Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 342)

For detailed information on configuring these protocols, refer to Configuring STP-Type Protocols on
page 407.
The Brocade VDX hardware handles Ethernet frames as follows:
• When the destination MAC address is not in the lookup table, the frame is flooded on all ports in the
same VLAN, except the ingress port.
• When the destination MAC address is present in the lookup table, the frame is switched only to the
correct egress port.
• When the destination MAC address is present in the lookup table, and the egress port is the same
as the ingress port, the frame is dropped.
• If the Ethernet Frame Check Sequence (FCS) is incorrect, because the switch is in cut-through
mode, a correctly formatted Ethernet frame is sent out with an incorrect FCS.
• If the Ethernet frame is too short, the frame is discarded and the error counter is incremented.
• If the Ethernet frame is too long, the frame is truncated and the error counter is incremented. The
truncated frame is sent out with an incorrect FCS.
• Frames sent to a broadcast destination MAC address are flooded on all ports in the same VLAN,
except the ingress port.
• When MAC address entries in the lookup table time out, they are removed. In this event, frame
forwarding changes from unicast to flood.
• An existing MAC address entry in the lookup table is discarded when a device is moved to a new
location. When a device is moved, the ingress frame from the new port causes the old lookup table
entry to be discarded and the new entry to be inserted into the lookup table. Frame forwarding
remains unicast to the new port.
• When the lookup table is full, new entries replace the oldest MAC addresses after the oldest MAC
addresses reach a certain age and time out. MAC addresses that still have traffic running are not
timed out.
• If the port is receiving jumbo frame packets and the port is not configured with the required MTU
size to support jumbo frames, then the port discards those frames and increments the over-sized
packet error counter.
• If the port is receiving valid multicast frames and the port is not part of a VLAN that is enabled for
IGMP snooping, then the frames are treated as broadcast frames.
• If the port is receiving multicast frames with a destination MAC address (multicast MAC address)
and destination IP address (multicast IP address) belonging to different group addresses or not
pointing to the same group, the frames are silently discarded by the port.
NOTE
New entries start replacing older entries when the lookup table reaches 90 percent of its 32Kb
capacity.
802.1Q VLAN tagging
The Layer 2 switch always tags an incoming frame with an 802.1Q VLAN ID. If the incoming frame is
untagged, then a tag is added according to the port configuration. A port can classify untagged traffic
to a single VLAN or to multiple VLANs. If the incoming frame is already tagged, then the port will either
forward or discard the frame according to allowed VLAN rules in the port configuration.
These are three examples of 802.1Q VLAN tagging:
• If the DCB port is configured to tag incoming frames with a single VLAN ID, then incoming frames
that are untagged are tagged with the VLAN ID.
• If the DCB port is configured to tag incoming frames with multiple VLAN IDs, then incoming frames
that are untagged are tagged with the correct VLAN ID based on the port setting.
• If the DCB port is configured to accept externally tagged frames, then incoming frames that are
tagged with a VLAN ID are passed through unchanged.
802.1Q VLAN tagging
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