Cisco VPN Services Port Adapter Configuration Guide OL-16406-01 Chapter 5 Configuring IPsec VPN Fragmentation and MTU Understanding IPsec VPN Fragmentation and MTU Fragmentation in Different Modes The fragmentation process differs depending on the IPsec VPN mode and whether GRE or virtual tunnel interface (VTI) is used.

MTU Size Issues Issues related to MTU size, PMTUD and packet fragmentation The Maximum Transmission Unit (MTU) is the largest number of bytes an individual datagram can have on a particular data For fragmentation of packets in VRF mode, the following are the MTU setting requirements and recommendations: • The MTU of the crypto interface VLAN. – You must configure this MTU to be less than or equal to the minimum MTU of the physical egress interfaces configured on the port VLAN, or packets will be dropped. Jan 13, 2019 · Default value of 1450 allows IPv4 packets to be transmitted over a link with MTU 1473 or higher without IP level fragmentation. This suggests we need to take in to account the UDP header bytes. So 1470 - 28 (for IPv4) = 1442. Dec 21, 2006 · A ping with data size of this VPN interface MTU size and 'Do not Fragment' bit set to the other end VPN device is failing. This may happen if there is a lesser MTU network which drops the 'Do not fragment' packets.

For more information about VPN fragmentation, refer to sk98074 - MTU and Fragmentation Issues in IPsec VPN. This hotfix , adds the new kernel parameter sim_ipsec_dont_fragment . If this parameter is enabled, then the behavior of Security Gateway with enabled SecureXL changes to the following:

Fragmentation and MTU. MTU/maximum transmission unit is the maximum length for an IP packet. MTU can vary based on configurations and interfaces. By default, a computer calculates an interface’s MTU based on the maximum size of the data portion of the data-link frame. Default MTU on Ethernet is 1500.

IKE Fragmentation - In some instances, key exchange packets can be large which will lead to packet loss as described above. By using an extension to the IKE protocol, it is possible for IPsec Peers to exchange large packets even when a trouble router exists between them.

Feb 05, 2018