This document discusses all the available IPsec fragmentation options in a Cisco VPN 3000 Concentrator. The IPsec fragmentation policy specifies how to treat packets that exceed the MTU setting when tunneling traffic through the public interface.
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The information in this document is based on the Cisco VPN 3000 Series Concentrator with version 4.7.
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The IPsec Fragmentation Policy feature provides a way to handle cases where a router or Network Address Translation (NAT) device between the VPN Concentrator and the VPN Client rejects or drops IP fragments. For example, a client wants to use FTP get from an FTP server behind a VPN Concentrator. The FTP server transmits packets that when encapsulated, exceed the MTU size of the VPN Concentrator on the public interface.
The fragmentation policy you set here applies to all traffic that travels out of the VPN Concentrator public interface to clients that run VPN Client software version 3.6 or later. The second and third options described here can affect performance. VPN Clients that run software versions earlier than 3.6 or Layer 2 Tunnel Protocol (L2TP) over IPsec clients can use only the "Do not fragment prior to IPsec encapsulation; fragment prior to interface transmission" option. The setting you configure applies to VPN Client software version 3.6 and later. The VPN Concentrator ignores the setting for VPN Clients that run software versions earlier than 3.6 and protocols other than IPsec. For these clients, the "Do not fragment prior to IPsec encapsulation; fragment prior to interface transmission" option applies.
Select Configuration > Interface > Ethernet > General to see the various options for IPsec fragmentation under Public Interface IPsec Fragmentation Policy.
These options determine how the VPN 3000 Concentrator processes these packets:
Do not fragment prior to IPsec encapsulation; fragment prior to interface transmission—The VPN Concentrator encapsulates all tunneled packets. After encapsulation, the VPN Concentrator fragments packets that exceed the MTU setting before transmitting them through the public interface. This is the default policy for the VPN Concentrator. This option works for situations where fragmented packets are allowed through the tunnel without hindrance. For the FTP example, large packets are encapsulated and then fragmented at the IP layer. Intermediate devices can drop fragments or just out-of-order fragments. Load-balancing devices can introduce out-of-order fragments.
Fragment prior to IPsec encapsulation with Path MTU Discovery (ICMP)—The VPN Concentrator fragments tunneled packets that exceed the MTU setting during encapsulation. For this option, the VPN Concentrator drops large packets that have the Do not Fragment (DF) bit set, and sends the "Packet needs to be fragmented but DF is set" ICMP message to the packet's initiator. The ICMP message includes the maximum MTU size allowed. Path MTU Discovery means that an intermediate device (in this case the VPN Concentrator) informs the source of the MTU permitted to reach the destination.
If a large packet does not have the DF bit set, the VPN Concentrator fragments prior to encapsulating. This creates two independent non-fragmented IP packets and transmits them out the public interface. This is the default policy for the VPN 3002 Hardware Client.
Fragment prior to IPsec encapsulation without Path MTU Discovery (Clear DF bit)—The VPN Concentrator fragments tunneled packets that exceed the MTU setting before encapsulating them. If the DF bit on these packets is set, the VPN Concentrator clears the DF bit, fragments the packets, and then encapsulates them. This action creates two independent non-fragmented IP packets leaving the public interface and successfully transmits these packets to the peer site by turning the fragments into complete packets to be reassembled at the peer site. In this example, the VPN Concentrator overrides the MTU and allows fragmentation by clearing the DF bit.
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