Layer 3 device connected to a port on a switch connected upstream as layer 2?

[CiscoPurpleBelt]

So if you have a multi-layer switch which is connected via layer 2 switchport to another switch heading to main campus and/or out to core network devices, can you install another device let's say router or FW on a port configured for Layer 3 and traffic will be able to communicate fine as long as the layer 3 IP on the device and "NO Switchport" are in same subnet?

[balaji.bandi · Accepted solution]

If i understand the question correctly. you looking to connect L3 device  like below :

L2 ------L2(L3 port)---L3 port point to point ?

[CiscoPurpleBelt]

Yes basically. So its DATA CENTER L2 port-----L2 port on Sw1-----L3 port on Sw1 (new device). Should work without real issues as long as in same subnet correct?

[balaji.bandi]

i do not see as issue on high level, end switch treat as end point with IP address.

[CiscoPurpleBelt]

End switch meaning the far right switch the layer 3 device is connected too? Yes it has an SVI and default gateway pointing towards Data Center.

[balaji.bandi]

If far right side switch has SVI, then you need to do trunking between the switch.

if the far end switch configured as no switchport treat as end device.

[CiscoPurpleBelt]

Yes far right device has SVI. You mean trunking between Sw1 and the switch to its left? I think I would rather just make layer 3 port with IP on Sw1 and give the FW/router an IP (all in same subnet as SVI) and connect it to that layer 3 port. This is what you mean correct?

[balaji.bandi · Accepted solution]

You can do both the ways.

make a trunk between switches, so you can have SVI to communicate with devices.

or if you require to be L3 

leave the left SW on l2 configured that port as access port -----configure L3 connected interface with no switch port and IP address of that VLAN.

To make more clarity make a small diagram of what you intend to do with the information.

[CiscoPurpleBelt]

Ok thanks!

Yes so plan is to do as such meaning no need to have to make trunks. See attached.

[balaji.bandi · Accepted solution]

yes the solution is possible as per your diagram.

[CiscoPurpleBelt]

Is it better to just configure the port that the layer 3 device (FW or router) is connecting to as a switchport VLAN or configure as Layer 3 with "no switchport"?

[Deepak Kumar · Accepted solution]

Hi,

Yes, It will work without any issue of there is no configuration or software glitch.

[CiscoPurpleBelt]

Is it better to configure the port that the layer 3 device (FW or router) is connecting to as a switchport VLAN or configure as Layer 3 with "no switchport"?

[Joseph W. Doherty]

If I understand your question, and diagram, no it wont work.

Two L3 end devices can intercommunicate across multiple L2 links, but once you make a L3 switch port, it will only intercommunicate with L2 links if they have a SVI defined on the same L3 switch (routing between networks).

e.g.

host x.1 <L2> SW <L2> SW <L2> x.2 host [okay]
host x.1 <L2> SW <L2> {SVI} x.2 SW y.1 <L3> y.2 host [okay]

[CiscoPurpleBelt]

Yes, so on the far right end switch (where the layer 3 FW or Router will connect) there is an SVI for that single subnet on that switch. So you saying L3 port on that switch port and l3 port on the FW/RT it should still work fine correct? Is it better to just leave the far right switch port as layer 2 (still have the interface of the Rt/FW connecting to that as layer 3)?