Extending the LAN with Wireless Solutions – Configuring Switches for WLAN Solutions

This section describes the Catalyst switch configurations required to support APs and WLCs. It is important to remember that you are not expected to perform any AP or WLC configurations. AP and WLC configurations will not be included in this chapter, or the remainder of this guide. The following configurations will be illustrated in this section:

• Configuring Switch Ports for Autonomous Access Points
• Configuring Switch Ports for Lightweight Access Points
• Configuring Cisco IOS DHCP for Lightweight Access Points
• Configuring Switch Ports for Wireless LAN Controllers

Configuring Switch Ports for Autonomous Access Points

An Autonomous AP is a standalone AP that is not controlled by a Wireless LAN Controller. While the configuration of the actual AP itself is beyond the scope of the SWITCH exam requirements, configuring the switch port to support the Autonomous AP is not. This configuration example will be based on the wireless LAN network illustrated below in Figure 9-15:

Fig. 9-15.  Autonomous Access Point Wireless Network Topology

NOTE: It should be assumed that VLAN 10 and VLAN 20 have already been configured on the Distribution Layer switches, as are the corresponding SVIs for those VLANs. The SSIDs are configured on the AP itself and not on the switch. Also, assume that all trunking between the Distribution and Access Layer switches is in place. This section focuses exclusively on the configuration of the FastEthernet0/1 interface on switch Access 1. The steps to configure the Access Layer switch to support the autonomous AP are as follows:

1. Select the desired interface and configure it as a Layer 2 port using the switchport interface configuration command.

2. Configure the trunking encapsulation for the switch port using the switchport trunk encapsulationinterface configuration command.

3. Configure the switch as a trunk port using the switchport mode trunk interface configuration command.

4. Explicitly only allow the configured VLANs using the switchport trunk allowed vlan interface configuration command.

5. Optionally, enable PortFast for the trunk port using the spanning-tree portfast trunk interface configuration command.

The following output illustrates how to configure the Fa0/1 interface on switch Access 1 to support the connected autonomous Access Point:

This configuration can be validated using the show interfaces [name] switchport command as illustrated in the following output:

It is important to always use the switchport trunk allowed vlan [range] command on the trunk link connected to an Autonomous AP. This command should always be used to permit only the VLANs that exist on the AP. If not, this may result in unexpected issues.

Enabling the PortFast feature on a trunk port can result in loops, as seen in the warning above. While this allows the port to transition to the forwarding state much faster, it is important to understand the topology before this is applied.

Configuring Switch Ports for Lightweight Access Points

Unlike Autonomous APs, Lightweight APs are completely controlled by the WLC. Because of this, the switch port that the LAP is connected to must always be configured as an access port. The switch access port can be assigned to any particular VLAN, though it is common practice to assign it to a designated ‘network management’ VLAN.

This is because, unlike an Autonomous AP, which must be manually configured, LAPs receive their configuration from the WLC. Therefore, information about VLANs mapped to any SSID is transported between the LAP and the WLC using either an LWAPP or a CAPWAP tunnel. This configuration example will be based on the wireless LAN network illustrated in Figure 9-16.

Fig. 9-16.  Lightweight Access Point Wireless Network Topology

NOTE: It should be assumed that connectivity between the Access and Distribution switches is already in place, as is connectivity between the Distribution switch and the WLC.

The following sequence of steps is required to configure a switch port to support an LAP:

1. Select the desired interface and configure it as a Layer 2 port using the switchport interface configuration command.

2. Assign the switch port to the desired VLAN using the switchport access vlan interface configuration command.

3. Configure the switch as a static access port using the switchport mode access interface configuration command.

4. Optionally, enable PortFast for the access port using the spanning-tree portfast interface configuration command.

The following output illustrates how to configure the Fa0/1 interface on switch Access 1 to support the connected LAP:

This configuration can be validated using the show interfaces [name] switchport command as illustrated in the following output:

Configuring Cisco IOS DHCP for Lightweight Access Points

LAPs require IP addressing information from a DHCP server. This includes not only their IP address and default gateway but also the IP address of the WLC controller that they will contact to derive their configuration. The LAP takes the following sequence of steps when it boots up:

1. The LAP obtains IP addressing information from the DHCP server. This information also contains the IP address of one or more WLCs.

2. The LAP sends a Join Request to the first WLC in the list (if more than one is available). If no response is received from the first WLC, the next is attempted, and so forth.

3. The WLC compares the images stored on the LAP to its local release. If these images are not the same, the LAP downloads and installs the updated image and reboots.

4. The WLC and the LAP build either a secure LWAPP or CAPWAP tunnel for management traffic and an unsecure LWAPP or CAPWAP tunnel for wireless clients.

In most networks, a dedicated DHCP server provides IP addressing information to all network hosts, which may include workstations, laptops, IP phones, and LAPs. However, it is also possible to configure the Cisco IOS DHCP server to perform the same functionality. Although basic IOS DHCP configuration is described in detail in the CCNA course, additional configuration is required to provide the LAPs with the IP address, or addresses, of one or more WLCs. This address must be provided using DHCP Option 43, which returns vendor-specific information to the LAPs.

RFC 2132 defines that DHCP servers must return vendor-specific information as DHCP Option 43. The RFC allows vendors to define encapsulated vendor-specific sub-option codes between 0 and 255. The sub-options are included in the DHCP offer as Type/Length/Value (TLV) blocks embedded within Option 43. The definition of the sub-option codes and their related message format is left to the different vendors.

When configuring a Cisco IOS DHCP server to provide IP addressing information to LAPs, the IP address, or addresses, or the WLC are defined within the DHCP using the option 43 [ascii|hex] [string] DHCP pool configuration command. The [ascii] keyword allows you to specify the IP address of the WLC using dotted-decimal notation, while the [hex] keyword allows you to specify the IP address of the WLC in Hexadecimal format. However, this option is beyond the scope of the SWITCH exam requirements and is not described in this section. The following output illustrates how to configure a Cisco IOS DHCP server for network 192.168.5.0/24, with a default gateway of 192.168.5.1, and specify three WLC IP addresses to be used by the LAPs:

Although this configuration cannot be validated using the show ip dhcp pool [name] command, it can be viewed by looking at the configuration as illustrated below:

NOTE: DHCP is required only if the LAP and the WLC are not in the same VLAN. If they are, the LAP simply Broadcasts out a Join Request and a WLC will respond to this request.

Configuring Switch Ports for Wireless LAN Controllers

As is the case with switch ports connected to Autonomous APs, switch ports connected to WLCs should always be configured as trunk links. However, unlike Autonomous APs, WLCs may have more than one physical interface and therefore an EtherChannel can be configured between the WLC and the switch; however, keep in mind that it will also still be a trunk port. The switch configuration required to support a WLC will be based on Figure 9-17 below:

Fig. 9-17.  Wireless LAN Controller Wireless Network Topology

NOTE: The topology above assumes that all pertinent configurations on the WLC have already been applied and the VLANs are already configured on the Distribution switches. In addition to this, the WLC is connected to only a single Distribution Layer switch (for simplicity) using a single interface. The steps required to configure the Distribution Layer switch to support the WLC are as follows:

1. Select the desired interface and configure it as a Layer 2 port using the switchport interface configuration command.

2. Configure the trunking encapsulation for the switch port using the switchport trunk encapsulationinterface configuration command.

3. Configure the switch as a trunk port using the switchport mode trunk interface configuration command.

4. Optionally, enable PortFast for the trunk port using the spanning-tree portfast trunk interface configuration command.

The following output illustrates how to configure the Gi2/1 interface on switch Distribution 1 to support the connected WLC:

This configuration can be validated using the show interfaces [name] switchport command as illustrated in the following output:

As is the case when you are configuring switch ports connected to support LAPs, it is important to always use theswitchport trunk allowed vlan [range] command on the trunk link connected to the WLC. This command should always be used to permit only the VLANs that exist on the WLC.

In addition, always remember that enabling the PortFast feature on a trunk port can result in loops, as seen in the warning above. While this allows the port to transition to the forwarding state much faster, it is important to understand the topology before this is applied to the port.