IBM SC30-3681-08 manual Overview of LAN Emulation

Overview of LAN Emulation

This ®eld can specify the upper bound on the size of a data frame that can be processed by the LE client, or it can be unspeci®ed. The LECS cannot assign a client to an ELAN with a maximum frame size larger than that speci®ed by the client. If the ELAN allows frames too large for the client to handle, the client cannot function on that ELAN.

Given this information, the LECS assigns the LE client to a LES. This is accomplished through the use of policies and policy values. A policy is a criterion that the LECS uses to make LE client-to-LES assignment decisions. A policy value is a (value, LES) pair that indicates that the speci®ed value should be assigned to the speci®ed LES. For example, a policy could be the MAC address of the LE client, and a policy value could be (MAC ADDR_A, LES_1). An LE client with MAC ADDR_A will be assigned to LES_1 if the LE client has not already been assigned to another LES because of a higher-priority policy. One set of policies and policy values applies to all the ELANs.

In accordance with the LE service MIB Speci®cation of the ATM forum, these are the six policies de®ned:

1.ATM address

2.MAC address

3.Route descriptor

4.ELAN type

5.Max frame size

6.ELAN name

Policies also have priorities. The LECS examines policies in prioritized order. Policies with smaller values in the priority ®eld are considered before policies with larger values in the priority ®eld. Policies with equal values in the priority ®eld are considered at the same time and ANDed together.

The LECS assigns an LE client to a LES when all of the policies at the current priority level are satis®ed and in agreement. The policies are satis®ed when there is a policy value that matches the corresponding ®eld in the con®guration request for each policy at the current level. The policies are in agreement when the set of matches include a LES that is common to all the policies. If these conditions are not met, the LECS considers the policies at the next priority level. If the LECS is unable to ®nd a LES at any priority level, an unsuccessful con®guration response is returned to the LE client.

To understand the meaning of agreement of the policies, consider this example of policies not in agreement. Suppose that the policies at priority 1 are a MAC address and an ELAN name. One of the policy values is (X©400000121225©, LES_A) and one is (ELAN 1, LES_B). If the LE client provides a LAN destination of X©400000121225©, the MAC address policy is satis®ed. If the LE client provides an ELAN name of ELAN 1, then the ELAN name policy is also satis®ed. In this case the policies at priority 1 are not in agreement because they refer to different LESs. In this example, the LECS would examine the policies at the next priority level.

After determining the correct LES for an LE client, the LECS returns a con®guration response to the LE client that includes the following information: LES ATM address, ELAN type, max frame size, and ELAN name. The con®guration response can also include type/length/value (TLV) parameters. TLVs provide a method to download optional or user-de®ned parameters to the LE client.

Chapter 20. Overview of LAN Emulation 261