Static Virtual LANs (VLANs)
Multiple VLAN Considerations
Example of an Unsupported Configuration and How To
Correct It
The Problem. In figure 2-9, the MAC address table for Switch 8000M will
sometimes record the 5300xl, 3400cl, or 6400cl as accessed on port A1 (VLAN
1), and other times as accessed on port B1 (VLAN 2):
Switch 8000M
VLAN 1
VLAN 2
5300xl Switch
Routing Enabled
(Same MAC address for all
VLANs.)
VLAN 1
VLAN 2
This switch has multiple
forwarding databases.
This switch has a single
forwarding database.
PC “A”
PC “B”
A1
B1
C1
D1
Figure 2-9. Example of Invalid Configuration for Single-Forwarding to Multiple-
Forwarding Database Devices in a Multiple VLAN Environment
In figure 2-9, PC “A” sends an IP packet to PC “B”.
1. The packet enters VLAN 1 in the Switch 8000 with the 5300xl’s MAC
address in the destination field. Because the 8000M has not yet learned
this MAC address, it does not find the address in its address table, and
floods the packet out all ports, including the VLAN 1 link (port “A1”) to
the 5300xl. The 5300xl then routes the packet through the VLAN 2 link to
the 8000M, which forwards the packet on to PC “B”. Because the 8000M
received the packet from the 5300xl on VLAN 2 (port “B1”), the 8000M’s
single forwarding database records the 5300xl as being on port “B1”
(VLAN 2).
2. PC “A” now sends a second packet to PC “B”. The packet again enters
VLAN 1 in the Switch 8000 with the 5300xl’s MAC address in the destina-
tion field. However, this time the Switch 8000M’s single forwarding data-
base indicates that the 5300xl is on port B1 (VLAN 2), and the 8000M drops
the packet instead of forwarding it.
3. Later, the 5300xl transmits a packet to the 8000M through the VLAN 1 link,
and the 8000M updates its address table to indicate that the 5300xl is on
port A1 (VLAN 1) instead of port B1 (VLAN 2). Thus, the 8000M’s informa-
2-19
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