Administrative Distance of IP Routing Protocols – Cisco

Administrative Distance (AD) is an 8 bit value/number (Cisco) that defines trustworthiness of a route. AD has only local significance, and is not advertised in routing updates. Smaller the AD value of route/prefix, more preferred it is.

Below is chart of AD values of protocols with respect to Cisco.

If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be chosen and placed in the routing table.

Example: Routers R2, R3, and R4 advertise prefix 5.5.5.5/32 to router R1 through protocols OSPF, EIGRP, and RIP respectively.  Router R1, who is running three protocols (OSPF, EIGRP, and RIP) will install the route in the routing table only via EIGRP because it has least AD value (90) compared of OSPF(110) and RIPv2(120).

R1#show ip protocols summaryIndex Process Name0     connected1     static2     ospf 13     rip4     eigrp 10 R1#sh ip route 5.5.5.5Routing entry for 5.5.5.5/32

  Known via “eigrp 10”, distance 90, metric 158720, type internal

  Redistributing via eigrp 10  Last update from 10.10.13.3 on FastEthernet1/1, 00:19:00 ago  Routing Descriptor Blocks:  * 10.10.13.3, from 10.10.13.3, 00:19:00 ago, via FastEthernet1/1      Route metric is 158720, traffic share count is 1      Total delay is 5200 microseconds, minimum bandwidth is 100000 Kbit      Reliability 255/255, minimum MTU 1500 bytes      Loading 1/255, Hops 2 R1#sh ip route      5.0.0.0/32 is subnetted, 1 subnets

D        5.5.5.5 [90/158720] via 10.10.13.3, 00:27:39, FastEthernet1/1

However, on R1, if we shut interface F1/1, the best route for prefix will now be learned via OSPF. Likewise, if we shut both interfaces F1/0 and F1/1, we can see that route will be learned via RIP.

Other useful commands:

R1#sh ip protocols*** IP Routing is NSF aware ***

Routing Protocol is “ospf 1”

  Outgoing update filter list for all interfaces is not set  Incoming update filter list for all interfaces is not set  Router ID 10.10.12.1  Number of areas in this router is 1. 1 normal 0 stub 0 nssa  Maximum path: 4  Routing for Networks:  Routing on Interfaces Configured Explicitly (Area 0):    FastEthernet1/0  Routing Information Sources:

    Gateway         Distance      Last Update

    5.5.5.5                 110          00:41:29    10.10.14.4           110          00:42:47    10.10.12.2           110          00:28:38    10.10.13.3           110          00:46:26

  Distance: (default is 110)

Routing Protocol is “rip”

  Outgoing update filter list for all interfaces is not set  Incoming update filter list for all interfaces is not set  Sending updates every 30 seconds, next due in 18 seconds    Gateway         Distance      Last Update  Invalid after 180 seconds, hold down 180, flushed after 240  Redistributing: rip  Default version control: send version 2, receive version 2    Interface             Send  Recv  Triggered RIP  Key-chain    FastEthernet1/0       2     2    FastEthernet1/1       2     2    FastEthernet2/0       2     2  Automatic network summarization is not in effect  Maximum path: 4  Routing for Networks:    10.0.0.0  Routing Information Sources:    Gateway         Distance      Last Update

    10.10.14.4           120      00:00:08

  Distance: (default is 120)

Routing Protocol is “eigrp 10”

  Outgoing update filter list for all interfaces is not set  Incoming update filter list for all interfaces is not set  Default networks flagged in outgoing updates  Default networks accepted from incoming updates  Redistributing: eigrp 10    Gateway         Distance      Last Update  EIGRP-IPv4 Protocol for AS(10)    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0    NSF-aware route hold timer is 240    Router-ID: 10.10.14.1    Topology : 0 (base)      Active Timer: 3 min      Distance: internal 90 external 170      Maximum path: 4      Maximum hopcount 100      Maximum metric variance 1  Automatic Summarization: disabled  Maximum path: 4  Routing for Networks:    10.0.0.0  Routing Information Sources:    Gateway         Distance      Last Update    10.10.13.3            90      00:28:52

  Distance: internal 90 external 170

Changing the default AD value

You can modify the administrative distance of a protocol through the distance command in the router configuration mode. For example, if we want to prefer route of prefix 5.5.5.5/32 via RIP we can do that by changing the default AD value of RIP to 80 as shown below:

R1#conf tR1(config)#router ripR1(config-router)#distance 80R1(config-router)#

We can see that the route for prefix 5.5.5.5/32 is now learned via RIP whose AD value is 80 (less than EIGRP’s AD 90).

R1#sh ip route 5.5.5.5Routing entry for 5.5.5.5/32

  Known via “rip”, distance 80, metric 2

  Redistributing via rip  Last update from 10.10.14.4 on FastEthernet2/0, 00:00:27 ago  Routing Descriptor Blocks:  * 10.10.14.4, from 10.10.14.4, 00:00:27 ago, via FastEthernet2/0      Route metric is 2, traffic share count is 1

Similarly, you can change  the default AD values of other protocols too.

One of the common reasons to change the AD of a route is when you use Static Routes to backup and existing IGP route. This is normally used to bring up a backup link when the primary fails.

For example, assume there is 4th low speed ISDN link on R1 to reach R5. We can use floating static route with AD 150 (a higher AD value compared to EIGRP, OSPF, and RIP)

R1(config)#ip route 5.5.5.5 255.255.255.255.255  Dialer 1   150  

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