Single-Forwarding Database Operation; Example Of An Unsupported Configuration And How; To Correct It - HP ProCurve Series 2810 Management And Configuration Manual

Single-Forwarding Database Operation

When a packet arrives with a destination MAC address that matches a MAC
address in the switch's forwarding table, the switch tries to send the packet
to the port listed for that MAC address. But, if the destination port is in a
different VLAN than the VLAN on which the packet was received, the switch
drops the packet. This is not a problem for a switch with a multiple-forwarding
database (refer to table 2-7, above) because the switch allows multiple
instances of a given MAC address; one for each valid destination. However, a
switch with a single-forwarding database allows only one instance of a given
MAC address. If (1) you connect the two types of switches through multiple
ports or trunks belonging to different VLANs, and (2) enable routing on the
switch having the multiple-forwarding database; then, on the switch having
the single-forwarding database, the port and VLAN record it maintains for the
connected multiple-forwarding-database switch can frequently change. This
causes poor performance and the appearance of an intermittent or broken
connection.

Example of an Unsupported Configuration and How

to Correct It

The Problem. In figure 2-1, the MAC address table for Switch 8000M will
sometimes record the multiple-forwarding database switch as accessed on
port A1 (VLAN 1), and other times as accessed on port B1 (VLAN 2):
VLAN 1
PC "A"
VLAN 1
Figure 2-1. Example of Invalid Configuration for Single-Forwarding to Multiple-
Forwarding Database Devices in a Multiple VLAN Environment
In figure 2-1, PC "A" sends an IP packet to PC "B".
Switch 8000M
VLAN 2
A1 B1
C1
D1
VLAN 2
Multiple-Forwarding
Database Switch
Routing Enabled
(Same MAC address for all
VLANs.)
Static Virtual LANs (VLANs)
Port-Based Virtual LANs (Static VLANs)
PC "B"
This switch has a single
forwarding database.
This switch has multiple
forwarding databases.
2-11