VLAN(4) Kernel Interfaces Manual VLAN(4) NAME vlan -- IEEE 802.1Q VLAN network interface SYNOPSIS pseudo-device vlan [count] DESCRIPTION The vlan driver demultiplexes frames tagged according to the IEEE 802.1Q standard into logical vlan network interfaces, which allows routing/bridg- ing between multiple VLANs through a single switch trunk port. To function, a vlan interface must be assigned a parent interface and nu- meric VLAN tag using ifconfig(8). A single parent can be assigned to mul- tiple vlan interfaces provided they have different tags. The parent inter- face is likely to be an ethernet card connected to a properly configured switch port. The VLAN tag should match one of those set up in the switched network. The vlan driver supports physical devices that do the VLAN demultiplexing in firmware. The link0 flag should be set on a vlan interface (not on its parent) using ifconfig(8) in that case to indicate that the hardware sup- port for the 802.1Q VLANs is present in its parent. Selecting the Right Network Interface Card to Run VLANs Through By now, the only NICs that have both hardware support and proper driver hooks for the 802.1Q VLAN technology in FreeBSD are bge(4), nge(4), ti(4), and txp(4). The rest of the ethernet NICs supported by FreeBSD can run VLANs using the software emulation in the vlan driver. However, most of them lack the ca- pability of transmitting and/or receiving oversized frames. Using such a NIC as a parent interface for a vlan implies a reduced MTU on the corre- sponding vlan interfaces. In the modern Internet, this is likely to cause tcp(4) connectivity problems due to massive, inadequate icmp(4) filtering that breaks the Path MTU Discovery mechanism. The NICs that support oversized frames are as follows: de(4) requires defining BIG_PACKET in the /usr/src/sys/pci/if_de.c source file and rebuilding the kernel. The hack works only for the 21041, 21140, and 21140A chips. fxp(4) supports long frames for the vlan natively. sis(4) supports long frames for the vlan natively. tl(4) does support long frames. tx(4) does support long frames. wx(4) does support long frames. xl(4) supports long frames only if the card is built on a newer chip (Cyclone and above). Note: Except for fxp(4), sis(4) and tx(4), none of the above drivers inform the vlan driver about their support for long frames. Just fix the MTU of a vlan interface if it appears to be lower than 1500 bytes after attaching the vlan to a parent interface that is known to support long frames. SEE ALSO ifconfig(8) BUGS No 802.1Q features except VLAN tagging are implemented. FreeBSD 4.5 July 25, 2001 VLAN(4)
NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | BUGS
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