Cisco Systems 7920 IP manual Layer 3 Roaming

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Layer 2 roaming with both static WEP and LEAP has been shown to provide acceptable QoS using either G.711 or G.729. If LEAP is being used, it is recommended that users are defined locally on the ACS server. This is because using off−box databases have potentially unknown response times, which could adversely effect overall QoS during Layer 2 roaming.

Layer 3 Roaming

Layer 3 roaming occurs when a client moves from an AP that covers IP subnet A to an AP that covers IP subnet B. At this point, the client would no longer have an IP address or default gateway that is valid within the new IP subnet. Since the clients￿ IP address and default gateway are no longer valid, its existing data sessions or VoIP calls fail since the remote client can no longer reach the local client.

There are the following ways that clients can work in an environment where they are roaming across Layer 3 boundaries:

∙Obtain a new IP address or default gateway in the new subnet through DHCP. While this is supported on some operating systems, it is not an effective model for VoIP since it would break all existing calls (signaling and media).

∙Mobile IP (MIP) is a mechanism where routers within the network allow end points to keep their existing IP address or default gateway, and the routers handle rerouting packets to the clients on their new IP subnet. MIP requires Home Agent and (optionally) Foreign Agent functionality on the routers, as well as a MIP client on the end points. This functionality is not supported on the Cisco 7920.

Proxy MIP (PMIP) provides similar functionality to MIP, except that the end points do not have to have an embedded MIP client. The MIP client is contained in a network device, such as an AP, and that device acts as a MIP proxy for the end points that reside behind it. PMIP is currently supported on Cisco APs, but it is currently not supported for VoIP applications.

The Cisco 7920 does not currently support native Layer 3 roaming using MIP. In addition, the current lack of VLAN support on the Cisco APs when using PMIP prevents the Cisco 7920s from being deployed for Layer 3 roaming. The use of VLANs to create isolated voice and data traffic is required by Cisco AVVID design guidelines.

Note: In multistory WLAN deployments where the WLANs on each floor have different subnets, particular care needs to be taken in the RF site survey to ensure that stations on one floor do not roam to WLANs on floors above or below.

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

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Contents Table of Contents Prerequisites IntroductionRequirements Wlan Overview Cisco 7920 IP Phone OverviewRF Overview Site Surveys and Design Recommendations Components UsedRssi VoIP QoS∙ Delay Variation Jitter Not to exceed 30ms Security for the Cisco 7920 and Wlan Networks Number of 802.11b Devices per AP Network SizingNumber of 802.11b IP Phones per AP Page Numbers of 802.11b Phones per Layer 2 Subnet or Vlan Understanding Roaming Terminology Understanding Layer 2 and Layer 3 RoamingLayer 2 Roaming Page Layer 3 Roaming VLANs Wlan QoS for VoIP Page Page Page Interconnecting WLANs to Cisco Campus Infrastructure Connecting APs to the Catalyst 3550 SMI or EMI Connecting APs to the Catalyst 2950 SI Connecting APs to the Catalyst 2950 EICaveats and Limitations Designing Around the Lack of Layer 3 Roaming Call Admission ControlOther Caveats and Limitations Related Information
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