Users Guide

Dell PowerConnect ArubaOS 5.0 | User Guide IP Mobility | 409
z Derives the address of the home agent for a mobile client from the HAT using the mobile client’s IP address.
If there is more than one possible home agent for a mobile client in the HAT, the proxy mobile IP module
uses a discovery mechanism to find the current home agent for the client.
z Detects when a mobile client has moved. Client moves are detected based on ingress port and VLAN changes
and mobility is triggered accordingly. For faster roaming convergence between AP(s) on the same controller, it
is recommended that you keep the “on-association” option enabled. This helps trigger mobility as soon as
802.11 association packets are received from the mobile client.
Proxy DHCP
When a mobile client first associates with a controller, it sends a DHCP discover request with no requested IP.
The controller allows DHCP packets for the client onto the configured VLAN where, presumably, it will receive
an IP address. The incoming VLAN becomes the client’s home VLAN.
If a mobile client moves to another AP on the same controller that places the client on a different VLAN than its
initial (home) VLAN, the proxy DHCP module redirects packets from the client’s current/visited VLAN to the
home VLAN. The proxy DHCP module also redirects DHCP packets for the client from the home VLAN to the
visited VLAN.
If the mobile client moves to another controller, the proxy DHCP module attempts to discover if the client has
an ongoing session on a different controller. When a remote controller is identified, all DHCP packets from the
client are sent to the home agent where they are replayed on the home VLAN. The proxy DHCP module also
redirects DHCP packets for the client from the home VLAN to the visited network. In either situation, operations
of the proxy DHCP module do not replace DHCP relay functions which can still operate on the client’s home
VLAN, either in the controller or in another device.
Revocations
A home agent or foreign agent can send a registration revocation message, which revokes registration service for
the mobile client. For example, when a mobile client roams from one foreign agent to another, the home agent
can send a registration revocation message to the first foreign agent so that the foreign agent can free any
resources held for the client.
Mobility Multicast
Internet Protocol (IP) multicast is a network addressing method used to simultaneously deliver a single stream of
information from one sender to multiple clients on a network. Unlike broadcast traffic, which is meant for all
hosts in a single domain, multicast traffic is sent only to those specific hosts who are configured to receive such
traffic. Clients who want to receive multicast traffic can join a multicast group via IGMP messages. Upstream
routers use IGMP message information to compute multicast routing tables and determine the outgoing
interfaces for each multicast group stream. ArubaOS supports mobility multicast enhancements that provide
uninterrupted streaming of multicast traffic, regardless of a client's location.
Proxy IGMP and Proxy Remote Subscription
The mobility controller is always aware of the client's location, so the controller can join multicast group(s) on
behalf of that mobile client. This feature, called Proxy IGMP, allows the controller to join a multicast group and
suppresses the client’s IGMP control messages to the upstream multicast router. (The client's IGMP control
messages will, however, still be used by controller to maintain a multicast forwarding table.) The multicast IGMP
traffic originating from the client will instead be sent from the controller’s incoming VLAN interface IP.
The IGMP proxy feature includes both a host implementation and a router implementation. An upstream router
sees a Dell controller running IGMP proxy as a host; a client attached to the controller would see the controller as