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VLAN Management
Page 4-23
By default, independent learning of addresses is enabled. Under independent
learning, all addresses learned in a VLAN are stored in an address table for
that VLAN only, and all forwarding decisions for that VLAN are made by
consulting that table. This can sometimes cause unexpected results if a port
is a member of more than one VLAN.
For example, assume a port is a member of VID 1 and VID 2, and VID 1 has
already learned MAC address X. Unicast traffic from the port that is destined
for X will be forwarded correctly in VLAN 1. But if the address is not
present in the VLAN 2 forwarding table, the frame will be flooded to all
ports in VLAN 2.
To prevent this undesired flooding, the address tables may be shared. When
shared learning is enabled, a single forwarding table is used by all VLANs
that are members of the shared group.
Each address database in the system is represented by a Filtering ID (FID).
For independent learning VLANs, the FIDs are assigned by the system
beginning from 0. Shared learning VLANs are represented by FIDs
beginning at 65.
Inter-Switch Links
An inter-switch link (ISL) is a port that connects VLANS that reside on two
different switches; it’s the means to share VLAN information between
switches on a network.
For example, consider the two-switch network in Figure 4-6, which connects
the ethernet segments, E-1 through E-9. Assume port 1 on each switch is set
up as default; it passes untagged frames. Also assume port 2 on each switch
is configured to only accept tagged frames, which limits traffic to VLAN 1.
In that case, VID 1 frames from E-1 will never reach E-5.
An ISL is necessary to connect VLAN 1 across the switches. This is done by
configuring port 1 as a member of VLAN 1 on both switch 1 and switch 2.
Both instances of port 1 must transmit tagged frames, and a typical ISL is
also configured to drop untagged frames. In that case, VLAN 1 is connected
across the switches.