[c-nsp] Using switchport 802.1q for a point-to-point instead of routed /30

Rick Kunkel kunkel at w-link.net
Mon Feb 8 14:08:26 EST 2010

Hello all...

Right now, I've got a bunch of customers connected to a bunch of 
switchports using different VLANs.  I've got 802.1q running between the 
switches, and then a router attached with a bunch of subinterfaces, one 
for each VLAN.  Assigned to each of these subinterfaces is the customer's 
gateway IP address.

So, for instance, have something like this for the customer port

interface FastEthernet 1/12
switchport access vlan 80

Then the switch is connected to a router, with an interface like this

interface GigabitEthernet 0/1.80
encapsulation dot1Q 80
ip address X.X.X.2

Pretty standard stuff....

So, now, we're opening another location, and we've got some customers 
interested in having some equipment in the first location and some in the 
second, and having it all be part of the same network.

The connection between the two location is ethnernet, and the hardware is 
(well, will be as soon as we upgrade out of a 7200) a 6509 on either side, 
and I think it'd be pretty cool to run an 802.1q trunk between them using 
6509 switchports instead of routed ports.  However, I've got some 
problems, or at least I'm having trouble wrapping my brain around some 

1. In the interests of keeping things simple, is it a "bad" idea to use an 
802.1q trunk for backbone connectivity?

2. I'd normally set up this kind of point-to-point link using a /30, using 
interfaces in "routed" mode, and assigning the addresses to the interfaces 
on each end of the link.  If using and 802.1q trunk with interafaces in 
"switchport" mode, would it be advisable to use loopback interfaces for 
these addresses instead?

3.  I'm used to having the customer's gateway set on that Gigabit 
subinterface, as above.  But if I want this customer to have their stuff 
on the same VLAN in both locations, AFAIK, I should set switchport access 
VLAN 80 on both their access ports.  I'm then stuck figuring out where to 
put the gateway address for their IP space.  Again, would loopback 
interfaces be good candidates for this?  Or perhaps a VLAN interface, as 
weird as that seems to me?

4.  My motivation for doing any of this in the first place, as opposed to 
a simple /30 point-to-point interface, is to allow customers to have 
access to layer 2 across our network, whether it be for internal use or 
for purchasing third-party connectivity.  Is it "acceptable" to use our 
single point-to-point ethernet for this, or should I be using a separate 
network for this entirely?



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