Specifications
Table Of Contents
- Table of Contents
- Preface Template Formats
- Web-based (GUI) Configuration
- Configuration
- Device Information
- System Information
- Serial Port Settings
- IP Address Settings
- IPv6 Address Settings
- IPv6 Route Settings
- IPv6 Neighbor Settings
- Port Configuration Folder
- Static ARP Settings
- User Accounts
- System Log Configuration Folder
- DHCP Relay Folder
- MAC Address Aging Time
- Web Settings
- Telnet Settings
- CLI Paging Settings
- Firmware Information
- SNTP Settings Folder
- SMTP Settings Folder
- SNMP Settings Folder
- Layer 2 Features
- Jumbo Frame
- VLANs
- 802.1Q Static VLAN
- Q-in-Q Folder
- 802.1v Protocol VLAN Folder
- GVRP Settings
- Asymmetric VLAN Settings
- MAC-based VLAN Settings
- PVID Auto Assign Settings
- Port Trunking
- LACP Port Settings
- Traffic Segmentation
- IGMP Snooping Folder
- MLD Snooping Settings
- Port Mirror
- Loopback Detection Settings Page
- Spanning Tree Folder
- Forwarding & Filtering Folder
- LLDP Folder
- Quality of Service (QoS)
- Security
- Access Control List (ACL)
- Monitoring
- Save and Tools
- System Log Entries
- Trap List
Chapter 3: Layer 2 Features
Extreme Networks EAS 100-24t Switch Software Manual
50
EtherType field is equal to 0x8100, the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained
in the following two octets and consists of three bits of user priority, one bit of Canonical Format
Identifier (CFI - used for encapsulating Token Ring packets so they can be carried across Ethernet
backbones), and twelve bits of VLAN ID (VID). The three bits of user priority are used by 802.1p. The
VID is the VLAN identifier and is used by the 802.1Q standard. Because the VID is twelve bits long,
4094 unique VLANs can be identified.
Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q
compliant network device to another with the VLAN information intact. This allows 802.1Q VLANs to
span network devices (and indeed, the entire network, if all network devices are 802.1Q compliant).
Unfortunately, not all network devices are 802.1Q compliant. These devices are referred to as tag-
unaware. 802.1Q devices are referred to as tag-aware.
Prior to the adoption of 802.1Q VLANs, port-based and MAC-based VLANs were in common use.
These VLANs relied upon a Port VLAN ID (PVID) to forward packets. A packet received on a given
port would be assigned that port's PVID and then be forwarded to the port that corresponded to the
packet's destination address (found in the Switch's forwarding table). If the PVID of the port that
received the packet is different from the PVID of the port that is to transmit the packet, the Switch will
drop the packet.
Within the Switch, different PVIDs mean different VLANs (remember that two VLANs cannot
communicate without an external router). So, VLAN identification based upon the PVIDs cannot create
VLANs that extend outside a given switch (or switch stack).
The tag is inserted into the packet header
making the entire packet longer by 4 octets.
All of the information originally contained
in the packet is retained.
The EtherType and VLAN ID are inserted
after the MAC source address, but before
the original EtherType/Length or Logical
Link Control. Because the packet is now a
bit longer than it was originally, the Cyclic
Redundancy Check (CRC) must be
recalculated.