Static Virtual LANs (VLANs)

Port-Based Virtual LANs (Static VLANs)

Multiple-Forwarding Databases* Single-Forwarding Database*

Series 2600/2600-PWR switches

Series 2510 switches

*To determine whether other vendors’ devices use single-forwarding or multiple-forwarding database architectures, refer to the documentation provided for those devices.

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 2-8, 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):

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