Cisco and Linux -- Vlans

Published on March 16 2014 by dellpe

I appear to have some fundamental misunderstanding of how VLANs work on Linux, and I'm hoping the good people here can educate me.

Cast: One Cisco WS-C3560X-48T-L, one VLAN, and one Linux box [1].

Cisco  ---------------  Linux
    ge0/1           eth0

The Cisco has a Vlan 37 interface, with IP address 10.40.37.252/24. I want to place 10.40.37.1/24 on the Linux box.

When the Cisco de-encapsulates vlan 37, everything works fine [2]:

# Cisco 
interface Vlan37
    ip address 10.40.37.252/24

interface GigabitEthernet 0/1
    switchport mode access
    switchport access vlan 37

# Linux
ip link set eth0 up
ip addr add 10.40.37.1/24 dev eth0

$ ping 10.40.37.252 && echo It works

However, when I set the port to trunking and assign vlan 37 on the Linux side, it stops working:

# Cisco
interface GigabitEthernet 0/1
    switchport trunk encapsulation dot1q
    switchport mode trunk
    ! [3] [4] [7]

# Linux
vconfig add eth0 37
ip link set eth0.37 up
ifconfig eth0 0.0.0.0 up # ensure no address
ip addr add 10.40.37.1/24 dev eth0.37

$ ping 10.40.37.252 || echo Why does this not work

What am I missing here?

Edit: Solutions:

Shane's question about the mac address table led me to a solution: Use "ip addr" to set different unique L2 (MAC) addresses on each of the VLAN sub-interfaces, and it suddenly works.

Another possible solution that I didn't try (because my hardware is too old) is using "ethtool" to disable VLAN offloading by the NIC itself, and forcing the kernel to deal with the tags.

Thank you Shane!

Edit: More info as per comments:

The overall goal is to have three vlans (public, private, oam&p) terminating on three individual IP addresses on the linux box, with different applications binding to the local addresses. I can expand further if necessary, but I'm trying to keep the problem description and discussion simple, since before I can have three vlans working, I kind of need one to be working. :)

Antoine --> ifup versus ifconfig makes no difference.

Pepoluan --> I'm assuming this is what you were looking for. Note the lack of references by phy drivers is apparently normal. [5]

$ lsmod | grep 802
    8021q   25545 1 cxgb3

Handyman -->

$ ifconfig eth0
    eth0  Link encap: Ethernet HWaddr 00:17:08:92:87:22
    UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
    RX packets:0 [...]
    TX packets:31932 errors:0 dropped:0 overruns:0 carrier:0 

$ ifconfig eth0.37
    eth0.37 Link encap: Ethernet HWaddr 00:17:08:92:87:22
    UP BROADCAST RUNNING MULTICAST MUT:1500 Metric:1
    RX packets: 0 [...]
    TX packets:32024 errors:90 dropped:0 overruns:0 carrier:0

$ cat /proc/net/vlan/config
    VLAN Dev Name | VLAN ID
    Name-Type: VLAN_NAME_TYPE_RAW_PLUS_VID_NO_PAD
    eth0.37 | 37 | eth0

Chuck --> wireshark and/or tcpdump do not show the tags, but this is apparently a normal limitation on Linux, due to the processing order of vlan handling and pcap in the kernel [6]. Also, the untagged VLAN is set to 1 [7].

[1] I've tried this with both CentOS 5.5 and Ubuntu 11.04, and both have the same issue.

[2] Note the configs are not a cut&paste, so any typos here are simply my bad memory.

[3] "nonegotiate" on or off has no effect on the problem.

[4] Vlan 37 is shown as active & non-pruned on the link, so "allowed" is not the problem.

[5] serverfault: Enabling 8021q on a nic

[6] http://wiki.wireshark.org/CaptureSetup/VLAN#Linux

[7] The native (untagged) VLAN is 1. Manually setting it with "switchport trunk native vlan 1" has no effect.