CBS250 meaning of "Switchport Mode" in GUI

[train00wreck]

Hey folks I'd like some clarity on what exactly the option "Switchport Mode" means in the GUI config of a CBS250. Specifically it's on the "VLAN Management -> VLAN Settings" page. The options are either Layer 2 or Layer 3, with a default of 2. The help file (unhelpfully) just says "Select either Layer 2 or Layer 3". I ask because I have recently enabled IPv4 L3 routing on the switch and added some VLAN SVIs. Everything seems to be working normally, but all of the Switchport Mode settings are "Layer 2".... I would have thought they should be Layer 3? But clearly I'm not understanding what this Mode feature actually does. Some info would be appreciated. Thanks

[Kasun Bandara]

@train00wreck hi, if you set switch port as Layer2, you can assign VLAN and make it access or trunk. if its Layer3, you can assign IP address to the port and use for routing. same line like SVI, this port will participate for routing but directly IP assigned to port.

[train00wreck]

Sorry, but what do you mean by "same line SVI, this port will participate for routing but directly IP assigned to port."? I think there may be a language barrier here.

[Kasun Bandara]

@pieterh hi, sorry my bad. its 'same like" 

[Jens Albrecht]

Hello @train00wreck,

let's have a look at both options:

Layer 2 port

This is the default setting for all ports on these switches.

The port switches incoming frames based on the destination MAC addresses.
All handling is done based on the Layer 2 information (MAC addresses) and therefore this setting is called a Layer 2 port.
The port learns the MAC addresses of connected devices and you can configure it as a trunk port to handle multiple Vlans.
So Layer 2 ports are used to connect to other switches using trunks and to connect end devices using access ports.

Layer 3 port

This port routes incoming packets based on the destination IP addresses.

The packets are handled based on the Layer 3 information (IP addresses) and hence this setting is called a Layer 3 port.
This port cannot be used to handle multiple Vlans and does not participate in spanning-tree.
So this port behaves like the port on a physical router and you can assign an IP address directly to this port.
It can be used to connect to a router if you terminate all your vlans on the switch using SVIs.

In a typical setup you connect all your switches using Layer 2 ports that are configured as trunks to transport multiple vlans. The routing between your vlans can be done on the switches using SVIs just as you did in your setup. Then you could use a Layer 3 port to connect to a router that is used as a gateway for internet access.

Hope this clarifies your doubts.

[pieterh]

as addition to @Jens Albrecht and @Kasun Bandara contributions

in the old days you had dedicated routers (L3) and dedicated switches (L2).
at present there are switches with router functionality (L3 switches)
the command disables the behavior of this port as switchport, and you enable the behavior as router port
each port has its own (dis)advantages and features and used for different purposes.

in other Cisco IOS devices this corresponds to the "no switchport" interface configuration command
a "no switchport" interface does NOT process VTP, BPDU packets etc. these are L2 bound
and (unless you create subinterfaces) also does not process dot1q vlan tagged packets -> only L3 packets (tcp/ip)

[Jens Albrecht]

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@pieterh wrote:

... and (unless you create subinterfaces) also does not process dot1q vlan tagged packets -> only L3 packets (tcp/ip)

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Quick note:

In general switches do not allow the configuration of subinterfaces if you configure an interface as a Layer 3 port.

If you need to run various subnets over a single link on a switch you have to configure the interface as a Layer 2 trunk port.
The configuration of subinterfaces aka the so-called router-on-a-stick is typically used on routers and firewalls.

[train00wreck]

Wow OK thanks for all the replies, it's a bit to take in. I definitely did notice that changing a port to Layer 3 seemed to disable trunking on that port. I guess I have 2 questions at this point then:

-What is the actual advantage of changing it to a Layer 3 port? Less resource usage on the switch, since it isn't processing L2 traffic (LLDP, spanning tree, etc.)

-WHERE on earth is this documented??

thanks!