VLAN Changes

[John Blakley]

I'm making a new VLAN, and have attached the following picture. Do I have to create a subinterface for traffic to still flow completely?

--John

[Jon Marshall]

John

If you create a new L2 vlan and assign switchports into it for those devices in that new vlan to be able to communicate with devices in other vlans you need to have a L3 interface for your new vlan.

Whether that L3 interface is a subinterface in a router-on-a-stick setup or whether it is a L3 SVI on a L3 switch doesn't really matter but you will need one.

Jon

[John Blakley]

So, I have a 3745 (router), and a 3750 (l3 switch). I *WILL* need to create a subinterface for VLAN25 on the router (router-on-a-stick), as well as assigning the address to VLAN25 on the 3750 (l3 switch)?

[Jon Marshall]

John

You don't necessarily need both. If you create a L3 SVI on the 3750 switch for the new vlan then you don't need a subinterface on the 3745 as long as routing is being exchanged between the 3745 and the 3750.

If you want to manage the 3750 using vlan 25 interface then yes you need to create on both.

If all your inter-vlan routing is happening via the 3745 at present then it may make sense to create a new subinterface for vlan 25 there but i would recommend using your 3750 for all inter-vlan routing as the 3750 would be more efficient.

However that is only a general recommendation, there may be things about your setup that make that difficult to implement.

Jon

[John Blakley]

Thanks Jon. So in theory, I should be able to add the address to VLAN25 on the 3750, and still be able to ping the 3745 from a server on VLAN25, without any change to the 3745. And people from the other side of the 3745 should be able to see the servers still.

[Richard Burts]

John

As Jon indicated it would be easier for us to answer this question if we knew more about how your environment was set up. Are you currently doing the routing for the VLANs on the 3745 (which implies a trunk from the 3750 to the 3745 and subinterfaces on the 3745) or are you routing for the VLANs on the 3750 (which implies that the link from 3750 to 3745 is an access port not a trunk and that the link from 3750 to 3745 is a separate subnet)?

Jon's answer is quite right that it is very possible to do the routing on the 3750. In that case you configure interface vlan 25 on the 3750, give it an IP address, and make sure that IP routing is enabled. If routing is enabled on the link between the 3750 and the 3745 then the servers should be able to ping the 3745 and users on the 3745 should be able to access the servers in VLAN 25. This does require that the 3745 have a route to the subnet of VLAN 25. That could either be from a routing protocol that is running or it could be from a static route.

HTH

Rick

[John Blakley]

Rick,

I've got the following:

hosts --> IPFR Cloud --> 3745 --> 3750 --> hosts

The 3745 has about 5 subnets on it's internal interface, one of which is on the same network as the new vlan I need to create on the 3750. (for instance: 10.1.2.1 is on 3745, and vlan1 has 10.1.2.2 on 3750)

Vlan1, currently, has the same subnets on it. There are five subnets on VLAN1. I want to remove the servers that are on a certain subnet to a VLAN, thus raising my original question as if I needed to create subinterfaces on the 3745.

The 3745 does NOT have subinterfaces on it; they're all on the primary internal interface. The 3750 does all of our routing (ip routing is enabled).

--John

[Jon Marshall]

John

You can create a L3 SVI on the 3750 for vlan 25. As Rick mentioned you need to make sure that the 3745 knows how to get to the new vlan 25 subnet which you can do with either a static route on the 3745 or use a routing protocol.

Jon