Cisco Cisco Aironet 340 Access Point-RP-TNC Guía Para Resolver Problemas

The first step in VPN setup is to accommodate for the use of the IP Security (IPSec) technology, which is
incorporated within the VPN technology. IPSec uses encryption technology to provide data confidentiality,
integrity, and authenticity between participating peers in a private network.

IPSec defines a new set of headers that are added to IP datagrams. These headers are placed after the IP
header and before the Layer 4 protocol (typically Transmission Control Protocol [TCP] or User Datagram
Protocol [UDP]). The result is that the packets go from the local network where the PC is installed through to
the internet. These packets are a larger size than non−encrypted packets. The increased size can cause
problems to devices that expect normal size packets, because the receiving devices see them as oversized
packets.

Figure 1 shows how the IPSec header fits within a normal packet.

In order to ensure that receiving devices do not perceive the packets as oversized, you must adjust the size of
the Maximum Transmission Unit (MTU) on the PC/host side. Adjust the total maximum size that the packet
can take so that it does not exceed the normal size of a non encrypted Ethernet packet. VPN applications
typically provides the option to customize the MTU size.

Complete these steps to adjust the MTU in a Cisco Systems VPN client within Microsoft Windows:

Choose Start > Programs > Cisco Systems VPN Client > Set MTU. This window opens:

1.