Adding 802.1Q Trunking to Debian GNU/Linux

Here's a quick guide to setting up 802.1Q trunking for VLANs on a Debian GNU/Linux box connected to one or more Cisco Catalyst switches, which could then be used as a cheap router replacement.

Configuration on the Debian box:

Add 8021q to /etc/modules so 802.1Q support is enabled at startup. To install it immediately:

# modprobe 8021q

Use vconfig to add the VLANs to the interface you'll be using (if vconfig is missing, run apt-get install vlan):

# vconfig add eth0 2

(In the above, eth0 is the physical interface and 2 is the ID of the VLAN)

Give the interface an IP. Choose an address in the range you've set aside for that particular VLAN. In this example, VLAN 2 is using

# ifconfig eth0.2 netmask


To have this happen on startup, do the following:
Edit /etc/network/interfaces to assign permanent settings to your new interface(s) – the virtual interface takes its name from the interface to which it is attached plus the VLAN ID: for example, setting up VLAN 2 via interface eth0 results in an interface called eth0.2.

auto eth0.2
iface eth0.2 inet static
vlan_raw_device eth0

Whichever method is used, check the VLAN interface has come up correctly:

# ifconfig eth0.2
eth0.2 Link encap:Ethernet HWaddr 00:c0:9f:9a:45:dc
inet addr: Bcast: Mask:
inet6 addr: fe80::2c0:9fff:fe9a:45dc/64 Scope:Link
RX packets:278 errors:0 dropped:0 overruns:0 frame:0
TX packets:293 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:29924 (29.2 KiB) TX bytes:34873 (34.0 KiB)

It's also important to make sure the trunking interface (eth0 in the above) has no IP assigned (or at least not one in any of the VLANs you've configured). To clear any address currently assigned, enter:

# ifconfig eth0

Configuration on the switch (C3500XL in this case):

First up, create at least one VLAN apart from the default VLAN1:
(NB: The process for creating VLANs differs depending on the model of switch used)
#vlan database
(vlan)#vlan 2
(vlan)#vlan 2 name testing

Set the switch to be a VTP server and name the VTP domain:

(vlan)#vtp server
(vlan)#vtp domain testing

Then, enable trunking on a port (24 in this example) and give all VLANs access to it:

(config)# interface fa0/24
(config)# switchport trunk encapsulation dot1q
(config)# switchport mode trunk
(config)# switchport trunk access vlan all

Check the port's configuration (important sections in bold):

#sh int fa0/24 switchport

Name: Fa0/24
Switchport: Enabled
Administrative mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1,2
Pruning VLANs Enabled: 2-1001

Priority for untagged frames: 0
Override vlan tag priority: FALSE
Voice VLAN: none
Appliance trust: none

Add some ports to the VLAN (repeat the below for each port, or use (for example) int range fa0/7 – 10 if supported by your switch:

(config)# int fa0/7
(config-int)# switchport access vlan 2

Check that the ports have been assigned correctly:

switch1#sh vl br
VLAN Name             Status    Ports
---- ---------------- --------- ---------------------------
1    default          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4,
                                Fa0/5, Fa0/8, Fa0/9, Fa0/12,
                                Fa0/13, Fa0/14, Fa0/15, Fa0/16,
                                Fa0/17, Fa0/18, Fa0/19, Fa0/20,
                                Fa0/21, Fa0/22, Fa0/23, Gi0/1,
2    testing          active    Fa0/6, Fa0/7, Fa0/10, Fa0/11

Connect the trunking port to the PC and hook up another device to one of the VLAN 2 ports.

On the switch, check the status of the interfaces (cropped; only relevant interfaces shown):

switch1#sh int status

Port    Name  Status       Vlan     Duplex Speed   Type
------- ----- ------------ -------- ------ ------- ----

Fa0/6         notconnect   2          Auto    Auto 100BaseTX/FX
Fa0/7         connected    2        A-Full   A-100 100BaseTX/FX
Fa0/10        notconnect   2          Auto    Auto 100BaseTX/FX
Fa0/11        notconnect   2          Auto    Auto 100BaseTX/FX
Fa0/24        connected    trunk    A-Full   A-100 100BaseTX/FX

They should be now able to ping each other, as below:

From the Client PC (connected to port 7):
# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_req=1 ttl=64 time=0.244 ms
64 bytes from icmp_req=2 ttl=64 time=0.264 ms
64 bytes from icmp_req=3 ttl=64 time=0.251 ms
64 bytes from icmp_req=4 ttl=64 time=0.238 ms
64 bytes from icmp_req=5 ttl=64 time=0.244 ms

From the Trunking PC (connected to port 24):
# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=0.153 ms
64 bytes from icmp_seq=2 ttl=64 time=0.196 ms
64 bytes from icmp_seq=3 ttl=64 time=0.192 ms

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