The switch delivers more services at access layer other than merely providing increased speeds and feeds. Wireless services is now integrated with the switch, which ensures that the access layer switch terminates the wireless users data plane, thereby delivering on the promise of Cisco's unified architecture. Unification implies that mobility services are provided to both wireless and wired stations.
The switch provides seamless roaming, which requires transparency of the network configuration and deployment options to the client.
From the end user's perspective, any mobility event must not change its IP address, its default router or DHCP server. This means that as stations roam, they must be able to
Send an ARP to their default router, or
Transmit a DHCP request to the server that had previously assigned their address.
From the infrastructure's perspective, as mobility events occur, the station's traffic must follow its current point of attachment, which can either be a mobility agent (MA) or mobility controller (MC). This must be true regardless of whether the station has moved to a network that is configured for a different subnet. The period from which the station is not receiving traffic following its mobility event must be as short as possible, even below 40 ms whenever possible, which includes any authentication procedures that are required.
From the infrastructure's perspective, the mobility management solution must have four main components, and all of these functions must be performed within the constraints of roaming:
Initial Association—This function is used to identify the user's new point of attachment in the network.
Context Transfer—This function is used to transfer state information associated with the station. This ensures that the station's static and real-time policies, including security and application ACLs, and services, remain the same across handoffs.
Handoff—This function is used to signal that the station's point of attachment has changed, and control of the station should be relinquished by the previous access switch.
Data Plane—This function is typically tied to the handoff process, and ensures that the station's traffic continues to be delivered and received from the station without any noticeable performance degradation.
Wired and Wireless Mobility
One of the key features of the Converged access solution (applicable to both the Cisco Catalyst 3850 Switch and Cisco WLC 5700 Series Controller) is its ability to provide a device with an IP address and maintain its session persistence, across mobility events from ethernet connections to wireless and vice-versa. This feature allows users to remain on an ethernet network when possible, and make use of the freedom of mobility associated with wireless when necessary.
This feature leverages support from both the client and the infrastructure and uses the two factor authentication-device and user. The device authentication credentials is cached in the mobility controller (MC). When a device transitions across link layers, the device credentials is validated, and if a match is found, the MC ensures that the same IP address is assigned to the new interface.
Features of Mobility
Mobility Controller (MC)—The controller provides mobility management services for inter-peer group roaming events. The MC provides a central point of contact for management and policy based control protocols, such as RADIUS. This eliminates the need for the infrastructure servers to maintain a user's location as it transitions throughout the network. The MC sends the configuration to all the mobility agents under its sub-domain of their mobility configuration, peer group membership and list of members. A sub-domain is synonymous to the MC that forms it. Each sub-domain consists of an MC and zero or more access switches that have AP's associated to them.
Mobility Agents (MA)— A mobility agent is either an access switch that has a wireless module running on it or an MC with an internal MA running on it. A mobility agent is the wireless component that maintains client mobility state machine for a mobile client that is connected via an AP to the device that the MA is running on.
Mobility Sub Domain— It is an autonomous portion of the mobility domain network. A mobility sub-domain comprises of a single mobility controller and its associated mobility agents (MAs).
Even when more than one mobility controller is present, only one MC can be active at any given time.
A mobility sub-domain is the set of devices managed by the active mobility controller. A mobility sub-domain comprises of a set of mobility agents and associated access points.
Mobility Group— A collection of mobility controllers (MCs) across which fast roaming is supported. The concept of mobility group is the same as a collection of buildings in a campus across which frequent roaming is expected.
Mobility Domain— A collection of mobility sub-domains across which mobility is supported. The term mobility domain may be the same as a campus network.
Mobility Oracle (MO)—The mobility oracle acts as the point of contact for mobility events that occur across mobility sub-domains. It also maintains a local database of each station in the entire mobility domain, their home and current sub-domain. A mobility domain includes one or more mobility oracle, though only one would be active at any given time.
Mobility Tunnel Endpoint (MTE)— The mobility tunnel endpoint (MTE) provides data plane services for mobile devices through the use of tunneling. This minimizes the impact of roaming events on the network by keeping the user's point of presence on the network a constant.
Point of Attachment— A station's point of attachment is where its data path is initially processed upon entry in the network. This could either be the access switch that is currently providing it service, or the wireless LAN controller.
Point of Presence— A station's point of presence is the place in the network where the station is being advertised. For instance, if an access switch is advertising reachability to the station via a routing protocol, the interface on which the route is being advertised is considered the station's point of presence.
Switch Peer Group (SPG)— A peer group is a statically created list of neighboring access switches between which fast mobility services is provided. A peer group limits the scope of interactions between switches during handoffs to only those that are geographically proximate.
Station—A user's device that connects to and requests service from the network. The device may have a wired, wireless or both interfaces.
Switch in the same SPG—A peer switch that is part of the peer group of the local switch.
Switch outside the SPG—A peer access switch that is not part of the local switch's peer group.
Foreign Mobility Controller— The mobility controller providing mobility management service for the station in a foreign mobility sub-domain. The foreign mobility controller acts as a liaison between access switches in the foreign sub-domain and the mobility controller in the home domain.
Foreign Mobility Sub-Domain— The mobility sub-domain, controlled by a mobility controller, supporting a station which is anchored in another mobility sub-domain
Foreign Switch— The access switch in the foreign mobility sub-domain currently providing service to the station.
Anchor Mobility Controller— The mobility controller providing a single point of control and mobility management service for stations in their home mobility sub-domain.
Anchor Mobility Sub-Domain— The mobility sub-domain, controlled by a mobility controller, for a station where its IP address was assigned.
Anchor Switch— The switch in the home mobility sub-domain that last provided service to a station.
Sticky Anchoring for Low Latency Roaming
Sticky Anchoring ensures low roaming latency from the client's point of presence is maintained at the switch where the client initially joins the network. It is expensive to apply client policies at a switch for a roaming client. There can be considerable delay as it involves contacting the AAA server for downloadable ACLs which is not acceptable for restoring time sensitive client traffic.
To manage this delay, when the client roams between APs connected to different switches , irrespective of whether it is an intra sub-domain roam or inter sub-domain roam, the client traffic is always tunneled to the switch where the client first associates. The client is anchored at its first point of attachment for its lifetime in the network.
This behavior is enabled by default. You can also disable this behavior to allow the client anchoring only for inter-subnet roams. This configuration is per WLAN config and is available under the WLAN config mode. The customer can configure different SSIDs for time sensitive and non time sensitive applications.
Bridge Domain ID and L2/L3 Roaming
Bridge domain ID provides the mobility nodes with information to decide on specific roam type, either as L2 or L3 roam. It also allows the network administrators to reuse the VLAN IDs across network distribution. When the VLAN IDs do not have the associated subnet configurations, they may require additional parameter to use in conjunction with VLAN ID. The network administrator ensures that the given VLAN under the same bridge domain ID are associated with the unique subnet. The mobility nodes will first check for the bridge domain ID for the given node and the VLAN ID associated with the client to identify the roam type. The bridge domain ID and the VLAN ID must be same to treat a roam as L2 roam.
The bridge domain ID is configured for each SPG when creating a SPG and later on the MC. The bridge domain ID could be same for more than one SPG and all the MAs under the SPG will share the same bridge domain ID. This information is pushed to the MAs as part of the configuration download when MA comes up initially. If the bridge domain ID is modified when the system is up, it will be pushed to all the MAs in the modified SPG and will take immediate effect for the future roams.
The MC can also have a bridge domain ID for it self, as the MC can also be part of a SPG.
Link Down Behavior
This section provides information about data synchronization between MA-MC and MC-MO when MC or MO faces downtime in absence of redundancy manager. When Keepalive is configured between MA-MC or MC-MO the clients database is synchronized between the MO and the MCs and the MC and its MAs respectively.
Platform Specific Scale Requirement for the Mobility Controller
The Mobility Controller (MC) role is supported on a number of different platforms like, the Cisco WLC 5700 Series, CUWN and Catalyst 3850 Switches. The scale requirements on these three platforms are summarized in the table below: