2 routers 1 subnet across a large network?

All,
I was wondering if I could pick your brains. I am not a l3 guy by any means, but if I have 2 different networks, 2 diff routers all draining out the same place, and I place a /24 on both routers, when I trace to an ip on that subnet sometimes it routers to router a, then sometimes it routes to router b. Is there a way to fix this without changing the subnet, or do I need to add a separate subnet? Thanks a ton!

Routers are used to define subnets. Having two on the same subnet is not good practice, unless you're not using the uplink port on one and using it as a wireless gateway or switch.

Create another subnet or use one as a switch (if capable).

There should be no way for it to change unless every computer is connected to both networks, in which case why are there two networks in the first place?

Lets throw the 3rd router in. This router is on a different subnet, but when I trace from this router to router a, sometimes it goes to router b. so lets say I have .1, .2, .3 on switches behind router a, then .4,.5,.6 on router b same subnet. Then I trace to .2 which has a gw of .1. .1 being router a, and .4 being router b.

So trace .2, it goes hop, hop (these routers are 100 miles apart separated by a l3 network). then dies at .4. Instead of routing to .1. Then sometimes it works just fine....

I am curious if when I trace the routers in between send it along its way, but since 2 routers know about the whole subnet, it gets confused and sends it to one router, which drops the packet, then the next time, it sends it to the correct router which in turn sends it to the correct switch...

Am I way off on this?

I am lost as to what exactly you have setup.
Could you diagram for us ? Maybe that would help.

router a has 172.29.1.1/24 it is located in jville off of this router are .2 which is sw1, .3 sw2, .4 sw3

router b has 172.29.1.20/24 it is located 100 miles away in btown and has .21 sw1, .22, sw2.

router c is another router on this network but not on this subnet. So that I can ping to the private addresses on different networks.

So what happened is this....
Router b had its own network 172.29.100.1/29, then someone added 172.29.1.20/24 to that interface.

So now from anywhere in this which spans hundreds of devices, when I try to trace .1, sometimes it goes to router a, but sometimes it goes to router b.

Dougan said:

All,
I was wondering if I could pick your brains. I am not a l3 guy by any means, but if I have 2 different networks, 2 diff routers all draining out the same place, and I place a /24 on both routers, when I trace to an ip on that subnet sometimes it routers to router a, then sometimes it routes to router b. Is there a way to fix this without changing the subnet, or do I need to add a separate subnet? Thanks a ton!

Click to expand...

That is the nature of TCP/IP, it chooses the best route for given conditions. Tracert to a location on the interw3b and you will see the same thing happening on a broader hop scale.

So because the router is advertising the same subnets people will trace one way one time, and another way another time..?

It's not because of the subnet, it's related to the response times at each 'hop' along the route. TCP/IP chooses the fastest route...as it's designed to do so there is nothing to fix. As long as the packets get to the destination who cares?

Thats the problem, we have monitoring setup and it is set to accept alarms from router a, so when it sends requests for that subnet, sometimes router a responds, sometimes it does not.

I think it has been solved... Thank you all for your help!

Man... I just drew up a diagram to see if what you were talking about was right...haha



What did you do to resolve the issue?

sounds like a routing issue to me

are you guys using rip / rip 2 or ospf?