Motorola WS5100 用户手册

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L3 roaming works with switches in the mobility domain to exchange mobility related control information. This 
includes IP addresses, Media Access Control (MAC) address information and the HS-VLAN-id of all MUs in 
the mobility-domain. A consistent peer configuration results in full-mesh sessions required for L3 roaming to 
work correctly. Peering sessions use Transmission Control Protocol (TCP) as the transport layer protocol to 
carry mobility update messages. TCP provides the following advantages:

• TCP retransmits lost messages thereby providing reliable connectivity

• TCP ensures ordered message delivery using sequenced numbers.

• TCP has a built-in “keep-alive” mechanism which helps detect loss of connectivity to the peer or peer 

failure.

In a layer 3 environment, the access port adoption process is somewhat unique, for more information, see 
Configuring Layer 3 Access Port Adoption on page 4-88.

MUs roam from AP to AP as an MU moves throughout a WLAN coverage area. To improve roaming 
performance, various fast roaming features are implemented:

• Pairwise Master Key (PMK) — Caching credentials are in the AP, so the MU does not need to re-

authenticate.

• PMK Opportunistic Caching — The MU starts transmitting on another AP in order for both AP's to connect 

to a common wireless switch.

• Switch to Switch Hand-Off — When an MU roams from a wireless switch in one subnet to a wireless 

switch in another subnet, the transport layer connections will be preserved as far as possible.

• PMK Pre-Authentication —The MU authenticates itself with the AP before roaming to it.

An associated MU (connected to a particular wireless switch) can roam to another access port connected to 
a different wireless switch. Both switches must be on the same L2 domain. Authentication information is not 
shared between the switches, nor is buffered packets on one switch transferred to the other switch. Pre-
authentication between the switch and MU allows faster roaming.

The wireless switch supports international roaming as per the 802.11d specification.

As a value added proprietary feature between Motorola infrastructure products and Motorola MUs, a move 
command has been introduced. This command permits an MU to roam between ports connected to the same 
wireless switch without the need to perform the full association and authentication defined by the 802.11 
standard. The move command is a simple packet up/packet back exchange with the access port. Verification 
of this feature is dependent on its implementation in one or more mobile units. 

The switch supports multiple Basic Service Set Identifiers (BSSIDs). An access port capable of supporting 
multiple BSSID's generates multiple beacons, one per BSSID. Hence, an AP that supports 4 BSSID's can send 
4 beacons. The basic requirement for supporting multiple BSSID's is multiple MAC addresses, since each 
BSSID is defined by its MAC address.