Cisco LAN2LAN Software

Document ID: 14999

Contents

Introduction
Prerequisites
      Requirements
      Components Used
Considerations
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Introduction

This document discusses LAN2LAN response time considerations.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Considerations

When you configure LAN²LAN Wide-Area Networks (WANs) with multiple links at low speeds, make sure you take into account the delays which will be experienced by interactive users. The line speed of the LAN²LAN links should be selected to provide the level of service desired. Where higher speed lines are not readily available, you can use rotary links to reduce the average utilization of the links and provide better response times.

In the three-LAN internet illustrated here, users on LAN 1 can access services on LAN 3 via Router B. When they do so, the packets in the interaction are delayed over a normal interaction on the same LAN because of the two hops in each direction on the low-speed communications lines.

These tables show some of the delays that can occur. The figures are in seconds and are given for the one-way trip for two different packet sizes, at five different link speeds over one and two hops, and for both lightly and heavily loaded links.

                            RouterB
                               |
                               |
                              LANB
                               |
                               |
                              LAN
                              100
                               |
                               |
                     +---------+---------+
                     |                   |
                     |                   |
           LAN1----LANB                 LANB----LAN3
                    |                     |
                    |                     |
                 RouterA               RouterC

Note that if a link is overloaded, the delay might exceed that shown. Since packets are discarded when the queue is full, in cases of extreme overloads the source might need to retry to send the packet.

In a NetWare environment, where the request/response mode is the norm, extreme overloads occur only if the number of simultaneous conversations and their resulting aggregate bandwidth requirement is grossly excessive compared to the bandwidth of the link. The possibility of this condition is best determined empirically.

Table: One-Way Delay for LAN²LAN segments (in seconds)
------------------------------+------------------------------
# of Hops   1          2      |  # of Hops   1          2
------------------------------|------------------------------
         Speed (Kbps)         |         Speed (Kbps)-
=============================================================
4.8        .16        .32     |  4.8        .92        1.84
9.6        .08        .16     |  9.6        .46        .92
19.2       .04        .08     |  19.2       .23        .46
56.0       .01        .02     |  56.0       .08        .16
64.0       .01        .02     |  64.0       .07        .14
------------------------------|------------------------------
Light load, 100-byte packet   |  Light load, 57-byte packet
------------------------------+------------------------------

Table: One-Way Delay for LAN²LAN segments (in seconds)
------------------------------+------------------------------
# of Hops   1          2      |  # of Hops   1          2
------------------------------|------------------------------
        Speed (Kbps)-         |         Speed (Kbps)
=============================================================
4.8        .32        .64     |  4.8        1.84       3.68
9.6        .16        .32     |  9.6        .92        1.84
19.2       .08        .16     |  19.2       .46        .92
56.0       .02        .04     |  56.0       .16        .32
64.0       .02        .04     |  64.0       .14        .28
------------------------------|------------------------------
Heavy load, 100-byte packet   |  Light load, 576-byte packet
------------------------------+------------------------------

Table: One-Way Delay for LAN²LAN
       segments (in seconds)
---------------------------------
# of Hops   1          2
---------------------------------
       Speed (Kbps)
=================================
4.8        .96        1.92
9.6        .48        .96
19.2       .24        .48
56.0       .04        .09
64.0       .04        .08
---------------------------------
Round trip, heavy load, 100-byte
request, 200 byte response
---------------------------------

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