Troubleshoot administrative distance (all routing protocols)

📘CCNP Enterprise – ENARSI (300-410)

1. What is Administrative Distance (AD)?

Administrative Distance (AD) is a number that routers use to rank the trustworthiness of routing information they receive from different routing protocols.

Think of AD as a “trust score”. Lower numbers mean the route is more trustworthy.
When a router receives multiple routes to the same destination from different sources (like OSPF, EIGRP, or static routes), it uses AD to decide which route to put into the routing table.

Default AD Values (for common protocols)

Routing Source	Default AD
Connected interface	0 (most trusted)
Static route	1
EIGRP (internal)	90
OSPF	110
RIP	120
EIGRP (external)	170
Unknown/Untrusted	255 (never used)

Lower AD wins. If two routes exist to the same network, the router picks the route with the lowest AD.

2. Why is AD important?

Routers often learn about networks from multiple sources:

Directly connected interfaces (AD = 0)
Static routes configured manually (AD = 1)
Dynamic routing protocols like OSPF, EIGRP, or RIP

If a router receives the same network from multiple sources, it needs to decide which route to trust. That’s where AD comes in.

Example in IT networks:

Your router learns about network 10.1.1.0/24 from OSPF (AD 110) and a static route (AD 1).
The router will choose the static route because AD 1 < AD 110.

3. How to Troubleshoot Administrative Distance

When routes don’t appear as expected in the routing table, AD might be the reason. Here’s how to troubleshoot:

Step 1: Check the Routing Table

Use commands like:

show ip route

Look for the source of the route:

C = Connected
S = Static
O = OSPF
D = EIGRP
R = RIP

Notice which route the router chose. The route not in the table might be less preferred because of a higher AD.

Step 2: Check AD Configuration

Some protocols allow you to manually change AD.

Static routes example:

ip route 192.168.10.0 255.255.255.0 10.0.0.2 200

Here, AD is set to 200. The router will use this route only if all other routes have AD > 200.

EIGRP or OSPF routes can also have AD changed if needed:

router ospf 1
 distance 90  # changes OSPF's AD

Changing AD is useful in dual-protocol environments to control which routes are preferred.

Step 3: Compare Competing Routes

Sometimes two protocols advertise the same network.

Example:
- OSPF advertises 10.2.2.0/24 (AD 110)
- RIP advertises 10.2.2.0/24 (AD 120)

Troubleshooting Tip:

Check show ip route 10.2.2.0 to see which route is in use.
If the route from OSPF is missing, verify:
- AD values
- Network advertisement
- Interface status

Step 4: Use Debug Commands

To see routing updates and AD decisions in real-time:

debug ip routing

This helps identify why a route was rejected due to a higher AD.

Step 5: Verify with Ping or Traceroute

Once AD issues are resolved:

Test connectivity with:

ping 10.2.2.1
traceroute 10.2.2.1

Ensures traffic is using the correct route.

4. Key Points to Remember for the Exam

AD is always about trust. Lower AD = more trusted.
Directly connected interfaces and static routes are usually preferred over dynamic protocols.
Dynamic protocols have default ADs: EIGRP < OSPF < RIP.
If a route is missing from the routing table, check AD first.
You can manually adjust AD to influence routing decisions.
Always verify routing decisions using show ip route.

5. Real IT Environment Scenario

In a corporate network:

Multiple routing protocols may run for redundancy.
Static routes might exist to critical servers.
AD ensures critical traffic uses the most reliable route.

For example:

Internet traffic might prefer a static default route (AD 1) over a dynamic route from BGP (AD 20) to reduce complexity.
Backup paths (OSPF or EIGRP) are only used if the primary route fails.

This ensures network stability and predictable routing behavior.

✅ Summary for the Exam

Understand what AD is and why it matters.
Know default AD values for all routing protocols.
Be able to troubleshoot when a route is missing using show ip route.
Know how to change AD manually if needed.
Verify network connectivity after adjusting routes.