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This document describes Domain Name System (DNS) server Round Robin functionality for the Cisco AS5800. The information in this document includes benefits, supported platforms, related documents, and so forth.
This document includes the following sections:
The Domain Name System (DNS) is a distributed database for objects in a computer network. The basic function of name servers is to provide information about network objects by answering queries. By using a name server approach, the network can be organized into a hierarchy of autonomous domains. The name space is organized as a tree that often resembles the organizations that are responsible for the administration boundaries.
You can configure the Network Registrar DNS server and zones by accepting the system defaults or changing them through the Network Registrar graphical user interface (GUI) or the (CLI) tool.
In a multiple server configuration without the DNS round robin functionality, the first host server/IP address is used for the whole time to live (TTL) of the cache, and uses the second and/or third only in the event of host failure. This presents a problem when a high volume of users all arrive at the first host during the TTL time. The network access server (NAS) then sends out a DNS query; the DNS servers reply with a list of the configured IP addresses to the NAS. The NAS then caches these IP addresses for a given time (for example, five minutes). All users that dial in during the five minute TTL time will all land on one host, the first IP address in the list.
In a multiple server configuration with the DNS round robin functionality, the DNS server returns the IP address of all hosts to rotate between the cache of host names. During the TTL of the cache, users would be distributed among the hosts. This functionality distributes calls across the configured hosts and reduces the amount of DNS queries.
A scheduling algorithm in which processes are activated in a fixed cyclic order. Those processes which cannot proceed because they are waiting for some event (for example, termination of a child process or an input/output operation), simply return control to the scheduler. The benefit of round-robin scheduling is its simplicity --- only the processes themselves need to know what they are waiting for or how to tell if it has happened. However, if the TTL of the process times out just before the event for which it is waiting occurs, then the event will not get handled until all the other processes have been activated.
The round robin algorithm specifies that the real server used for a new connection to the virtual server is chosen from the server farm in a circular fashion. Sending out packets from a queue until the byte count on the queue is met, then moving on to the next queue. This ensures that no queue gets starved.
Enabling DNS Round Robin functionality ensures successive clients resolving the same name will connect to different IP addresses. This operation causes the Network Registrar DNS server to rearrange the order of the records each time it is queried.
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Cisco IOS IP and IP Routing Configuration Guide, Release 12.1
Cisco AS5800
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This section documents new and modified commands. All other commands used with these features are documented in the Cisco IOS Release 12.1 command references.
To enable round-robin functionality on a DNS server(s), use the ip domain round-robin global configuration command. Use the no form of the command to disable round-robin functionality.
ip domain round-robinSyntax Description
This command has no arguments or keywords.
Defaults
Round Robin is not enabled.
Command Modes
Global configuration
Command History
| Release | Modification |
|---|---|
12.1(1)AA | This command was first supported on the Cisco AS5800. |
Examples
This configuration allows a Telnet to "www.company.com" to connect to each of the three IP addresses specified in the following order: The first time the Telnet command is given, it would connect to "10.0.0.1"; the second time the command is given, it would connect to "20.0.0.1"; and the third time the command is given, it would connect to "30.0.0.1". In each case, the other two addresses would also be tried if the first one failed since this is the normal operation of the Telnet command.
ip host www.company.com 10.0.0.1 20.0.0.1 30.0.0.1 ip domain round-robin
Related Commands
| Command | Description |
|---|---|
ip domain-list | Defines a list of default domain names to complete unqualified host names. |
ip domain-lookup | Enables the IP Domain Name System-based host name-to-address translation. |
ip domain-name | Defines a default domain name that the Cisco IOS software uses to complete unqualified host names. |
ip host | Defines a static host name-to-address mapping in the host cache. |
Posted: Thu Apr 13 16:26:12 PDT 2000
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