Table Of Contents
Pre-Fragmentation for IPsec VPNs
First Published: March 1, 2002
Last Updated: March 16, 2011
The Pre-Fragmentation for IPsec VPNs feature increases performance between Cisco IOS routers and VPN clients by delivering encryption throughput at maximum encryption hardware accelerator speeds for packets that are near the maximum transmission unit (MTU) size. Packets are fragmented into equally sized units to prevent further downstream fragmentation.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Pre-Fragmentation for IPsec VPNs" section.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Restrictions for Pre-Fragmentation for IPsec VPNs
Take the following information into consideration before this feature is configured:
•Pre-fragmentation for IPsec VPNs operates in IPsec tunnel mode and IPsec tunnel mode with GRE, but not with IPsec transport mode.
•Pre-fragmentation for IPsec VPNs configured on the decrypting router in a unidirectional traffic scenario does not improve the performance or change the behavior of either of the peers.
•Pre-fragmentation for IPsec VPNs occurs before the transform is applied if compression is turned on for outgoing packets.
•Pre-fragmentation for IPsec VPNs functionality depends on the egress interface crypto ipsec df-bit configuration and the incoming packet "do not fragment" (DF) bit state. See Table 1.
Information About Pre-Fragmentation for IPsec VPNs
Pre-fragmentation for IPsec VPNs
When a packet is nearly the size of the MTU of the outbound link of the encrypting router and it is encapsulated with IPsec headers, it is likely to exceed the MTU of the outbound link. This causes packet fragmentation after encryption. The decrypting router must then reassemble these packets in the process path, which decreases the decrypting router's performance.
The Pre-fragmentation for IPsec VPNs feature increases the decrypting router's performance by enabling it to operate in the high-performance CEF path instead of the process path. An encrypting router can predetermine the encapsulated packet size from information available in transform sets, which are configured as part of the IPsec security association (SA). If it is predetermined that the packet exceeds the MTU of the output interface, the packet is fragmented before encryption. This function avoids process-level reassembly before decryption and helps improve decryption performance and overall IPsec traffic throughput.
Note The pre-fragmentation feature is turned off by default for tunnel interfaces. To receive pre-fragmentation performance benefits, turn pre-fragmentation on after ensuring that the tunnel interfaces have the same MTU on both ends.
Crypto maps are no longer used to define fragmentation behavior that occurred before and after encryption. Now, IPsec Virtual Tunnel Interface (also referred to as Virtual-Template interface) (VTI) fragmentation behavior is determined by the IP MTU settings that are configured on the VTI.
See the IPsec Virtual Tunnel Interface feature document for more information on VTIs.
Note If fragmentation after-encryption behavior is desired, then set the VTI IP MTU to a value that is greater than the egress router interface IP MTU. Use the show ip interface tunnel command to display the IP MTU value.
How to Configure Pre-Fragmentation for IPsec VPNs
This section contains the following task:
Configuring Pre-Fragmentation for IPsec VPNs
Perform this task to configure Pre-Fragmentation for IPsec VPNs.
2. configure terminal
3. interface type number
4. ip mtu bytes
Related Topic Document Title
Cisco IOS commands
IPsec Virtual Tunnel Interface feature document
MIB MIBs Link
To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:
Feature Information for Pre-Fragmentation for IPsec VPNs
Table 2 lists the release history for this feature.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 2 lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
© 2002-2011 Cisco Systems, Inc. All rights reserved.