On Thu, Aug 21, 2008 at 09:50:35AM +0000, Stuart Henderson wrote:
| > you can stack vlans. aka
| >
| > ifconfig vlan2 vlan 2 vlandev em0
| > ifconfig vlan4 vlan 4 vlandev vlan2
| >
| 
| I think you have to take a hit on MTU, so this is probably better
| on gigabit interfaces that are configured for jumbo frames (using
| ifconfig to increase MTU configures them for jumbos).
| 
| Cisco does something similar (qinq) but iirc they use a different
| ethertype, so it may not be directly compatible.

The standard specifies 0x88a8 for 'qinq' or 'stacked vlans' or
802.1ad (Provider Bridges). OpenBSD uses 0x8100 for both the outer and
the inner ethertype (which may give interoperability issues with other
vendors, don't know, don't have gear that supports it).

Attached is a patch that adds ETHERTYPE_8021AD to ethertypes.h.

Cheers,

Paul 'WEiRD' de Weerd

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Index: ethertypes.h
===================================================================
RCS file: /cvs/src/sys/net/ethertypes.h,v
retrieving revision 1.9
diff -u -r1.9 ethertypes.h
--- ethertypes.h	5 May 2008 13:40:17 -0000	1.9
+++ ethertypes.h	21 Aug 2008 11:22:02 -0000
@@ -300,6 +300,7 @@
 #define	ETHERTYPE_LANPROBE	0x8888	/* HP LanProbe test? */
 #define	ETHERTYPE_PAE		0x888E	/* 802.1X Port Access Entity */
 #define	ETHERTYPE_AOE		0x88A2	/* ATA over Ethernet */
+#define	ETHERTYPE_8021AD	0x88A8	/* 802.1ad VLAN stacking */
 #define	ETHERTYPE_LLDP		0x88CC	/* Link Layer Discovery Protocol */
 #define	ETHERTYPE_LOOPBACK	0x9000	/* Loopback */
 #define	ETHERTYPE_LBACK		ETHERTYPE_LOOPBACK	/* DEC MOP loopback */