Wireless bridging provides a simple method for connecting building
sites without cabling or can be used as a backup to existing wired links. If
you have hundreds of nodes or bandwidth-hungry applications and data
transmitting between sites, bridging your networks will require more than 11
Mbps provided by the 802.11b standard. However, by using the following
Cisco-tested design, you can easily and effectively aggregate and load balance
the bandwidth of three 802.11b-compliant Cisco Aironet® bridges to support up
to a 33-Mbps half-duplex connection between bridge locations.
The use of standard technology and protocols including virtual LANs
(VLANs), VLAN trunks, equal-cost load balancing, and routing protocols makes
this design easy to configure and troubleshoot. More importantly, it makes
support from the Cisco Technical Assistance Center (TAC) possible.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
Load balancing is a concept that allows a router to take advantage of
multiple best paths (routes) to a given destination. When a router learns
multiple routes to a specific network -- via static routes or through routing
protocols -- it installs the route with the lowest administrative distance in
the routing table. If the router receives and installs multiple paths with the
same administrative distance and cost to a destination, load balancing will
occur. In this design, the router will see each wireless bridge link as a
separate, equal-cost link to the destination.
Note: The use of equal-cost load balancing and the routing protocols
mentioned in this article are a Cisco-supported means of aggregating Cisco
Aironet bridges for additional throughput between sites or as a redundant
failover wireless bridge link.
If your design requires failover capabilities, the use of a routing
protocol is required. A routing protocol is a mechanism to communicate paths
between routers and can automate the removal of routes from the routing table,
which is required for failover capabilities. Paths can be derived either
statically or dynamically through the use of routing protocols such as Routing
Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced
IGRP, and Open Shortest Path First (OSPF). The use of dynamic routes for load
balancing over equal-cost wireless bridge routes is highly recommended because
it is the only means available for automatic failover. In a static
configuration, if one bridge fails, the Ethernet port of the other bridge will
still be active and packets will be lost until the problem is resolved.
Therefore, the use of floating static routes will not work for failover
With routing protocols there is a tradeoff between fast convergence and
increased traffic needs. Large amounts of data traffic between sites can delay
or prevent communication between routing protocol neighbors. This condition can
cause one or more of the equal-cost routes to be removed temporarily from the
routing table, resulting in inefficient use of the three bridge links.
The design presented here was tested and documented using Enhanced IGRP
as the routing protocol. However, RIP, OSPF, and IGRP could also be used. The
network environment, traffic load and routing protocol tuning requirements will
be unique to your situation. Select and configure your routing protocol
The active forwarding algorithm determines the path that a packet
follows while inside a router. These are also referred to as
switching algorithms or switching
paths. High-end platforms have typically more powerful forwarding
algorithms available than low-end platforms, but often they are not active by
default. Some forwarding algorithms are implemented in hardware, some are
implemented in software, and some are implemented in both, but the objective is
always the same -- to send packets out as fast as possible.
Process switching is the most basic way of handling a packet. The
packet is placed in the queue corresponding to the Layer 3 protocol while the
scheduler schedules the corresponding process. The waiting time depends on the
number of processes waiting to run and the number of packets waiting to be
processed. The routing decision is then made based on the routing table and the
Address Resolution Protocol (ARP) cache. After the routing decision has been
made, the packet is forwarded to the corresponding outgoing interface.
Fast switching is an improvement over process switching. In fast
switching, the arrival of a packet triggers an interrupt, which causes the CPU
to postpone other tasks and handle the packet. The CPU immediately does a
lookup in the fast cache table for the destination Layer 3 address. If it finds
a hit, it rewrites the header and forwards the packet to the corresponding
interface (or its queue). If not, the packet is queued in the corresponding
Layer 3 queue for process switching.
The fast cache is a binary tree containing destination Layer 3
addresses with the corresponding Layer 2 address and outgoing interface.
Because this is a destination-based cache, load sharing is done per destination
only. If the routing table has two equal cost paths for a destination network,
there is one entry in the fast cache for each host.
Both fast switching and Cisco Express Forwarding (CEF) switching were
tested with the Cisco Aironet bridge design. It was determined that Enhanced
IGRP dropped neighbor adjacencies under heavy loads less often using CEF as the
switching path. The main drawbacks of fast switching include:
The first packet for a particular destination is always process
switched to initialize the fast cache.
The fast cache can become very big. For example, if there are
multiple equal-cost paths to the same destination network, the fast cache is
populated by host entries instead of the network.
There's no direct relation between the fast cache and the ARP table.
If an entry becomes invalid in the ARP cache, there is no way to invalidate it
in the fast cache. To avoid this problem, 1/20th of the cache is randomly
invalidated every minute. This invalidation/repopulation of the cache can
become CPU intensive with very large networks.
CEF addresses these issues by using two tables: the forwarding
information base table and the adjacency table. The adjacency table is indexed
by the Layer 3 addresses and contains the corresponding Layer 2 data needed to
forward a packet. It is populated when the router discovers adjacent nodes. The
forwarding table is an mtree indexed by Layer 3 addresses. It is built based on
the routing table and points to the adjacency table.
While another advantage of CEF is the ability to allow load balancing
per destination or per packet, the use of per-packet load balancing is not
recommended and was not tested in this design. Bridge pairs may have different
amounts of latency, which could cause problems with per-packet load balancing.
Quality of Service (QoS) features can be used to increase the
reliability of routing protocols. In situations with heavy traffic loads,
congestion management or avoidance techniques can prioritize routing protocol
traffic to ensure timely communication.
Setting the Fast Ethernet bridge ports and associated Layer 2 switch
ports to 10-Mbps full duplex will increase reliability by causing congestion to
be queued at the switch instead of the bridge, which has limited buffers.
For designs that require the emulation of full duplex links, it's
possible to configure the administrative distance of the equal-cost links
between sites to create two unidirectional links. With this design, the third
bridge set could be used as a failover link or not be installed at all. Note
that this specific design was not tested.
Configure bridge pair 1 to have a relatively low administrative
Configure bridge pair 2 to have a relatively high administrative
Configure bridge pair 3 to have a relatively medium administrative
Configure bridge pair 1 to have a relatively high administrative
Configure bridge pair 2 to have a relatively low administrative
Configure bridge pair 3 to have a relatively medium administrative
Traffic will flow from site 1 to site 2 across bridge pair 1 and from
site 2 to site 1 across bridge pair 2. In the event that either bridge pair
fails, bridge pair 3 will work as the failover link. See your specific routing
protocol documentation for more information on how to configure the
EtherChannel® is another technology that can be used to aggregate
bridges into a virtual single link. Using EtherChannel for this purpose is not
recommended, however, as it is not a supported design by Cisco and the Cisco
TAC. Furthermore, you will be unable to manage some bridges via TCP/IP due to
the way EtherChannel works. The port aggregation protocol (PagP) is not a
tunable protocol and failover support is limited.
There are few wieless attributes that need to be taken care in order to
increase the wireless bandwidth .
802.11n technology povides higher data rates of up to 600 Mbps. It can
interoprate with 802.11b and 802.11g clients. Refer toConfigure
802.11n on the WLC for more information on 802.11n.
As a general rule, as clients move farther away from the Access Point,
signal strength increases and therefore the data rates decreases. If the client
is closer to the AP, then the data rate is higher.
QoS is a technique that is used in order to prioritize certain packets
over other packets. For example, a voice application heavily depends on QoS for
uninterrupted communication. As of late WMM and 802.11e have emerged
specifically for wireless application. Refer to
Wireless LAN Controller Command Reference, Release 6.0 for more
In an environemnt where homogeneous clients are found to exist, data
rates are higher than in a mixed environment. For example, the presence of
802.11b clients in a 802.11g environment, 802.11g has to implement a protection
mechanism in order to co-exist with the 802.11b client, and therefore results
in decreased data rates.
The following information is specifically related to the actual testing
of the aggregation of three Cisco Aironet 350 Series bridges. The equipment
used included six Cisco Aironet 350 bridges, two Cisco Catalyst® 3512 XL
switches, and two Cisco 2621 routers. This design may also be used with two
bridge pairs instead of three. The test design used Enhanced IGRP as the
routing protocol with equal-cost load balancing, and CEF as the forwarding
Most likely you will be using some hardware other than the specific
models tested. Here are some guidelines when choosing the equipment to be used
to aggregate bridges.
The routers used for testing had two Fast Ethernet (100-Mbps) ports and
supported 802.1q trunking and CEF-based switching. It's possible to use a
single 100-Mbps port to trunk all traffic to and from a switch. However, the
use of a single Fast Ethernet port was not tested and could interject unknown
issues or negatively impact performance. A router with four Fast Ethernet ports
would not require the use of a VLAN trunking protocol. Other router
For 802.1q trunking support, Cisco 2600 and 3600 Series routers
require Cisco IOS® Software Release 12.2(8)T or higher.
If the routers don't support 802.1q trunking, check if they support
ISL trunking, a Cisco proprietary trunking mechanism that can be used in place
of 802.1q. Before you configure the routers, verify that your switch supports
For Cisco 2600 and 3600 Series routers, IP Plus code is required for
802.1q trunk support (this would be a cost upgrade from IP code).
Depending on the hardware and its intended use, the base flash and
DRAM may need to be increased. Take into consideration additional
memory-intensive processes such as CEF tables, routing protocol requirements,
or other processes running on the router that are not specifically related to
the bridge aggregation configuration.
CPU utilization may be a consideration depending on the configuration
and features used on the router.
(registered customers only)
for Cisco IOS Software support for
IEEE 802.1q VLAN trunking on your specific hardware platform.
The switches in the tested design require support for VLANs and 802.1q
trunking. Using inline power-enabled switches such as the Cisco Catalyst
3524PWR when using Cisco Aironet 350 Series bridges is recommended, as this
will make the setup less cumbersome. To collapse the switch and routing
functionality into a single box, the Catalyst 3550 was tested and works quite
Using Cisco Aironet 340 Series bridges will work as well, but the
configuration would be slightly different since the Cisco Aironet 340 uses
10-Mbps half duplex Ethernet ports and a different operating system.
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