IP address on physical interface vs vlan interface

[RonaldNutter]

I am migrating from a 6509 to a stack of 3750's and have run into something new.  I have an interface that has an ip address assigned to it. 

This is probably a simple question but one that I havent been able to find an answer for.  Why would you assign a ip address to an individual interface versus creating vlan, assigning the ip address to the vlan interface and then assigning a physical interface to the vlan ?

I am sure there are good reasons for this, would like to know to increase my understanding.

Ron

[phiharri]

Greetings Ron,

When you know there will only ever be a need for one physical interface in the subnet (typical example being a WAN uplink) and your switch is capable, I would turn the question around and ask why create an SVI for just one interface?

Reasons that come to mind - less overhead in terms of Layer 2 protocols - no need to run an STP instance or consider VTP implications of creating that new VLAN, more of the relevant config in one place, no worry about pruning the VLAN off of trunks (you'd likely want the subnet removed from your routing table if the physical interface went down), faster link availability (no STP convergence), better handling of /31 addressing..

I'm sure others on this forum can think of situations where one config style would be preferable to the other. 'switchport' vs 'no switchport' config should be pretty much consistent between 6500 and 3750, is there a specific issue you're facing?

Cheers,

/Phil

[RonaldNutter]

Dont have a specific issue.  I was trained to always use a SVI and hadnt seen it done another way.  What I am running up against is that there have been several people before me and everybody did things their own way.  For example, each remote office is a single vlan.  Curious thing is that each office uses a different vlan # when there is no benefit since the vlan info doesnt go past the router.

I am configuring a 3750 to replace a 6509 that that is being retasked and sent to another office.  I have configured the port the same way the 6509 equivalent is just to maintain consistentcy. 

Ron

[ebarticel]

Hi Ron,

Just to put my bit in. Sometime it is recommended to have your network segmented by vlans, for security and better bandwidth utilization. Remote offices should be on their own vlan, you don't want the main site to get all this broadcasts if you have a slower link connecting them. Also you may want to separate the receptionist from the finance ( you don't want them poking around there) and vice-versa.

Eugen

[glen.grant]

I think the general  rule would be if it's on a L3 switch and there is a possibility it would need to be on more than 1 interface then create an SVI . If you have links between routing devices then a routed port is the way to go.

[chandru_29]

Is thats all .Are there no other reasons to use a SVI vs routed port ?

[devils_advocate]

You are going from a 6500 to a 3750 stack....seems a strange way of doing things

I would always create an SVI, especially on a L3 switch which has L2 switchports available because you may need to use them in future which means downtime if you ever need to move the IP address from a Routed port to an SVI. If the address is already on an SVI and you need an additional port in the Vlan, there is no downtime as you can just do a L2 switchport.

Cheers