The no switchport command converts a Layer 2 (L2) switch port into a Layer 3 (L3) routed port on a multilayer switch. By default, ports on a Cisco switch operate at Layer 2, processing traffic based on MAC addresses and participating in VLANs. The no switchport command effectively disables this default behavior and makes the port function like a router interface, processing traffic based on IP addresses.
Detailed explanation
Key functions and characteristics
When applied to an interface on a Layer 3-capable switch, the no switchport command performs several critical functions:
- Enables L3 capabilities: The most significant effect is that it enables the port to handle Layer 3 functions. This allows you to assign a specific IP address and subnet mask to the physical interface, which is not possible on a standard L2 switch port.
- Creates a routed port: The interface ceases to be a member of any VLAN, including the default VLAN 1. Instead, it becomes a "routed port," directly connected to an IP subnet.
- Disables L2 protocols: L2-specific protocols and features, such as Dynamic Trunking Protocol (DTP) and Spanning Tree Protocol (STP), are disabled on the interface.
- Facilitates routing: Once the port is configured with an IP address, it can participate in L3 routing. If
ip routingis enabled globally on the switch, this port can function as a direct, router-like connection for a given subnet.
Use cases and scenarios
Connecting to a router or firewall
One of the most common applications of the no switchport command is creating an L3 link between a multilayer switch and another L3 device, like a router or firewall. This provides a direct, point-to-point connection for routing traffic between different networks.
Configuration example:
Switch(config)# interface gigabitEthernet 1/0/1
Switch(config-if)# no switchport
Switch(config-if)# ip address 10.10.10.1 255.255.255.252
Switch(config-if)# no shutdown
Layer 3 EtherChannels
For improved performance and redundancy, multiple routed ports can be bundled together into a Layer 3 EtherChannel (also known as a PortChannel). This is accomplished by applying the no switchport command to both the physical member interfaces and the logical port-channel interface.
Configuration example:
Switch(config)# interface range gigabitEthernet 1/0/1 - 2
Switch(config-if-range)# no switchport
Switch(config-if-range)# channel-group 1 mode active
Switch(config-if-range)# exit
Switch(config)# interface port-channel 1
Switch(config-if)# no switchport
Switch(config-if)# ip address 10.20.20.1 255.255.255.252
Switch(config-if)# no shutdown
Intra-switch routing
In some campus or data center designs, the no switchport command can be used on a switch's uplink ports to provide high-speed routing services without the need for a separate router. This is part of a larger L3 switching strategy that moves routing to the distribution or access layer, improving network performance.
Alternatives: Routed ports vs. SVIs
It's important to differentiate no switchport routed ports from Switch Virtual Interfaces (SVIs), as both can be used for Layer 3 switching.
| Feature | Routed Port (no switchport) |
Switch Virtual Interface (SVI) |
|---|---|---|
| Interface type | A physical switch port is reconfigured as an L3 interface. | A virtual L3 interface is created within a VLAN. |
| VLAN association | Not a member of any VLAN. | Directly associated with a specific VLAN. |
| Use case | Ideal for point-to-point L3 links to other routers or L3 switches, or for dedicated, L3-only connections. | Primarily for inter-VLAN routing, serving as the default gateway for devices within a specific VLAN. |
| Command | Switch(config-if)# no switchport. |
Switch(config)# interface vlan [vlan-id]. |
Potential pitfalls and considerations
- Requires a Layer 3 switch: The
no switchportcommand is only available on multilayer switches with L3 routing capabilities and the appropriate IOS software image. - Breaks L2 forwarding: Once applied, the interface no longer functions as a traditional switch port. It will not forward L2 frames to other ports in the same VLAN.
- VTP limitations: When using
no switchport, some switches internally allocate hidden VLANs to support the L3 port. This can cause issues in environments using VTP (VLAN Trunking Protocol), as it can conflict with externally configured VLANs and fail to propagate new ones.