Cisco recommends setting the GRE tunnel IP MTU to 1400 bytes when GRE is combined with IPsec, to absorb both overheads.
Checked 2026-10-02
- Vendor
- Cisco
- Product
- Cisco IOS XE
- Subsystem
- GRE over IPsec
- Deployment
- on-premises
- Region
- not restricted
- Release range
- as of 2023-05-17 document update
- Checked
- 2026-10-02
Sources
- Resolve IPv4 Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPsec — Cisco Systems · tier 2 current vendor documentation · GRE over IPsec recommendations
Cite this note
APA
WarmTransfer. (2026, October 2). Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-cisco-gre-ipsec-mtu-1400. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-cisco-gre-ipsec-mtu-1400
BibTeX
@misc{warmtransfer-claim-sip-fragmentation-troubleshooting-cisco-gre-ipsec-mtu-1400,
title = {Troubleshooting failed calls caused by large SIP messages over UDP: source note sip-fragmentation-troubleshooting-cisco-gre-ipsec-mtu-1400},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-fragmentation-troubleshooting-cisco-gre-ipsec-mtu-1400},
note = {Source note sip-fragmentation-troubleshooting-cisco-gre-ipsec-mtu-1400, checked 2026-10-02}
}