Two routing protocols with the same AD

I came across an interesting question the other day (though I can't recall where): what happens when two routing protocol are given the same administrative distance? Administrative distance (AD) on IOS routers is used to prefer one routing protocol over another when both protocols carry a route to the same destination. Each routing protocol supported by IOS is assigned a default administrative distance:

• Connected routes - 0
• Static routes - 1
• EIGRP Summary - 5
• External BGP - 20
• EIGRP - 90
• OSPF - 110
• IS-IS - 115
• RIP - 120
• Internal BGP - 200

For example, because 90 is lower than 120, a router will favor an EIGRP route to 172.16.0.0./16 over a RIP route to 172.16.0.0/16 (note that these routes have an identical length; more-specific routes will always be favored over less-specific routes regardless of the AD set for a protocol). Most deployments leave these administrative distances unmodified, but they can be changed for each protocol using the distance command under the appropriate process configuration.

To observe what happens when two protocols are assigned the same administrative distance, we can setup a simple lab:

R1 has two equal connections to R2: EIGRP is run over one while OSPF is run over the other. R2 advertises a route to 172.16.0.0/16 via both of these protocols. Normally, the EIGRP route would be favored as it has, by default, a lower administrative distance of 90, versus OSPF's 110. However, we can tweak the ADs of both protocols to be equal:

R1(config)# router eigrp 1
R1(config-router)# distance 100 10.0.0.2 0.0.0.0
R1(config-router)# router ospf 1
R1(config-router)# distance 100 10.0.0.6 0.0.0.0

The configurations above instruct the router to assign an AD of 100 to EIGRP routes learned from 10.0.0.2 and OSPF routes learned from 10.0.0.6. Since both routes for 172.16.0.0/16 are assigned the same AD, which will win the spot in the routing table? You might assume that the OSPF route would be chosen as it would have a lower numeric metric than the EIGRP route, which it does, but in fact the EIGRP route is favored:

R1# show ip route
Codes: 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

Gateway of last resort is not set

D    172.16.0.0/16 [100/409600] via 10.0.0.2, 00:20:11, FastEthernet0/0
 10.0.0.0/30 is subnetted, 2 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0
C       10.0.0.4 is directly connected, FastEthernet0/1

The D in front of the 172.16.0.0/16 route above shows that the EIGRP route is active in the routing table, with an administrative distance of 100. We can confirm that the OSPF route is still being received by cutting off the EIGRP neighbor:

R1(config)# interface f0/0
R1(config-if)# shutdown
R1(config-if)# ^Z
R1# show ip route
Codes: 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

Gateway of last resort is not set

O    172.16.0.0/16 [100/11] via 10.0.0.6, 00:00:16, FastEthernet0/1
 10.0.0.0/30 is subnetted, 1 subnets
C       10.0.0.4 is directly connected, FastEthernet0/1

What's interesting is that here we've seen one route (from EIGRP) favored over another (from OSPF) with the same AD and a lower metric. It seems like IOS might follow the same order of preference as outlined in the above list of ADs, but we've only pitted EIGRP against OSPF here. If anyone feels compelled to experiment with assigning other protocols the same AD please share your observations in the comments.

• R1.txt
• R2.txt