Administrative distance

[marcos754]

How do I get two routing entries by default from ospf and eigrp in my routing table? What makes one protocol precede the other is its administrative distance or there is another parameter that I need to modify.

[Meheretab Mengistu]

Hi,

In the route selection process, the first thing which will be looked is the longest match. The second thing is administrative distance. If you learn the same route from OSPF and EIGRP, the route entry from EIGRP will be in your routing table as its administrative distance is smaller. If the routes are learned from the same process, then the one with the lower metric value will be selected.

Here are some results from a lab environment:-

If you learn routes from two different protocols (say EIGRP & OSPF) with the same AD, routes learned from the lower default AD (in this case, EIGRP) will be in the routing table. I believe, the tiebreaker is the default AD value.

P.S.:- Please do not forget to rate helpful solutions and solve the thread once you get the correct solution.

HTH,

Meheretab

[marcos754]

After configuring the same administrative distance to the two protocols, eigrp is the only one that appears in my routing table.

E*EX and O*E2

Those two entries put them in the same routing table.

In addition to the AD there are other parameters to configure.

[Meheretab Mengistu]

Hi Marco,
Thank you for this interesting scenario. I modified my original response as it was not correct. In order to do load balancing, the route entries need to be from the same protocol & same process.

HTH,
Meheretab

[Julio E. Moisa · Accepted solution]

Hi,

Having 2 route entries with the same subnet mask and AD, and learnt through 2 different routing protocols is not possible, it definitely was generating loops or routing problems. That is the importance of the administrative distance, the lowest AD will be preferred if both routes match with the same network characteristics. 

Now if you want to know if the route is learnt via EIGRP you can use: show ip eigrp topology. 

[marcos754]

Gracias por su aporte!!!!!

[Julio E. Moisa]

Para servirte Marcos, 

Ten un buen día  :-)

saludos.