User guide

4-6 B
LACK
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IAMOND
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Tagging is a process that inserts a marker (called a tag) into the Ethernet frame. The tag
contains the identification number of a specific VLAN, called the VLANid.
The use of 802.1Q tagged packets may lead to the appearance of packets
slightly bigger than the current IEEE 802.3/Ethernet maximum of 1518 bytes.
This may affect packet error counters in other devices, and may also lead to
connectivity problems if non-802.1Q bridges or routers are placed in the path.
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Tagging is most commonly used to create VLANs that span systems. The
system-to-system connections are typically called trunks. Using tags, multiple VLANs
can span multiple systems using one or more trunks. In a port-based VLAN, each
VLAN requires its own pair of trunk ports, as shown in Figure 4-3. Using tags, multiple
VLANs can span two systems with a single trunk.
Another benefit of tagged VLANs is the ability to have a port be a member of multiple
VLANs. This is particularly useful if you have a device (such as a server) that must
belong to multiple VLANs. The device must have a NIC that supports 802.1Q tagging.
A single port can be a member of only one port-based VLAN. All additional VLAN
membership for the port must be accompanied by tags. In addition to configuring the
VLAN tag for the port, the server must have a Network Interface Card (NIC) that
supports 802.1Q tagging.
A
SSIGNING A
VLAN T
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Each VLAN may be assigned an 802.1Q VLAN tag. As ports are added to a VLAN with
an 802.1Q tag defined, you decide whether each port will use tagging for that VLAN.
The default mode of the BlackDiamond is to have all ports assigned to the VLAN
named default with an 802.1Q VLAN tag (VLANid) of 1 assigned.
Not all ports in the VLAN must be tagged. As traffic from a port is forwarded out of
the system, the BlackDiamond determines (in real time) if each destination port should
use tagged or untagged packet formats for that VLAN. The BlackDiamond adds and
strips tags, as required, by the port configuration for that VLAN.
Packets arriving tagged with a VLANid that is not configured in the BlackDiamond
will be discarded.