ip routing and gateway of Multilayer switch

[qaribovrauf7]

hello everybody, when we making ip routing on multilayer switches, we give to pc default gateway of vlans which we created(for example VLAN 2 and VLAN 3 on layer 2 switches for sharing  each other) but when i changed ip  default gateway on layer 3 switches interface vlan 1 or interfaces( fasteth or gig) is not working, although ip default gateway is door or devices for connecting different networks, how it works???

[M02@rt37]

Hello @qaribovrauf7,

Layer 2 switches operate at the data link layer and are responsible for forwarding Ethernet frames between connected devices. When you create VLANs on a layer 2 switch, you are essentially dividing the switch into multiple virtual switches, each of which operates as a separate broadcast domain. To enable communication between VLANs, you need to configure inter-VLAN routing on a layer 3 switch.

Layer 3 switches operate at the network layer and are responsible for forwarding IP packets between connected subnets. When you configure IP routing on a layer 3 switch, you are essentially telling the switch how to forward packets between different subnets. To enable communication between VLANs, you need to configure a virtual interface for each VLAN on the layer 3 switch.

Default gateway is the IP address of a device that enables communication between different networks. When a device wants to communicate with a device on a different network, it sends the packet to its default gateway, which then forwards the packet to the destination network. In the context of a multilayer switch, the default gateway for devices on a particular VLAN is the IP address of the virtual interface on the layer 3 switch that is connected to that VLAN.

Now, to address your specific question: When you change the IP address of the default gateway on a layer 3 switch, you are essentially changing the address that devices on a particular VLAN will use to reach devices on other networks. If the new IP address is not configured correctly or is not reachable from the devices on that VLAN, then communication will not be possible.

Additionally, if you are experiencing issues with the FastEth. or Gig. interfaces, you may need to check your configuration to ensure that the interfaces are properly configured and that they are correctly connected to the devices on the VLANs.

[Richard Burts]

There is some ambiguity in the original post about default gateway. It seems to assume that default gateway is a single thing. But for a layer 3 switch there are 2 things that are default gateways. One is the layer 2 default gateway that is provided to clients using dhcp (which is the IP of the vlan interface where the client is connected) and if there are multiple vlans then each vlan will have its own unique default gateway. The other default gateway is for the layer 3 forwarding logic of the switch.

Part of the ambiguity may relate to the command ip default-gateway. This command is used on layer 2 switches to allow the management interface of the switch to communicate with remote networks (and it has nothing to do with what default gateway the clients use). When we configure a switch as layer 3 (using the command ip routing) then the ip default-gateway command is ignored and the switch looks for a layer 3 gateway (either configuration of ip route 0.0.0.0 0.0.0.0 or configuration of a dynamic routing protocol which will advertise the default route).

The original post says "when i changed ip default gateway on layer 3 switches" I am not clear which type of default gateway this is. Perhaps the original poster can provide some clarification?

[MHM Cisco World]

ip default GW mainly used for traffic originate from SW 
you need IP route if you want to routing traffic pass through the SW 

[Flavio Miranda]

Hello

 If you have L3 switch and L2 switch the best way to create a geway to vlan is by using the protocol HSRP.

You add 2 L3 multlayer switches, create vlan 2 and 3 in both switches,  then use HSRP to create a VIP between then.

 Use command ip routing to route between vlan on the Layer3 switch

 Connect Layer2 switch on both L3 switches and pass the vlan in the trunk between them . And then create the vlan in Layer2 switch with vlan X command

That it.

ip default gateway is used in layer2 switches to send all managemt trafic to the layer3 switch.  It does not apply to user data traffic.