Administrative distance
Administrative distance (AD) is a crucial concept in Cisco networking that determines the trustworthiness or reliability of routing information received from different routing protocols. When a router learns about the same destination network from multiple routing sources, it uses administrative distance to decide which route to install in the routing table. Each routing protocol is assigned a default administrative distance value, with lower values indicating more trustworthy sources. Connected interfaces have an AD of 0, making them the most trusted. Static routes have an AD of 1, followed by EIGRP summary routes at 5. BGP has an AD of 20 for external routes, while EIGRP internal routes carry an AD of 90. OSPF routes have an AD of 110, IS-IS has 115, and RIP has 120. External EIGRP routes carry an AD of 170, and unknown or unbelievable routes have an AD of 255, meaning they will never be used. When multiple routing protocols advertise the same network, the router compares their administrative distances and selects the route with the lowest AD value for the routing table. For example, if both OSPF (AD 110) and RIP (AD 120) advertise a path to network 10.0.0.0, the router chooses the OSPF route because it has a lower administrative distance. Network administrators can modify administrative distance values to influence routing decisions and create backup routes. This is particularly useful in scenarios where you want to prefer one routing protocol over another or create floating static routes that only activate when the primary route fails. Understanding administrative distance is essential for troubleshooting routing issues and designing resilient networks. It helps explain why certain routes appear in the routing table while others from different sources do not. The show ip route command displays the administrative distance value in brackets alongside the metric for each route entry.
Administrative Distance: Complete CCNA Guide
Administrative Distance (AD) is a numerical value that routers use to rank the trustworthiness of routing information received from different routing protocols or sources. The lower the administrative distance, the more trustworthy the route is considered.
When a router learns about the same destination network from multiple sources (such as OSPF, EIGRP, and static routes), it uses administrative distance to determine which route to install in the routing table.
Why is Administrative Distance Important?
In real-world networks, multiple routing protocols often run simultaneously. Administrative distance provides a consistent method for:
• Route Selection - Determining which route to prefer when multiple paths exist
• Network Stability - Ensuring predictable routing behavior
• Migration Planning - Facilitating smooth transitions between routing protocols
• Backup Routes - Creating floating static routes for redundancy
Default Administrative Distance Values (Memorize These!)
• 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
• External EIGRP: 170
• Internal BGP (iBGP): 200
• Unknown/Unreachable: 255
How Administrative Distance Works
When a router receives routing updates:
1. It compares the AD of each source advertising the same destination
2. The route with the lowest AD is selected and placed in the routing table
3. Routes with higher AD values are kept as backup (if configured)
4. If AD values are equal, the metric determines the best path
Floating Static Routes
A floating static route is a static route configured with a higher AD than the primary route. It only becomes active when the primary route fails.
Example:
ip route 10.0.0.0 255.0.0.0 192.168.1.1 150
This static route has AD 150, so it will only be used if OSPF (AD 110) or EIGRP (AD 90) routes to the same destination fail.
Exam Tips: Answering Questions on Administrative Distance
Tip 1: Memorize the AD values
Focus on the most common ones: Connected (0), Static (1), EIGRP (90), OSPF (110), RIP (120). Create mnemonics if needed.
Tip 2: Remember - Lower is Better
The route with the lowest AD wins. A connected route (AD 0) will always be preferred over any routing protocol.
Tip 3: AD vs Metric Confusion
AD is used to compare routes from different sources. Metric is used to compare routes from the same source. Do not confuse these concepts.
Tip 4: Watch for Trick Questions
Questions may present scenarios where EIGRP has a worse metric than OSPF. Remember: AD is checked first, then metric. EIGRP (AD 90) beats OSPF (AD 110) regardless of metric.
Tip 5: Floating Static Route Questions
When asked about backup routes, look for static routes with AD values higher than the primary routing protocol.
Tip 6: Recognize Route Source in Output
In show ip route output:
• C = Connected
• S = Static
• D = EIGRP
• O = OSPF
• R = RIP
Tip 7: External EIGRP Trap
External EIGRP has AD 170, which is higher than RIP (120). This is a common exam trap.
Tip 8: Verification Commands
Know these commands:
• show ip route
• show ip protocols
• show ip route [destination]
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