Cisco 4000 & 3000 series ip route & ip default gateway

Question

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

Answer 1

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