How to configure Administrative Distance

When two or more sources are giving you information about a certain prefix you need to choose which information you are going to use. For example OSPF might tell you to go “left” if you want to reach network X, EIGRP might tell you that you need to go “right”. Who do you trust? OSPF or EIGRP? You can’t put both in the routing table for network X.

The administrative distance solves this problem. When two sources give us information about the exact same network we’ll have to make a decision and it’s done by looking at the administrative distance. Let me show you the different values:

Source Administrative Distance
Directly connected 0
Static route 1
EIGRP summary 5
External BGP 20
EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
ODR 160
External EIGRP 170
Internal BGP 200
Unknown 255

The lower the better…as you can see EIGRP has a lower administrative distance (90) than OSPF (110) so we will use EIGRP in my example.

Keep in mind:

  • The administrative distance is only local and can be different for each router.
  • The administrative distance can be modified.

Especially when we use redistribution we sometimes have to change the administrative distance. Let me show you how you can do this:

R1(config)#router eigrp 12      
R1(config-router)#distance eigrp 90 160

Above we have EIGRP and with the distance command I can change the administrative distance for EIGRP globally. Internal EIGRP will keep its AD of 90 but external EIGRP will have an AD of 160. You will see this change in the routing table:

R1#show ip route eigrp 
     3.0.0.0/24 is subnetted, 1 subnets
D EX    3.3.3.0 [160/1734656] via 192.168.12.2, 00:00:30, FastEthernet0/0
D EX 192.168.23.0/24 [160/1734656] via 192.168.12.2, 00:00:30,FastEthernet0/0

You can verify it by looking at the routing table, the external networks on router R1 now have an AD of 160.

We can change the AD of the other routing protocols as well, here are some examples:

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Forum Replies

  1. Thanks Rene - “AD is not advertised and is only local to the router” and “AD of 255 can be used to prevent sub-optimal routing or loops,” are both great nuggets of information! All the best for Christmas - Gareth.

  2. Hello Shivam

    To change the AD value of EIGRP use the following command:

    R1(config)#router eigrp 12      
    R1(config-router)#distance eigrp 100 160
    

    The first value (100) is the configured AD for internal EIGRP, and the second value (160) is the configured AD for external EIGRP.

    For OSPF use the following command:

    R1(config)#router ospf 1
    R1(config-router)#distance ospf external 150 inter-area 80 int
    ... Continue reading in our forum

  3. Thanku so much laz , now my point is clear .
    Rgds
    Shivam

  4. Hi Laz ,
    How r you ? As per thoroughly read the forum of related post . i Came at this point where these statement is not relate with my understanding of this topic .

    “You should be careful when configuring ADs. The ADs used should be the same across all your routers so that routing loops are not created**.”

    that Administrative Distance is a value that is valid only locally. That means that any changes you make to AD will change the way that the specific router handles routing, but will not change how other routers handle routing.

    Both statement is contradict

    ... Continue reading in our forum

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