Adjust Administrative Distance for Route Selection in Cisco IOS Routers

Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. The smaller the administrative distance value, the more reliable the protocol.

Note: When you change the default distances, it can lead to routing loops in the network. Change the administrative distance with caution and only after you have thought through what you want to achieve.

Network Diagram

Here the router R1 and R2 are connected via parallel serial lines. The routers R1 and R2 are configured with BGP and OSPF. The default administrative distance of OSPF is 110 while that of the eBGP is 20. With the command distance, we changed the AD value of BGP to 190. Prior to this command, the router R2 preferred the BGP routes over OSPF since they had the default AD values configured. After the AD value of the BGP changed, the OSPF routes takes the precedence.

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Configurations

This document uses these configuration

R1 Configuration
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 !
!
interface Loopback10
 ip address 10.10.10.10 255.255.255.255
 !
!
interface Loopback20
 ip address 20.20.20.20 255.255.255.255
 !
!
interface Loopback30
 ip address 30.30.30.30 255.255.255.255
 !
!
interface Serial1/0
 ip address 100.100.100.1 255.255.255.0
 serial restart-delay 0
 clock rate 64000
 !
!
interface Serial1/1
 ip address 192.168.12.1 255.255.255.0
 serial restart-delay 0
 clock rate 64000
 !
!
router ospf 10
 router-id 1.1.1.1
 log-adjacency-changes
 network 1.1.1.1 0.0.0.0 area 0
 network 10.10.10.10 0.0.0.0 area 0
 network 20.20.20.20 0.0.0.0 area 0
 network 100.100.100.1 0.0.0.0 area 0
!
router bgp 123
 no synchronization
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 network 10.10.10.10 mask 255.255.255.255
 network 20.20.20.20 mask 255.255.255.255
 network 30.30.30.30 mask 255.255.255.255
 neighbor 2.2.2.2 remote-as 100
 neighbor 2.2.2.2 ebgp-multihop 5
 neighbor 2.2.2.2 update-source Loopback0
 no auto-summary
!

R2 Configuration
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 !
!
interface Serial1/0
 ip address 100.100.100.2 255.255.255.0
 serial restart-delay 0
 clock rate 64000
 !
!
interface Serial1/1
 ip address 192.168.12.2 255.255.255.0
 serial restart-delay 0
 clock rate 64000
 !

!
router ospf 10
 router-id 2.2.2.2
 log-adjacency-changes
 network 2.2.2.2 0.0.0.0 area 0
 network 100.100.100.2 0.0.0.0 area 0
!
router bgp 100
 no synchronization
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 123
 neighbor 1.1.1.1 ebgp-multihop 5
 neighbor 1.1.1.1 update-source Loopback0
 distance 190 1.1.1.1 0.0.0.0 

Changed the AD value of BGP as 190!

 no auto-summary
!

On Router R2

When the distance command is not applied on Router R2

Show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       I - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, + - replicated route

Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
O        1.1.1.1 [110/65] via 100.100.100.1, 00:00:03, Serial1/0
      2.0.0.0/32 is subnetted, 1 subnets
C        2.2.2.2 is directly connected, Loopback0
      10.0.0.0/32 is subnetted, 1 subnets
B        10.10.10.10 [20/0] via 1.1.1.1, 00:00:03        

 BGP Router Preffered Over OSPF

      20.0.0.0/32 is subnetted, 1 subnets
B        20.20.20.20 [20/0] via 1.1.1.1, 00:00:03
 
 BGP Router Preffered Over OSPF

      30.0.0.0/32 is subnetted, 1 subnets
B        30.30.30.30 [20/0] via 1.1.1.1, 00:00:03
      100.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        100.100.100.0/24 is directly connected, Serial1/0
L        100.100.100.2/32 is directly connected, Serial1/0
      192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.12.0/24 is directly connected, Serial1/1
L        192.168.12.2/32 is directly connected, Serial1/1

When the distance command is applied on Router R2

Show ip route
R2#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       I - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, + - replicated route

Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
O        1.1.1.1 [110/65] via 100.100.100.1, 00:00:03, Serial1/0
      2.0.0.0/32 is subnetted, 1 subnets
C        2.2.2.2 is directly connected, Loopback0
      10.0.0.0/32 is subnetted, 1 subnets
O        10.10.10.10 [110/65] via 100.100.100.1, 00:00:03, Serial1/0
           
By increasing the AD of External BGP, OSPF takes precedence

      20.0.0.0/32 is subnetted, 1 subnets
O        20.20.20.20 [110/65] via 100.100.100.1, 00:00:03, Serial1/0
           
By increasing the AD of External BGP, OSPF takes precedence

      30.0.0.0/32 is subnetted, 1 subnets
B        30.30.30.30 [190/0] via 1.1.1.1, 00:00:03
      100.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        100.100.100.0/24 is directly connected, Serial1/0
L        100.100.100.2/32 is directly connected, Serial1/0
      192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.12.0/24 is directly connected, Serial1/1
L        192.168.12.2/32 is directly connected, Serial1/1

 

 

http://www.cisco.com/c/en/us/support/docs/ip/ip-routed-protocols/113153-adjust-ad-00.html

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