KB.15.15

1. The packet enters VLAN 1 in the Switch 8000 with the 8212zl switch'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 8212zl switch. The 8212zl 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 8212zl switch on VLAN 2 (port "B1"), the 8000M's single forwarding

database records the 8212zl 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 8212zl switch's MAC address in the destination field. However, this time the

Switch 8000M's single forwarding database indicates that the 8212zl is on port B1 (VLAN

2), and the 8000M drops the packet instead of forwarding it.

3. Later, the 8212zl switch transmits a packet to the 8000M through the VLAN 1 link, and the

8000M updates its address table to indicate that the 8212zl switch is on port A1 (VLAN 1)

instead of port B1 (VLAN 2). Thus, the 8000M's information on the location of the 8212zl

switch changes over time. For this reason, the 8000M discards some packets directed through

it for the 8212zl switch, resulting in poor performance and the appearance of an intermittent

or broken link.

Figure 9 Invalid forwarding

The solution

1. Use only one cable or port trunk between the single-forwarding and multiple-forwarding

database devices

2. Configure the link with multiple, tagged VLANs.

3. To increase the network bandwidth of the connection between devices, use a trunk of multiple

physical links.

Now, the 8000M forwarding database always lists the 8212zl MAC address on port A1, and

the 8000M will send traffic to either VLAN on the 8212zl.

Figure 10 A solution for single-forwarding to multiple-forwarding database devices in a multiple

VLAN environment

Correcting an unsupported configuration 65