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 6600 will sometimes record the switch as accessed on port A1 (VLAN 1), and other times as accessed on port B1 (VLAN 2):

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PC “A”

 

 

 

 

 

 

 

 

A1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PC “B”

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

C1

 

D1

 

VLAN 1

 

VLAN 2

 

6120 Switch

(Same MAC address for all

VLANs.)

This switch has a single forwarding database.

This switch has multiple forwarding databases.

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 6600 with the 6120 switch’s MAC address in the destination field. Because the 6600 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 6120 switch. The 6120 switch then routes the packet through the VLAN 2 link to the 6600, which forwards the packet on to PC “B”. Because the 6600 received the packet from the 6120 switch on VLAN 2 (port “B1”), the 6600’s single forwarding database records the 6120 switch 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 6600 with the 6120 switch’s MAC address in the destination field. However, this time the Switch 6600’s single forwarding database indicates that the 6120 is on port B1 (VLAN 2), and the 6600 drops the packet instead of forwarding it.

3.Later, the 6120 switch transmits a packet to the 6600 through the VLAN 1 link, and the 6600 updates its address table to indicate that the 6120 switch is on port A1 (VLAN 1) instead of port B1 (VLAN 2). Thus, the 6600’s information on the location of the 6120 switch changes over time. For this

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