Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 2 Cisco Unified Wireless Technology and Architecture
Broadcast and Multicast on the WLC
WLC Broadcast and Multicast Details
Broadcast and multicast traffic often require special treatment within a WLAN network because of the
additional load placed on then WLAN as a result of this traffic having to be sent at the lowest common
bitrate. This is done to ensure that all associated wireless devices are able to receive the
broadcast/multicast information.
additional load placed on then WLAN as a result of this traffic having to be sent at the lowest common
bitrate. This is done to ensure that all associated wireless devices are able to receive the
broadcast/multicast information.
The default behavior of the WLC is to block broadcast and multicast traffic from being sent out the
WLAN to other wireless client devices. The WLC can do this without impacting client operation because
most IP clients do not send broadcast/multicast type traffic for any reason other than to obtain network
information (DHCP) and resolve IP addresses to MAC addresses (ARP).
WLAN to other wireless client devices. The WLC can do this without impacting client operation because
most IP clients do not send broadcast/multicast type traffic for any reason other than to obtain network
information (DHCP) and resolve IP addresses to MAC addresses (ARP).
DHCP
The WLC acts as a DHCP relay agent for associated WLAN clients. It unicasts client DHCP requests to
a locally configured or upstream DHCP server except during L3 client roaming, which will be discussed
in more detail below. DHCP server definitions are configured for each dynamic interface, which in turn
is associated with one or more WLANs. DHCP relay requests are forwarded via the dynamic interfaces
using the source IP address of a given dynamic interface. Because the WLC knows which DHCP server
to use for a given interface/WLAN, there is no need to broadcast client DHCP requests out its wired and
wireless interfaces.
a locally configured or upstream DHCP server except during L3 client roaming, which will be discussed
in more detail below. DHCP server definitions are configured for each dynamic interface, which in turn
is associated with one or more WLANs. DHCP relay requests are forwarded via the dynamic interfaces
using the source IP address of a given dynamic interface. Because the WLC knows which DHCP server
to use for a given interface/WLAN, there is no need to broadcast client DHCP requests out its wired and
wireless interfaces.
The method above accomplishes the following:
•
It eliminates the need for DHCP requests to be broadcasted beyond the WLC.
•
The WLC becomes part of the DHCP process, thereby allowing it to learn the MAC / IP address
relationships of connected WLAN clients, which in turn allows the WLC to enforce DHCP policies
and mitigate against IP spoofing or denial-of-service (DoS) attacks.
relationships of connected WLAN clients, which in turn allows the WLC to enforce DHCP policies
and mitigate against IP spoofing or denial-of-service (DoS) attacks.
•
It allows the WLC to relay DHCP reply messages using a virtual DHCP server IP address rather than
the actual IP address of a DHCP server The aforementioned behavior is configured via the WLC’s
CLI, and is enabled by default. The virtual address is shared by all WLCs that comprise a mobility
group. The benefit of DHCP proxy is realized during L3 client roaming or when a client roams
across an AP group boundary. In these cases, the WLC will receive a client DHCP renewal request
upon which it will verify the client is roaming within the mobility group and allow the client to
renew (keep) it’s existing IP address/subnet assignment even though the client roamed to a new
subnet on the foreign WLC. See
the actual IP address of a DHCP server The aforementioned behavior is configured via the WLC’s
CLI, and is enabled by default. The virtual address is shared by all WLCs that comprise a mobility
group. The benefit of DHCP proxy is realized during L3 client roaming or when a client roams
across an AP group boundary. In these cases, the WLC will receive a client DHCP renewal request
upon which it will verify the client is roaming within the mobility group and allow the client to
renew (keep) it’s existing IP address/subnet assignment even though the client roamed to a new
subnet on the foreign WLC. See
Note
The virtual IP/Proxy DHCP behavior described above is required if the asymmetrical mobility tunnel
method is configured (default), see Roaming section above. Otherwise, if the symmetrical tunnel method
is used, WLC based DHCP proxy is not necessary because client traffic and DHCP requests are always
tunneled back to the anchor controller.
method is configured (default), see Roaming section above. Otherwise, if the symmetrical tunnel method
is used, WLC based DHCP proxy is not necessary because client traffic and DHCP requests are always
tunneled back to the anchor controller.
ARP
Before a WLAN client can send IP packets to any other IP address, it needs to know the MAC address
of the target client to forward the frame to. To accomplish this, a client will broadcast an ARP query,
requesting the MAC address for the IP host that it wishes to communicate with, see
of the target client to forward the frame to. To accomplish this, a client will broadcast an ARP query,
requesting the MAC address for the IP host that it wishes to communicate with, see
.