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config-if-ssid)#
infrastructure-ssid
R2(config-if-ssid)#
no shut
5.3 Identify the basic parameters to configure on a wireless network
294
Chapter 5
Explain and select the appropriate administrative tasks
So, what we see here is that everything is pretty commonplace until we get to the SSID configuration.
This is the Service Set Identifier that creates a wireless network that hosts can connect to.
Unlike access points, the interface on the router is actually a routed interface, which is the
reason why the IP address is placed under the physical interface—typically, the IP address
would be placed under the management VLAN or Bridge-Group Virtual Interface (BVI).
That
guest-mode
line means that the interface will broadcast the SSID so that wireless
hosts will understand that they can connect to this interface.
Authentication open
means just that . . . no authentication. (Even so, you still have to
type that command in at minimum to make the wireless interface work.)
Last, the
infrastructure-ssid
indicates that this interface can be used to communicate to
other access points, or other devices on the infrastructure—to the actual wired network itself.
But wait, we’re not done yet—we still need to configure the DHCP pool for the wireless clients:
R2#
config t
R2(config)#
ip dhcp pool Admin
R2(dhcp-config)#
network 10.1.8.0 255.255.255.0
R2(dhcp-config)#
default-router 10.1.8.1
R2(dhcp-config)#
exit
R2(config)#ip dhcp excluded-address 10.1.8.1
R2(config)#
Creating DHCP pools on a router is actually a pretty simple process. To do so, you just
create the pool name, add the network/subnet and the default gateway, and exclude any
addresses you don’t want handed out (like the default gateway address). And you’d usually
add a DNS server as well.
Understand that the pool is basically attached to an interface that has an address from the
same subnet created by the DHCP pool. In the above example, this is interface dot11radio
0/3/0. We can easily create another pool and have it connected with a LAN interface as in
FastEthernet 0/0 by assigning an address on FastEthernet 0/0 that is from the subnet pool.
Service Sets
There are typically two types of wireless networks that you can create with wired networks:
Basic Service Set (BSS)
Extended Service Set (ESS)
Both types of networks define what we call a Service Set ID (SSID) that’s used to advertise
your wireless network so hosts can connect to the access point (AP). And you can have multiple
SSID’s configured on an access point for security reasons. For example, you can designate
that one SSID is open access for a public hot spot, while another SSID can use WEP or WPA2
for the employees that work at this public hot spot. The SSID name is broadcasted out the AP
by default so the clients can find the AP and connect to the wireless network, and of course you
can turn this feature off for security reasons.
295
BSS/IBSS
A BSS only involves a single access point. You create a BSS and by bringing up an AP and creating
a name for the service set ID (SSID). Users can then connect to and use this SSID to access
the wireless network, which provides connectivity to the wired resources. When the AP connects
to a wired network, it then becomes known as an Infrastructure basic service set, or IBSS. Keep
in mind that if you have a BSS/IBSS, users won’t be able to maintain network connectivity when
roaming from AP to AP because each AP is configured with a different SSID name.
BSS wireless networks are also really helpful if you happen to have a couple hosts that need
to establish wireless communication directly between just them. You can also make this happen
through something we call ad-hoc networking, but if you have an AP between the hosts
it’s just called a BSS.
Figure 5.5 shows a basic service set using one SSID:
FIGURE 5 . 5 Basic Service Set (BSS)
ESS
Mobile wireless clients can roam around within the same network if you set all your access points
to the same Service Set ID (SSID). Doing this creates an extended service set (ESS). Figure 5.6 shows
four AP’s configured with the same SSID in an office thereby creating the ESS network:
For users to be able to roam throughout the wireless network—from AP to AP without losing
their connection to the network—all APs must overlap by at least 10% or more, and the channels
on each AP shouldn’t be set the same either. And remember, in an 802.11b/g network, there are
only three non-overlapping channels (1, 6, 11) so design is super important here!
Exam Objectives
Remember how to set a service set identifier (SSID) on a wireless routed interface. From
the interface mode of the wireless routed interface, used the ssid ssid-name command. This
is the service set identifier that creates a wireless network that hosts can connect to.
5.3 Identify the basic parameters to configure on a wireless network
296 Chapter 5 Explain and select the appropriate administrative tasks
FIGURE 5 . 6 Extended Service Set (ESS)
Remember how to configured a wireless interface on a router to allow hosts to communicate
to a wired infrastructure. Under the Router(config-if-ssid)# command prompt, use
the command infrastructure-ssid to indicate that this interface can be used to communicate
to other access points, or to the wired network.
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