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List:       linux-vlan
Subject:    [VLAN] Was Re: FAQ? Now HOWTO!
From:       sxtaj () alaska ! edu sxtaj () alaska ! edu
Date:       1999-12-09 5:59:42
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Well, I guess I needed to acquaint myself with the terminology a bit, and
also with the software also. I did indeed get what I want working. Here's
a quicky HOWTO:

The goal:


 -------  802.1q    -------     ISL   -------    ISL   --------
| Linux |<-------->| 2924XL|<------->|  5500 |<------>|  7507  |->W
|       | VLAN12   |       | VLAN12  |       |   ISL  |        |->A
|       | VLAN42   |       | VLAN42  |       |<------>|        |->N
|       |          |       | VLAN210 |       |        |        |
 -------            -------  etc.     -------          --------
10.10.12.82         | | | |           | | | |         10.10.12.1
10.10.42.47         v v v v           v v v v         10.10.42.1
                  other hosts        other hosts      10.10.210.1
                                         &            etc.
                                      switches


I defined 2 VLANs on the 802.1q trunk between the Cisco Catalyst 2924XL
and me (the Linux machine). I could have defined more, but I didn't see
the need since I was just needed to test the software. These VLANs are
also defined on the 2924XL and Cisco Catalyst 5500 and correspond to IP
subnets routed by the Cisco 7507 via ISL (interswitch link, Cisco
proprietary?) trunks to the 5500.

On the Linux machine after patching the kernel source, configuring and
making the kernel, and then making the vconfig application, this is what I
did:

# vconfig add eth0 12
# ifconfig -i vlan0000 10.10.12.82 netmask 255.255.255.0

This then made a "sub-interface" on the existing ethernet interface. I use
the term sub-interface since I'm used to Cisco terminology and it makes
sense to me to consider this as such. The VLAN number was chosen because
its already defined on the existing LAN network consisting of the 2924XL,
5500, etc, and this VLAN is the 10.10.12.0 subnet according to the 7507's
configuration. One *huge* discovery I made (stupid error) was there *has*
to be a running eth0 configuration as seen by ifconfig. I gave this
interface a bogus (to this network) address. There might be a way to have
a physical interface up, but no IP address given, but I am unsure if this
is possible. I tend to think not.

I didn't bother giving it an ifconfig vlan0000 up since at least for me
ifconfig puts this interface in the up condition upon creation.

Then on the 2924XL I created an 802.1q trunk:

interface f0/1
switchport mode trunk
switchport trunk allowed vlan 12,42
switchport trunk encapsulation dot1q

So this got trunk up and allowing the specified VLANs to be carried on it.
Luckily the 2900XL series can do 802.1q trunking, though they do ISL by
default. Here I have to explicitely configure it to be dot1q(802.1q).

Now I wanted to create another VLAN interface on the Linux machine and
again this corresponded to an existing VLAN on the network.

# vconfig add eth0 42
# ifconfig -i vlan0001 10.10.42.47 netmask 255.255.255.224

Don't worry about the subnet mask, that's particular to our
configuration here. As in the previous part I created the sub-interface
and then ifconfig'd it to give it an IP address. 

Now since I had already configured the 2924 to allow me to trunk VLAN 42,
I didn't need to do anything more. I could go hog wild and add a million
other VLAN sub-interfaces (but in this case with how we are defining
things I'd have to explicitely add them to the catalyst trunk definitions
to be able to use them) and be able to talk directly to all VLAN/Subnets
on the LAN, but in our case that's the job of the 7507.

If you were not so lucky to have such hardware, then it should be possible
to have the Linux machine do the routing, and voila! you have a
routed-switched network.

Enjoy,

tim

--------------------------------------------------------------------------------
Timothy Jones
Network Communication Specialist
University of Alaska Information Technology Services
907-474-2414
Public PGP key @ http://radius.sons.alaska.edu/TimothyJones.asc

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