can be manually changed to adjust route preferences.
✅ Routing table shows only the most trusted route (lowest AD).
✅ AD only matters when multiple routing sources exist for the same network.

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9. Example Routing Table View

R1# show ip route
Codes: C - connected, S - static, O - OSPF, R - RIP, D - EIGRP

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.1.1.0/24 is directly connected, GigabitEthernet0/0
S       10.2.2.0/24 [1/0] via 192.168.1.2
O       10.3.3.0/24 [110/2] via 192.168.1.3, 00:00:23, GigabitEthernet0/1
D       10.4.4.0/24 [90/156160] via 192.168.1.4, 00:00:15, GigabitEthernet0/2

In this table:

• [1/0] → 1 is the Administrative Distance (Static route)
• [110/2] → 110 is the Administrative Distance (OSPF)
• [90/156160] → 90 is the Administrative Distance (EIGRP)

The router used these ADs to decide which routes to install into the table.

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10. Summary

Concept	Explanation
Purpose	Determines which route source is most trusted when multiple routes exist.
Low AD = High Trust	The router always picks the route with the lowest AD.
Default AD Values	Must be memorized for exam.
Custom AD	Can be manually adjusted to control route preference.
Used only for same destination networks	AD is compared only when multiple paths lead to the same network.

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