What is Administrative distance?

A preference value for each route source, used to decide which route to install in the routing table when routes to the same destination with the same prefix length are learned from several sources, such as static routes, OSPF and RIP. Lower values are preferred. It is not used to choose between routes with different prefix lengths; that is decided by longest prefix match. The name and default values differ between equipment vendors.

Japanese name: アドミニストレーティブディスタンス

Administrative distance appears in 2 practice questions across 1 topic, including Network Design and Operations.

Field: Network Design and Operations

Try it: questions about Administrative distance

A router has learned routes to the same destination, 192.168.50.0/24, from both OSPF and RIP. On this router, the administrative distance, which expresses the preference for each route source (lower is preferred), is 110 for OSPF and 120 for RIP. Which route goes into the routing table?

ABoth routes are installed and used alternately for each packet
BOnly the RIP route is installed
COnly the OSPF route is installed

Show answer and explanation

Answer: C. Only the OSPF route is installed

Routing protocols express route quality (the metric) in different ways, such as OSPF cost and RIP hop count, so their metrics cannot be compared directly. When routes to the same destination with the same prefix length are learned from several sources, the router therefore installs the one with the lower administrative distance, a measure of how much each source is trusted. Here OSPF's 110 is lower than RIP's 120, so the OSPF route goes into the routing table. Directly connected networks commonly get 0 and static routes 1, giving lower values to what people configure, but the name and default values differ between equipment vendors.

A router's routing table holds three routes: a static route 10.0.0.0/8 (next hop router A, administrative distance 1), 10.1.0.0/16 learned by OSPF (router B, 110) and 10.1.1.0/24 learned by RIP (router C, 120). To which router is a packet for 10.1.1.5 sent?

ARouter A (the static route)
BRouter B (the OSPF route)
CRouter C (the RIP route)

Show answer and explanation

Answer: C. Router C (the RIP route)

The three routes have different prefix lengths, so each is installed in the routing table as a separate route. When forwarding a packet, the router chooses the matching route with the longest prefix and does not compare administrative distances. 10.1.1.5 matches all three, but the longest, the /24 RIP route, is chosen and the packet goes to router C. Administrative distance is used only to decide which of several routes to the same destination with the same prefix length goes into the routing table.

Administrative distance in the explanations

When routes to the same destination with the same prefix length are learned from several sources, the router therefore installs the one with the lower administrative distance, a measure of how much each source is trusted.
A router has learned routes to the same destination, 192.16…
Administrative distance is used only to decide which of several routes to the same destination with the same prefix length goes into the routing table.
A router's routing table holds three routes: a static route…

Questions that mention Administrative distance (2)

Books on Network Design and Operations Ad

Search Amazon for semiconductor Network Design and Operations

As an Amazon Associate I earn from qualifying purchases.

Comparisons that include this term

Back to the glossary