Pre-Tunnel fragmentation at ASR 9000 XR
XR already supports fragmentation at interface level. SecGW in ASR 9000 has the advantage of using the XR functionality because the packets are always forwarded via XR.
The MTU size can be configured at the VSM interface used for clear traffic. The MTU size should be the PMTU of the encrypted network subtracted by the outer IP header size and crypto overhead (which is up to 100 bytes). If PMTU of the encrypted network is 1400 then the VSM clear interface MTU size must be 1300 for pre-tunnel fragmentation to work.