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How the MAP Locates a Wireless Switch

After an access point is converted to a MAP, it goes through a process to locate a Wireless Switch. This

section describes the complete boot and configuration process used by Distributed MAPs in MSS Version

3.2 to locate a Wireless Switch.

DHCP Exchange and IP Configuration

Before the MAP can locate a Wireless Switch, it must first obtain an IP address. It does

this through the DHCP protocol.

The MAP sends a DHCP Discover message to the subnet broadcast address. If a DHCP

server is present on the subnet, or through a router configured to relay DHCP, the server

replies with a unicast DHCP Offer message. The Offer message must contain the

following parameters:

IP address for the MAP

Domain name of the network

IP address of the network's DNS server

IP address of the subnet's default gateway

Optionally, the Offer message can also contain a list of Wireless Switch IP addresses or

hostnames, in DHCP option 43. The MAP sends a DHCP Request to one of the DHCP

servers that sent an Offer message, and receives an Ack message from the server. The

MAP then configures its network connection with the information contained in the Ack

message from that server.

Find WX Broadcast

This is the first method the MAP uses to try to contact a Wireless Switch.

The MAP sends a Find WX message to UDP port 5000 on the subnet broadcast address.

Wireless switches in the same IP subnet as the MAP can receive the message. Each

switch that receives the broadcast Find WX message replies immediately or after a short

delay:

If the MAP is configured as a Distributed MAP on a Wireless Switch and the

connection bias is high, the switch immediately sends a Find WX Reply message.

If the MAP is configured as a Distributed MAP on a Wireless Switch but the

connection bias is low, the switch waits one second and then sends a Find WX Reply

message. The delay allows switches with high bias for the MAP to respond first.

If a Wireless Switch that receives the Find WX message does not have the Distributed

MAP in its configuration but another WX switch in the same Mobility Domain does,

the switch waits two seconds and then sends a Find WX Reply message with the

IP address of the best switch to use. The determination of best switch is based on the

bias settings for the MAP on each switch and on the capacity of each switch to add

new active MAP connections.

After a Wireless Switch replies to the broadcast Find WX Request, the MAP sends a

unicast Find WX Request to the Wireless Switch's system IP address suggested in the

Find WX Reply message, to request a software image and configuration. After the MAP

receives the image and configuration, the boot and configuration process is complete.

However, if no WX switches reply, the MAP resends the Find WX broadcast up to 11

more times. If no Wireless Switches reply after all 12 attempts, the MAP attempts to find

a WX based on DHCP option 43 values received in the DHCP Ack.

DHCP Option 43

This is the second method the MAP uses to try to contact a Wireless Switch.

This method applies only if the DHCP Ack contained WX IP addresses or hostnames in

option 43. If the Ack did not have this information, the MAP moves directly onto the

third method.