Purpose of primary and secondary interfaces on 4900 vlans

[Liesl]

Hey all,
i have read all the relevant threads on vlan setup on 3Com 4900s and have found them quite helpful.

My question is what is the purpose for setting up primarry and secondary interfaces on the same vlan? I came to this company to meet one vlan setup (other than vlan1) with 3 interfaces, one primary and two secondary i.e. 192.9.200.1, 192.9.201.1 and 192.9.202.1

What is the reason for doing this instead of setting up three distinct vlans? All ports are assigned to this vlan.

One last question is if I wanted to add another 4900 to the network and incorporate the subnet 192.9.203.0, would I be able to define the 192.9.200.0 subnet again on this second 4900 or would there be a conflict between the two?

Thanks

 

[Liesl]

Well I guess no one knows the purpose of secondary adn primary interfaces huh? I still have no further info on this myself.

With regards to the other question...yes it is possible and there is no conflict.

 

[SheffieldDave]

The purpose of the secondary interface:

We have technical teams which are situated in waggons. (Track system building, overhead contact line building, signal technicans)
These teams have a lot workstations, servers and measuring instruments with fixed IP-Adresses on board.(For example 10.100.100.x)
They move from station Ato station B.
As soon as they arrive in station B, I add a secondary interface with 10.100.100.x on the VLAN which connects to the waggon and they have access to the whole network.

Otherwise I had to create a new VLAN on the router and setup this VLAN on every switch on the route to the waggon.
And that's a lot of work compared to simply adding a second interface on a VLAN.

Sheffield

 

[Liesl]

That's really interesting Sheffield but why dont you just add a second VLAN on the 4900. You wont have to define it on any other switches or any other routers either...that is what I have done. Wont that be the same thing as adding a second interface on an existing vlan? That is where my confusion lies...

 

[SheffieldDave]

It would be easy to create a new VLAN if the waggon directly connects to the SSIII 4900.
But in my case the waggon is far away from the SSIII 4900 and every packet hops trough several switches.
Example:
SSIII 4900
Switch 1
Switch 2
Switch 3
Switch 4
Switch 5
Switch 6
Waggon (Workstation)
If I'd choose to create a own VLAN, I have to do this on the SSIII 4900, switch 1, switch 2, switch 3, switch 4, switch 5 and untagg the new VLAN on switch 6.

But when I create a second interface on an existing VLAN, I only have to do this on the SSIII 4900.
That's all!
There is no need to setup a new VLAN on switch 1, switch 2,....
And that spares a lot of time.

 

[Liesl]

I see. My workstations hop through several switches too but I suspect the difference is that I use my 4900 as the gateway....so I dont have to define the vlans on the other switches (neither the other 4900s or the 4400s)....do you use your 4900 as a gateway?

 

[SheffieldDave]

I don't use the SSIII 4900 for routing because I also have to route DEC and have to provide QOS on some VLAN's.
And so all routing actvities were executed trough 3COM Netbuilder's and CISCO routers.
On these devices you can also specify a second interface on a VLAN.

I use the SSIII 4900 switches for simple layer 2 switching.