Setting Priority

An Accelar switch can operate in either of two modes of traffic priority: Best Effort mode or Priority mode. The factory default setting is Best Effort mode. The following differences exist between these modes:

• In Best Effort mode, all traffic is treated with the same priority.
• In Priority mode, high-priority traffic flows through the switch fabric using a high-priority data path. Output buffers are reserved for high-priority traffic.

High-priority traffic can be enabled on a per-port, per-MAC, per-VLAN, or per-flow basis as follows:

• When a port is set to High Priority, all traffic received on this port is assigned a high priority.
• When a MAC address is set to High Priority mode, all traffic from the MAC address is assigned a high priority.
• When a VLAN is set to High Priority mode, frames received on any of the active ports of the VLAN and classified as belonging to that VLAN are assigned a high priority.
• An IP flow record can be used to assign high switching priority to an IP packet based on its source and destination IP addresses, protocol type source port number, and destination port number.

When a high-priority frame is sent out a trunk port, the 3-bit User Priority field in the IEEE VLAN tag is set to 7. A normal priority frame has a User Priority of 0. Any received tagged frames with a User Priority greater than 0 are treated as high priority.

There are six ways to prioritize traffic:

• By port ("By Port")

In Device Manager, you can specify that any traffic that originates or emanates from a particular port can be treated as high priority. For example, a port that supports a video server could be given a higher priority than data traffic that originates from other ports.

• By MAC address ("By MAC address")

You can set the priority to high or low for either a learned or a static entry.

• By VLAN ("By VLAN")

You can configure any VLAN to be a high-priority VLAN. When you create a high-priority VLAN under Accelar VLAN Manager (for example, a video VLAN) any traffic through this VLAN is treated with priority.

• By IP flow

You can use IP flows to identify a particular stream of traffic at the IP layer and at the TCP/UDP layer. Setting IP flows as a priority method uses the following addresses and ports:

­ Source IP address
­ Destination IP address
­ Source IP port (IP connotation of port; that is, the port used for Telnet, FTP, NFS)
­ Destination IP port
• By IP filters

When using IP filters, matching frames have high priority. Refer to the chapter on IP filters in Reference for the Accelar 1000 Series Management Software Routing Operations for information about how to use filters to set priority.

• By 802.1p priority bits (in the VLAN tag)

These bits have a range of 0 to 7.

If a tagged frame is received with bits set and the priority is greater than 0, priority is high. If the priority is set to 0, the frame has normal priority.

When sending a tagged frame, if transferring through the priority queue, set the priority bits to 7.

The following sections describe assigning priority to traffic in these areas.