3Com 3CRWE725075 Handbuch für die Umstellung - Seite 3
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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.
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