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X.25 Protocols

X.25 Load Balancing with Hunt Groups

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Document ID: 14212


Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Background Information
Configure
      Network Diagram
      Configurations
Verify
Troubleshoot
Related Information

Introduction

This document describes the use of "hunt groups" as a method of load balancing among a large number of virtual circuits (VCs). The feature was introduced in Cisco IOS® Software Release 12.0(3)T.

Prerequisites

Requirements

Before attempting this configuration, ensure that you have an understanding of these topics:

  • X.25 basics

  • X.25 route command

Components Used

The information in this document is based on these software and hardware versions:

  • Cisco IOS Software Release 12.2(10b)

  • Cisco 2500 series routers

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Note: Although the configuration in this document was tested using 2500 series routers running Cisco IOS Software Release 12.2(10b), the document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, refer to Cisco Technical Tips Conventions.

Background Information

As the number of users accessing the same host grows, competition for VCs becomes a problem. Internet service providers (ISPs) have needed to increase the number of users they can support by increasing the number of X.25 lines to the host.

In order to support a large number of VCs to a single destination, the configuration of more than one serial interface to that destination is required. By configuring a serial interface to support X.25, a fixed number of VCs can be made available for use.

However, the current X.25 method for allocating VCs across multiple serial lines fills one serial line to its VC capacity before a second line is utilized. As a result, the first serial line frequently carries maximum data traffic before it runs out of VCs.

When a facility called a hunt group is used as the method for X.25 load balancing, a switch treats a pool of X.25 lines directed to the same host as one address, and it assigns VCs on an "idle logical channel" basis. With this feature, X.25 calls can be load balanced among all configured outgoing interfaces to fully use and balance all managed lines.

Configure

In this section, you are presented with the information to configure the features described in this document.

Note: To find additional information on the commands used in this document, use the Command Lookup Tool (registered customers only) .

Network Diagram

This document uses the network setup shown in the diagram below.

x25_load_balancing-1.gif

Configurations

This document uses the configurations shown below.

System 1 (Sys1)

 
hostname Sys1
!
interface Serial0
 no ip address
 encapsulation x25
 x25 address 7777

Router A

 
 Current configuration: 
 hostname RouterA
 ! 
 x25 routing 
 ! 
 ! 
 interface Serial2/0 Serial0 
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 dce 
  no ip mroute-cache 
  x25 address 34567 
  x25 nvc 8 
  clockrate 56000 
 ! 
 interface Serial2/1 Serial1 
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 dce 
  no ip mroute-cache 
  x25 address 45678 4441 
  clockrate 56000 
 ! 
 interface Serial2/2 Serial2
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 dce 
  no ip mroute-cache 
  x25 address 12345 4442 
  x25 nvc 8 
  clockrate 56000 
 ! 
 interface Serial2/3 Serial3 
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 dce 
  no ip mroute-cache 
  x25 address 23456 4443
  x25 nvc 8 
  clockrate 56000 
 ! 

 ! 
 x25 route .* input-interface Serial2/0 substitute-dest 222 hunt-group lse 
 ! 
 x25 hunt-group lse vc-count interface Serial1 interface Serial2 interface Serial3
  int Serial2/2
  int Serial2/3 
  int Serial2/1 
 x25 route .* input-interface Serial0 substitute-dest 222 hunt-group lse

Router B

 
 hostname RouterB
 ! 
 x25 routing xot-use-interface-defaults 
 ! 
 interface Serial2/0
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 
  no ip mroute-cache 
  x25 address 1111 
  x25 nvc 8 
 ! 

 interface Serial2/1 Serial1
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 
  no ip mroute-cache 
  x25 address 222 
  x25 nvc 8 
 ! 
 interface Serial2/2 Serial2
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 
  no ip mroute-cache 
  x25 address 222 
  x25 nvc 8 
 ! 
 interface Serial2/3 Serial3 
  no ip address 
  no ip directed-broadcast 
  encapsulation x25 
  no ip mroute-cache 
  x25 address 222 
  x25 nvc 8 
 ! 
 x25 route .* interface Serial2/0

Verify

This section provides information you can use to confirm your configuration is working properly.

The simplest way to verify the setup is to initiate multiple packet assembler/disassembler (PAD) sessions across the parallel links. In the example shown below, three PAD sessions are opened to the X.25 address 222 from the device Sys1.


!--- Initiate the first session by issuing the pad command.

Sys1# pad 222
Trying 222...Open

User Access Verification

Password:
RouterB>

!--- After successful authentication, the command prompt of the target device 
!--- (Router B, in this case) is displayed. Enter Ctrl-Shift-6, followed by x.
!--- This returns you to the device Sys1, from which another PAD session can be 
!--- initiated to the X.25 address 222.

Sys1# pad 222
Trying 222...Open

User Access Verification

Password:
RouterB>

!--- To initiate the third session, again enter Ctrl-Shift-6, followed by x.
!--- This returns you to Sys1 where the third PAD session can be initiated,
!--- as shown below.

Sys1# pad 222
Trying 222...Open

User Access Verification

Password:
RouterB>

To verify that traffic is being distributed over the parallel paths, issue the show x25 vc command on Router A or Router B. The output below is from Router B.

Note: Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.

RouterB# show x25 vc
SVC 1,  State: D1,  Interface: Serial1
   Started 00:01:43, last input 00:01:41, output 00:01:41

   Line: 5 vty 0 Location: Host: 7777
   7777 connected to 222 PAD <--> X25

   Window size input: 2, output: 2
   Packet size input: 128, output: 128
   PS: 4 PR: 2 ACK: 2 Remote PR: 4 RCNT: 0 RNR: no
   P/D state timeouts: 0 timer (secs): 0
   data bytes 62/62 packets 4/10 Resets 0/0 RNRs 0/0 REJs 0/0 INTs 0/0
SVC 1, State: D1, Interface: Serial2
   Started 00:01:38, last input 00:01:35, output 00:01:35

   Line: 6 vty 1 Location: Host: 7777
   7777 connected to 222 PAD <--> X25

   Window size input: 2, output: 2
   Packet size input: 128, output: 128
   PS: 4 PR: 2 ACK: 2 Remote PR: 4 RCNT: 0 RNR: no
   P/D state timeouts: 0 timer (secs): 0
   data bytes 62/62 packets 4/10 Resets 0/0 RNRs 0/0 REJs 0/0 INTs 0/0
SVC 1, State: D1, Interface: Serial3
   Started 00:01:34, last input 00:01:31, output 00:01:31

   Line: 7 vty 2 Location: Host: 7777
   7777 connected to 222 PAD <--> X25

   Window size input: 2, output: 2
   Packet size input: 128, output: 128
   PS: 4 PR: 2 ACK: 2 Remote PR: 4 RCNT: 0 RNR: no
   P/D state timeouts: 0 timer (secs): 0
   data bytes 62/62 packets 4/10 Resets 0/0 RNRs 0/0 REJs 0/0 INTs 0/0
   RouterB#

The output above shows that three switched virtual circuits (SVCs) are initiated, one each over Serial1, Serial2 and Serial3. If a fourth session is initiated from the device Sys1, the fourth SVC is built over interface Serial1. As this is the second VC built over Serial1, it is numbered SVC 2.

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Debugging for X.25 can be enabled by issuing the debug x25 events command. This command shows X.25 traffic without normal data and relative rate (RR) packets.

Note: Before issuing debug commands, refer to Important Information on Debug Commands.

RouterB# debug x25 events
   X.25 special event debugging is on

Note: The output of issuing the debug x25 events command on Router B is shown below. In this case, three PAD sessions are initiated.

Note: Some of the output below appears on two lines due to space limitations.

RouterB#
   
!--- The first PAD session is initiated over interface Serial1.

   *Mar 1 02:39:50.731: Serial1: X.25 I R1 Call (19) 8 lci 1
   *Mar 1 02:39:50.731: From (4): 7777 To (3): 222
   *Mar 1 02:39:50.735: Facilities: (6)
   *Mar 1 02:39:50.739: Call redirection/deflection notice, reason 0x80 specified by 
    source(3): 222
   *Mar 1 02:39:50.743: Call User Data (4): 0x01000000 (pad)
   *Mar 1 02:39:50.751: Serial1: X.25 O R1 Call Confirm (3) 8 lci 1
 
  !--- The second PAD session is initiated over interface Serial2.

   *Mar 1 02:40:08.491: Serial2: X.25 I R1 Call (19) 8 lci 1
   *Mar 1 02:40:08.495: From (4): 7777 To (3): 222
   *Mar 1 02:40:08.499: Facilities: (6)
   *Mar 1 02:40:08.499: Call redirection/deflection notice, reason 0x80 specified by 
    source(3): 222
   *Mar 1 02:40:08.503: Call User Data (4): 0x01000000 (pad)
   *Mar 1 02:40:08.515: Serial2: X.25 O R1 Call Confirm (3) 8 lci 1
   
!--- The third PAD session is initiated over interface Serial3.

   *Mar 1 02:40:26.323: Serial3: X.25 I R1 Call (19) 8 lci 1
   *Mar 1 02:40:26.327: From (4): 7777 To (3): 222
   *Mar 1 02:40:26.331: Facilities: (6)
   *Mar 1 02:40:26.331: Call redirection/deflection notice, reason 0x80 specified by 
    source(3): 222
   *Mar 1 02:40:26.335: Call User Data (4): 0x01000000 (pad)
   *Mar 1 02:40:26.347: Serial3: X.25 O R1 Call Confirm (3) 8 lci 1
   RouterB#

Related Information

Updated: Feb 07, 2007 Document ID: 14212