Administrative Distance
If you find yourself forgetting the default administrative distance values after you’ve completed an exam, this post is for you.
When routers learn of a route from more than one source, they need a way to pick the “best” one. Administrative Distance is used when such a situation arises. Lower administrative distances are considered more reliable than higher AD’s. The following table is a quick reference for those studying for exams and need to memorize the default administrative distances on Cisco gear:
| Source | Default Distance | Notes |
|---|---|---|
| Connected Interface | 0 | |
| Static Route | 1 | Can also act like a connected route when pointing to an interface. |
| EIGRP Summary | 5 | |
| External BGP | 20 | |
| Internal EIGRP | 90 | |
| IGRP | 100 | |
| OSPF | 110 | |
| IS-IS | 115 | |
| RIP | 120 | |
| EGP | 140 | |
| On Demand Routing (ODR) | 160 | |
| External EIGRP | 170 | For example, routes redistributed into EIGRP |
| Internal BGP | 200 | |
| Unknown | 255 | Unknown is when the router does not believe the source of the route, so the route will not be installed. |
In certain situations, you may need to change an AD from its default value. You can use the “distance” command in the routing protocol to change the administrative distance to whatever you want within the allowable range:
R1(config-router)#distance ? <1-255> Administrative distance
Also, when using static routes, you can modify the
administrative distance by specifying it at the end of the individual static route statement:
R1(config)#ip route 192.168.100.0 255.255.255.0 10.37.1.1 199
Comments
Perfect timing .. as i was
Perfect timing .. as i was about to google for such a table!
Thanks!
Marcin