[c-nsp] Concept of VLANs and Trunks

[Joost greene]

Hi,

I am writing this after reading alot about VLANs in different books, i
always get confused when people speak about it and the different ways its
configured along with trunking and changing the interface type, i'll list my
confusion points and i hope someone can shed some light

Ports on switches has different types, access or trunk

Access for PCs or routers (no vlans transferred)

Trunk ports are uplinks between switches and they are made to move vlan
information  between switches, so if Switch-A has port 2 in vlan 5 and port
2 on switch-b is in vlan5 too, automatically they can speak to each other if
port 3 on both switches is a trunk port, all i need to do is configure trunk
interfaces on both switches.

Now lets say that on a new router or switch you configure subinterfaces and
vlan tagging, do we do this because we are making sure if we will need vlans
in the future we dont need a service outage ? i mean i see configurations on
devices with subinterfaces and vlan tagging while there is no vlans
configured ?

The two routers below are directly connected with a cross-over cable,
Router-A is the PE of this PoP, which happends to be colocated with Router-B
and the data center, B doesnt speak any MPLS or carry VRF info, but B is
connected to data-center switches which of course has VLAN info but i dont
think there is a reason to carry them to the router even.



Router-B: Internet Gateway of a data center, co-located with Router-A
(Below)

!
interface FastEthernet2/1.5
 description ***Link"dial" VRF ****
 encapsulation isl 5
 ip address 172.31.255.254 255.255.255.252

!
interface FastEthernet2/1.6
 description ***Subinterface for Telindus Management***
 encapsulation isl 6
 ip address 172.17.255.254 255.255.255.252
 no ip redirects
 no cdp enable
!

Router-A (PE of this PoP) colocated with Router-B:

interface FastEthernet2/1.5
 description *******LINK FOR "dial" VRF *******
 encapsulation isl 5
 ip vrf forwarding dial
 ip address 172.31.255.253 255.255.255.252
 no ip redirects


So why is it configured with isl tagging, and i see engineers here configure
it like that anytime they need to add a new interface, there wont be routing
tables problems i understand.


Another case is when someone is  configuring 6500 with GigE module and says
that for vlan tagging, they cant configure subinterfaces, and that .1q tag
must be configured differently from normal switches ? please explain

Thanks in advance