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 di…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. The route with the lowest administrative distance is considered the most reliable and will be preferred. Each routing protocol and routing source has a default administrative distance value assigned by Cisco. Static routes configured by an administrator have an AD of 1, indicating high trustworthiness since they are manually configured. EIGRP internal routes have an AD of 90, OSPF routes have an AD of 110, IS-IS routes have an AD of 115, and RIP routes have an AD of 120. External EIGRP routes have an AD of 170, and eBGP routes have an AD of 20. Connected interfaces have an AD of 0, representing the most trusted routing information since the router knows these networks exist because they are attached to its own interfaces. Understanding administrative distance becomes essential when implementing route redistribution between different routing protocols. For example, if a router receives information about network 10.0.0.0 from both OSPF (AD 110) and RIP (AD 120), it will prefer the OSPF route because 110 is lower than 120. Network administrators can modify default administrative distance values when needed to influence routing decisions. This is particularly useful in scenarios where you want to create backup routes or implement specific traffic engineering policies. The floating static route concept utilizes this by configuring a static route with a higher AD than the primary dynamic routing protocol, ensuring it only becomes active when the primary route fails. Administrative distance operates locally on each router and is not advertised to other routers in the network.
Administrative Distance and Forwarding Decisions
Why Administrative Distance is Important
Administrative Distance (AD) is a critical concept in networking because routers often learn about the same destination network through multiple routing protocols or sources. The router needs a reliable method to determine which route to install in the routing table when multiple paths exist. AD provides this decision-making framework, ensuring predictable and consistent routing behavior across your network.
What is Administrative Distance?
Administrative Distance is a numerical value ranging from 0 to 255 that indicates the trustworthiness or reliability of a routing information source. The lower the AD value, the more trustworthy the source is considered. When a router receives routing information about the same destination from different sources, it will prefer the route with the lowest AD.
Default Administrative Distance Values
You must memorize these key AD values for the CCNA exam:
When a router needs to forward a packet, it follows this process:
1. The router performs a longest prefix match lookup in the routing table 2. If multiple routes to the same destination exist with different prefix lengths, the most specific (longest) prefix wins 3. If multiple routes exist with the same prefix length but from different routing sources, the route with the lowest AD is installed in the routing table 4. Routes with higher AD values are kept as backup routes
Example Scenario
If a router learns about network 10.1.1.0/24 through: • OSPF (AD 110) • RIP (AD 120) • Static route (AD 1)
The static route will be installed in the routing table because it has the lowest AD value of 1.
Modifying Administrative Distance
Administrators can change AD values to influence route selection. This is useful when you want a specific routing protocol to be preferred over another in certain situations. For static routes, you can configure a floating static route by setting a higher AD value, which serves as a backup when the primary route fails.
Exam Tips: Answering Questions on Administrative Distance
Tip 1: Memorize the Common AD Values Focus on Connected (0), Static (1), EIGRP (90), OSPF (110), and RIP (120). These appear most frequently on the exam.
Tip 2: Remember Lower is Better The route with the lowest AD value will always be preferred. Connected routes at AD 0 are the most trusted.
Tip 3: Understand Floating Static Routes Questions often ask about backup routes. A floating static route has a manually configured higher AD than the primary routing protocol.
Tip 4: AD vs Metric Confusion AD is used to choose between different routing sources. Metric is used to choose between routes learned from the same routing protocol. Do not confuse these concepts.
Tip 5: Watch for Trick Questions An AD of 255 means the route is considered unreachable and will never be installed in the routing table.
Tip 6: Verify Command Knowledge Know that show ip route displays the AD in brackets, formatted as [AD/Metric]. For example, [110/20] means AD of 110 and metric of 20.