The Cisco Content Switching Module (CSM) supports load-balancing
algorithms with the predictor command. Issue the
predictor command in the Server Load Balancing (SLB)
serverfarm configuration submode to specify the load-balancing algorithm for
the server farm.
For more information on document conventions, see the
Technical Tips Conventions.
There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware
The information presented in this document was created from devices in
a specific lab environment. All devices used in this document started with a
cleared (default) configuration. If you are working in a live network, ensure
that you understand the potential impact of any command before using it.
The default algorithm,
, keyword directs the network
connection to the next server, and treats all servers as equals, regardless of
the number of connections or response time. Although the CSM round-robin
predictor appears similar to a Domain Name System (DNS) round-robin, it is
superior because no propagation delay or caching hinders the algorithm.
network connections to the server with the fewest connections. Although it may
not be intuitively obvious that the
predictor would provide
effective load balancing, in fact, it is quite successful. On Web sites where
there is a collection of servers with similar performance, the
predictor is effective in
smoothing distribution when a server becomes bogged down. On Web sites where
there are large differences in the capacity of various servers, the
predictor is also very
effective. In maintaining the same number of connections to all servers, those
servers that are capable of processing (and thus terminating) connections the
fastest receive more connections over time. A server deemed to be twice as
powerful as another server receives about twice as many connections per second.
keyword allows you
to assign a performance weight to each server. Weighted load balancing is
similar to the function of the
servers with a higher weight value receive a larger percentage of connections
at any one time. CSM administrators can assign a weight to each real server,
and the CSM uses this weight to determine the percentage of the current number
of connections to give each server.
Issue the weight command in the SLB real
server configuration submode to configure the capacity of the real servers in
relation to the other real servers in the server farm. Issue the
no form of this command to change the server's
weight to its default capacity.
weighting-value is the value to use for the server farm predictor
algorithm. The range is from 1 to 100. The default weight is 8. For example, in
a configuration with five servers, the percentage of connections is calculated
Server Number Number of Connections
Weight of server 1 7
Weight of server 2 8
Weight of server 3 2
Weight of server 4 2
Weight of server 5 5
Total weight of all servers 24
This distribution results in server 1 getting 7/24 of the current
number of connections, server 2 getting 8/24, server 3 getting 2/24, and so on.
If a new server, server 6, is added with a weight of 10, it receives 10/34, and
Source or destination IP hash method maps the IP address in the request
to a real in a server farm. The configuration options are shown below.
predictor ip-hash netmask [source | destination] [netmask]
!--- Source IP hash or predictor hash address.
The default (netmask) is 255.255.255.255. The hash value is produced
with the following algorithm:
ip_addr = (Src_or_Dest_Ip_addr & Cfg_Netmask) hash_index =
(ip_addr) + (ip_addr >> 8) + ip_addr >> 16) + (ip_addr >>
URL hashing maps the URL (or portion) for the request to a real in a
server farm. To specify only a portion of the URL, you need to configure the
starting and/or ending keyword strings. This configuration is per vserver
url-hash begin-pattern str [end-pattern str]
All characters in the keywords are included in the hash-value. In order
to map it to a real, the CSM looks at the first few bits in the hash-value. If
this real is disabled, the CSM finds the next available real in the list. In
release 2.2(3) and later, the CSM looks at the higher bits in the hash-value if
the first hash bits mapped to a disabled real. If the CSM cannot find an
enabled real, it looks sequentially for the next available real.
keyword to tell
the CSM to forward traffic in accordance with its internal routing tables.
command has no effect when the
command is configured; this is because the servers
cannot be configured.