Enterasys Networks Security Router X-PeditionTM - Manual (Page 121)

IP Routing Protocols
XSR User’s Guide 5-19
Figure 5-1 802.1Q VLAN Tag
The reserved Tag Type denotes the associated Ethernet frame type of the VLAN Tag while the
remaining 16 tag bits comprise this control data:
• a 3-bit value indicating the user priority of the Ethernet frame for QoS purposes
• a 1-bit Canonical Format Indicator (CFI) denoting the presence of a Routing Information Field
• a 12-bit VLAN Identifier (VLAN ID) used for network assignment
The XSR lets you configure up to 4094 VLANs using the vlan command, with IDs 0 and 4095
reserved. VLAN ID 0 is used to represent the default VLAN. VLANs provide for eight priority
levels but the XSR does not act on priority bits. Any VLAN packets originating from the XSR are
assigned a priority of 0.
Figure 5-2 depicts a typical VLAN routing setup. The LAN that a device belongs to depends on
the settings of the interfaces on the VLAN switch rather than the actual systems layout.
Figure 5-2 Typical Configuration of VLAN Routing
Forwarding VLAN, PPPoE over VLAN
It is possible for a single physical Ethernet interface to support a mixture of Ethernet, PPPoE,
Ethernet VLAN, and PPPoE over VLAN traffic, as illustrated in
Figure 5-3 . Note that
GigabitEthernet sub-interfaces 3.1 - 3.4 are all sub-interfaces associated with different VLAN IDs.
Priority 802.1Q Tag Type CFI VLAN Identifier
WAN
XSR
VLAN Switches
VLAN 300
Physical layout
Logical layout
VLAN 200 VLAN 100