Static Virtual LANs (VLANs)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Port-Based Virtual LANs (Static VLANs)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch 8000M

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PC “A”

 

 

 

 

 

 

 

 

A1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B1

 

 

 

 

 

 

 

 

PC “B”

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

C1

 

D1

 

VLAN 1

 

VLAN 2

 

Multiple-Forwarding

Database Switch

Routing Enabled

(Same MAC address for all

VLANs.)

This switch has a single forwarding database.

This switch has multiple forwarding databases.

Figure 2-8. Example of Invalid Configuration for Single-Forwarding to Multiple- Forwarding Database Devices in a Multiple VLAN Environment

In figure 2-8, PC “A” sends an IP packet to PC “B”.

1.The packet enters VLAN 1 in the Switch 8000 with the multiple-forwarding database switch 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 multiple-forwarding database switch. The multiple- forwarding database switch 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 multiple-forwarding database switch on VLAN 2 (port “B1”), the 8000M’s single forwarding database records the multiple-forwarding database 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 8000 with the multiple-forwarding database switch’s MAC address in the destination field. However, this time the Switch 8000M’s single forwarding database indicates that the multiple-forward­ ing database switch is on port B1 (VLAN 2), and the 8000M drops the packet instead of forwarding it.

3.Later, the multiple-forwarding database switch transmits a packet to the 8000M through the VLAN 1 link, and the 8000M updates its address table to indicate that the multiple-forwarding database switch is on port A1 (VLAN 1) instead of port B1 (VLAN 2). Thus, the 8000M’s information on the location of the multiple-forwarding database switch changes over time. For this reason, the 8000M discards some packets directed through it for the multiple-forwarding database switch, resulting in poor perfor­ mance and the appearance of an intermittent or broken link.

2-13