Cisco 4000 & 3000 series ip route & ip default gateway

[Karim Elsadek]

Dear all ;

I have Enterprise Network Contain :

- Foundry Big Iron work on layer 3 , and Combination of 4000 and 3000 and 2000 cisco switches work on layer 2;

management network for the switches is 192.168.100.0

Foundry Big Iron is 192.168.100.1

i use ip default-gateway 192.168.100.1 on all cisco switches.

all other subnets route through the foundry.

but when i ping from any subnet (192.168.15.0) to any 3000 or 2000 switches series it replay and when i ping to 4000 series it don't replay.

when i use default route command on 4000 series it replay.

i need some explanation

Kind Regards
 

[paul driver · Accepted solution]

Helo

on the 4000 disable ip routing and give it a default-gateway instead

no ip routing

ip default-gateway x.x.x.x

res

paul

[Karim Elsadek]

Hi ,

thank you for your replay, 

i will try it but i want to ask about this commands , if i disable ip route can any issues occurs through my network.

kind regards

[paul driver]

Hello

Ip routing forwards layer 3 traffic - basically acting as a router

disabling this feature makes the switch a HOST device just like any other device so now this switch needs an default gateway just like any PC does to be enable to reach remote networks  and it does this by forwarding traffic to this gateway which acts as.a proxy to outside networks

If you define an default or static  route with ip routing enabled and have ip default gateway, the default/static route will take precedence But if you disable ip routing this default /static route will be removed and the default gateway will take over

Res

paul

[Karim Elsadek]

Thank you for your explain.

I applied this procedure and every thing work well.

but i want to ask about the effect of ip route on 400 switch in spite of i use it as layer 2 switch.

[Reza Sharifi]

Hi,

Please rate and mark the post as answered so others can benefit from it.

Thanks,

[Richard Burts]

In the original post it describes the Cisco switches as operating as layer 2 switches and I assume that in terms of how they are configured to forward traffic all the Cisco switches including the 4000 are configured only for layer 2 forwarding. But with IP routing enabled (as seems to have been the case on the 4000) the behavior of the switch becomes a bit different. In particular is the difference in how you identify the default route. For a switch that is layer 2 only you use the default-gateway command (which apparently worked fine for the 2000 and 3000 switches). But when you enable IP routing then the switch does not use the default-gateway to learn its default route and would look for some other mechanism to learn its default route. I am assuming that there was not any other mechanism and so the 4000 basically did not have a default route. When you configure no ip routing (which was good advice from Paul) then the 4000 stops working as layer 2 switch and at that point will use default-gateway to determine its default route.

HTH

Rick

[Reza Sharifi · Accepted solution]

Hi,

On layer-2 device with no IP routing enabled usually all you need is the IP default-gateway command.  It maybe that the 4000 has IP routing enabled and that is why it requires default route instead of default-gateway.  You can try disabling IP routing on it and see if IP default-gateway can do the job.  Careful as this change will cause outage to your network or to the device.

HTH 

[Karim Elsadek]

Hi ,

thanks for your replay , 

you said that if i disable routing on switch it will cause outage on my network .why? And how i can solve this issue ?

also what is the difference between default gateway and default route.

kind regards

[Reza Sharifi]

Hi,

since the 4000 is a layer-2 device only,  there will be short outage (one minute or so) on the management vlan between the time you disable IP routing and enable ip default-gateway.  

HTH

[Karim Elsadek]

Hi ;

thanks for your explain

now I understood what you mean.

kind regards