Enterasys Networks 2H258, 2E253, 2H253, 2H252 manual Layer, About the IP TOS Rewrite Function

Priority Classification Configuration Screen

The following example shows how the precedence concept can be applied:

Example

A network administrator has defined the following two classifications involving priorities:

•All frames with an IP TOS value of AA (Layer 3, precedence level 5a) are assigned to priority 7.

•All frames with a TCP source port number of 80 (Layer 4, precedence level 4a) are assigned to priority 3.

In this example, a frame that is received with a TOS value of AA, and a TCP port number of 80, will be assigned to priority 3. This is because the TCP port number classifications take precedence over IP TOS classifications.

It is important to remember that the SmartSwitch device will classify frames based on one of the classification options.

8.6.2About the IP TOS Rewrite Function

The Type of Service (TOS) field [also known as the Differential Services (DF) field in RFC 2474] is an 8-bit field. It is located in the IP packet and used by a device to indicate the precedence or priority of a given frame (see Figure 8-7). The TOS layer-3 priority indicator enables the ability to signal the frame priority from end to end as the frame makes its way through the network.

Figure 8-7 Datagram, Layer 2 and Layer 3
Layer 2	Layer 3

Destination Source Address Address

(DA) (SA)

VLAN Priority

Version Addr Length

Type of Service (8-bits)

3559_1

This IP TOS Rewrite feature enables a Network Administrator to assign Layer 3 TOS characteristics to incoming frames and setting the switch to rewrite the 8-bit TOS value in the Layer 3 information portion of incoming frames.

The IP TOS Rewrite feature enables you to configure the switch to:

•Insert a user-defined 8-bit value into the layer-3 TOS field.

•Write over an existing TOS value. This is useful when the Network Administrator wants to enforce a specific priority policy in the network.

802.1p Configuration Menu Screens 8-29