BugZero | Cisco BugID CSCwi49884 - TCP MSS is changed back to the default value when ...

Cisco - Defect ID: CSCwi49884

TCP MSS is changed back to the default value when a VTI or loopback interface is created

Cisco - Defect ID: CSCwi49884

TCP MSS is changed back to the default value when a VTI or loopback interface is created

Last updated on July 9th, 2025

BugZero Risk Score
6.1 Medium

Overall: 6.1

Severity: 6.4

Lifecycle: 9.1

Popularity: 5.1

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Bug Details

Description

Symptom

When a new virtual tunnel interface (VTI) or loopback interface is created, the ASA will utilize a TCP MSS value of 1380 even when a custom value is set with the 'sysopt connection tcpmss ####' command. This is noticed only in a packet-capture as the custom value will remain in the CLI.

Conditions

Configure a non-default MSS value such as "sysopt connection tcpmss 1360". Configure a VTI and/or loopback interface (no IP address required and does not have to be in use). Initiate traffic behind this firewall that would contain a TCP SYN such as SSH to another host, and the SYN will contain an MSS of 1380 rather than the custom value.

Workaround

Reapply the custom value with "sysopt connection tcpmss ####" after the creation of a new VTI/Loopback interface. Downgrade to 9.17 or below.

Further Problem Description

Change history

2025-07-21 Added: 7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.4.2.2, 7.6.0

2025-07-20 Added: 7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.4.2.2, 7.6.0

2025-07-19 Added: 7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.4.2.2, 7.6.0

2025-07-18 Added: 7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.4.2.2, 7.6.0

2025-07-17 Added: 7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.4.2.2, 7.6.0

2025-03-27 Added: 9.10.1, 9.10.1.10, 9.10.1.11, 9.10.1.17, 9.10.1.2, 9.10.1.22, 9.10.1.27, 9.10.1.30, 9.10.1.32, 9.10.1.37, 9.10.1.40, 9.10.1.42, 9.10.1.44, 9.10.1.7, 9.12.1, 9.12.1.2, 9.12.1.3, 9.12.2, 9.12.2.1, 9.12.2.4, 9.12.2.5, 9.12.2.9, 9.12.3, 9.12.3.12, 9.12.3.2, 9.12.3.7, 9.12.3.9, 9.12.4, 9.12.4.10, 9.12.4.13, 9.12.4.18, 9.12.4.2, 9.12.4.24, 9.12.4.26, 9.12.4.29, 9.12.4.30, 9.12.4.35, 9.12.4.37, 9.12.4.38, 9.12.4.39, 9.12.4.4, 9.12.4.40, 9.12.4.41, 9.12.4.47, 9.12.4.48, 9.12.4.50, 9.12.4.52, 9.12.4.54, 9.12.4.55, 9.12.4.56, 9.12.4.58, 9.12.4.62, 9.12.4.65, 9.12.4.67, 9.12.4.7, 9.12.4.8, 9.13.1, 9.13.1.10, 9.13.1.12, 9.13.1.13, 9.13.1.16, 9.13.1.19, 9.13.1.2, 9.13.1.21, 9.13.1.7, 9.14.1, 9.14.1.10, 9.14.1.15, 9.14.1.19, 9.14.1.30, 9.14.1.6, 9.14.2, 9.14.2.13, 9.14.2.15, 9.14.2.4, 9.14.2.8, 9.14.3, 9.14.3.1, 9.14.3.11, 9.14.3.13, 9.14.3.15, 9.14.3.18, 9.14.3.9, 9.14.4, 9.14.4.12, 9.14.4.13, 9.14.4.14, 9.14.4.15, 9.14.4.17, 9.14.4.22, 9.14.4.23, 9.14.4.24, 9.14.4.6, 9.14.4.7, 9.15.1, 9.15.1.1, 9.15.1.10, 9.15.1.15, 9.15.1.16, 9.15.1.17, 9.15.1.21, 9.15.1.7, 9.16.1, 9.16.1.28, 9.16.2, 9.16.2.11, 9.16.2.13, 9.16.2.14, 9.16.2.3, 9.16.2.7, 9.16.3, 9.16.3.14, 9.16.3.15, 9.16.3.19, 9.16.3.23, 9.16.3.3, 9.16.4, 9.16.4.14, 9.16.4.18, 9.16.4.19, 9.16.4.27, 9.16.4.9, 9.17.1, 9.17.1.10, 9.17.1.11, 9.17.1.13, 9.17.1.15, 9.17.1.20, 9.17.1.30, 9.17.1.33, 9.17.1.39, 9.17.1.45, 9.17.1.46, 9.17.1.7, 9.17.1.9, 9.18.1, 9.18.1.3, 9.18.2, 9.18.2.5, 9.18.2.7, 9.18.2.8, 9.18.3, 9.18.3.39, 9.18.3.46, 9.18.3.53, 9.18.3.55, 9.18.3.56, 9.18.4, 9.18.4.22, 9.18.4.5, 9.18.4.8, 9.19.1, 9.19.1.12, 9.19.1.18, 9.19.1.22, 9.19.1.24, 9.19.1.27, 9.19.1.5, 9.19.1.9, 9.20.1, 9.20.1.5, 9.20.2, 9.20.2.10, 9.20.2.21, 9.20.2.22, 9.20.3, 9.20.3.4, 9.20.3.7, 9.20.3.9, 9.22.1.1, 9.22.1.2, 9.8.1, 9.8.1.5, 9.8.1.7, 9.8.2, 9.8.2.14, 9.8.2.15, 9.8.2.17, 9.8.2.20, 9.8.2.24, 9.8.2.26, 9.8.2.28, 9.8.2.33, 9.8.2.35, 9.8.2.38, 9.8.2.45, 9.8.2.8, 9.8.3, 9.8.3.11, 9.8.3.14, 9.8.3.16, 9.8.3.18, 9.8.3.21, 9.8.3.26, 9.8.3.29, 9.8.3.8, 9.8.4, 9.8.4.10, 9.8.4.12, 9.8.4.15, 9.8.4.17, 9.8.4.20, 9.8.4.22, 9.8.4.25, 9.8.4.26, 9.8.4.29, 9.8.4.3, 9.8.4.32, 9.8.4.33, 9.8.4.34, 9.8.4.35, 9.8.4.39, 9.8.4.40, 9.8.4.41, 9.8.4.43, 9.8.4.44, 9.8.4.45, 9.8.4.46, 9.8.4.48, 9.8.4.7, 9.8.4.8, 9.9.1, 9.9.1.2, 9.9.1.3, 9.9.1.4, 9.9.1.5, 9.9.2, 9.9.2.1, 9.9.2.14, 9.9.2.18, 9.9.2.235, 9.9.2.25, 9.9.2.27, 9.9.2.32, 9.9.2.36, 9.9.2.40, 9.9.2.47, 9.9.2.50, 9.9.2.52, 9.9.2.56, 9.9.2.59, 9.9.2.61, 9.9.2.66, 9.9.2.67, 9.9.2.74, 9.9.2.80, 9.9.2.83, 9.9.2.85, 9.9.2.9

Links

Original Vendor Announcement

Relevant Products

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Affected versions:7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.4.2.2, 7.6.0, 9.10.1, 9.10.1.10, 9.10.1.11, 9.10.1.17, 9.10.1.2, 9.10.1.22, 9.10.1.27, 9.10.1.30, 9.10.1.32, 9.10.1.37, 9.10.1.40, 9.10.1.42, 9.10.1.44, 9.10.1.7, 9.12.1, 9.12.1.2, 9.12.1.3, 9.12.2, 9.12.2.1, 9.12.2.4, 9.12.2.5, 9.12.2.9, 9.12.3, 9.12.3.12, 9.12.3.2, 9.12.3.7, 9.12.3.9, 9.12.4, 9.12.4.10, 9.12.4.13, 9.12.4.18, 9.12.4.2, 9.12.4.24, 9.12.4.26, 9.12.4.29, 9.12.4.30, 9.12.4.35, 9.12.4.37, 9.12.4.38, 9.12.4.39, 9.12.4.4, 9.12.4.40, 9.12.4.41, 9.12.4.47, 9.12.4.48, 9.12.4.50, 9.12.4.52, 9.12.4.54, 9.12.4.55, 9.12.4.56, 9.12.4.58, 9.12.4.62, 9.12.4.65, 9.12.4.67, 9.12.4.7, 9.12.4.8, 9.13.1, 9.13.1.10, 9.13.1.12, 9.13.1.13, 9.13.1.16, 9.13.1.19, 9.13.1.2, 9.13.1.21, 9.13.1.7, 9.14.1, 9.14.1.10, 9.14.1.15, 9.14.1.19, 9.14.1.30, 9.14.1.6, 9.14.2, 9.14.2.13, 9.14.2.15, 9.14.2.4, 9.14.2.8, 9.14.3, 9.14.3.1, 9.14.3.11, 9.14.3.13, 9.14.3.15, 9.14.3.18, 9.14.3.9, 9.14.4, 9.14.4.12, 9.14.4.13, 9.14.4.14, 9.14.4.15, 9.14.4.17, 9.14.4.22, 9.14.4.23, 9.14.4.24, 9.14.4.6, 9.14.4.7, 9.15.1, 9.15.1.1, 9.15.1.10, 9.15.1.15, 9.15.1.16, 9.15.1.17, 9.15.1.21, 9.15.1.7, 9.16.1, 9.16.1.28, 9.16.2, 9.16.2.11, 9.16.2.13, 9.16.2.14, 9.16.2.3, 9.16.2.7, 9.16.3, 9.16.3.14, 9.16.3.15, 9.16.3.19, 9.16.3.23, 9.16.3.3, 9.16.4, 9.16.4.14, 9.16.4.18, 9.16.4.19, 9.16.4.27, 9.16.4.9, 9.17.1, 9.17.1.10, 9.17.1.11, 9.17.1.13, 9.17.1.15, 9.17.1.20, 9.17.1.30, 9.17.1.33, 9.17.1.39, 9.17.1.45, 9.17.1.46, 9.17.1.7, 9.17.1.9, 9.18.1, 9.18.1.3, 9.18.2, 9.18.2.5, 9.18.2.7, 9.18.2.8, 9.18.3, 9.18.3.39, 9.18.3.46, 9.18.3.53, 9.18.3.55, 9.18.3.56, 9.18.4, 9.18.4.22, 9.18.4.5, 9.18.4.8, 9.19.1, 9.19.1.12, 9.19.1.18, 9.19.1.22, 9.19.1.24, 9.19.1.27, 9.19.1.5, 9.19.1.9, 9.20.1, 9.20.1.5, 9.20.2, 9.20.2.10, 9.20.2.21, 9.20.2.22, 9.20.3, 9.20.3.4, 9.20.3.7, 9.20.3.9, 9.22.1.1, 9.22.1.2, 9.8.1, 9.8.1.5, 9.8.1.7, 9.8.2, 9.8.2.14, 9.8.2.15, 9.8.2.17, 9.8.2.20, 9.8.2.24, 9.8.2.26, 9.8.2.28, 9.8.2.33, 9.8.2.35, 9.8.2.38, 9.8.2.45, 9.8.2.8, 9.8.3, 9.8.3.11, 9.8.3.14, 9.8.3.16, 9.8.3.18, 9.8.3.21, 9.8.3.26, 9.8.3.29, 9.8.3.8, 9.8.4, 9.8.4.10, 9.8.4.12, 9.8.4.15, 9.8.4.17, 9.8.4.20, 9.8.4.22, 9.8.4.25, 9.8.4.26, 9.8.4.29, 9.8.4.3, 9.8.4.32, 9.8.4.33, 9.8.4.34, 9.8.4.35, 9.8.4.39, 9.8.4.40, 9.8.4.41, 9.8.4.43, 9.8.4.44, 9.8.4.45, 9.8.4.46, 9.8.4.48, 9.8.4.7, 9.8.4.8, 9.9.1, 9.9.1.2, 9.9.1.3, 9.9.1.4, 9.9.1.5, 9.9.2, 9.9.2.1, 9.9.2.14, 9.9.2.18, 9.9.2.235, 9.9.2.25, 9.9.2.27, 9.9.2.32, 9.9.2.36, 9.9.2.40, 9.9.2.47, 9.9.2.50, 9.9.2.52, 9.9.2.56, 9.9.2.59, 9.9.2.61, 9.9.2.66, 9.9.2.67, 9.9.2.74, 9.9.2.80, 9.9.2.83, 9.9.2.85, 9.9.2.9

Fixed versions: No known fixed versions

Relevant Products

Click on a version to see all relevant bugs

Fixed versions: No known fixed versions

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OPERATIONAL DEFECT DATABASE

Cisco - Defect ID: CSCwi49884

TCP MSS is changed back to the default value when a VTI or loopback interface is created

Cisco - Defect ID: CSCwi49884

TCP MSS is changed back to the default value when a VTI or loopback interface is created

Last updated on July 9th, 2025

BugZero Risk Score6.1 Medium

Bug Details

Symptom

Conditions

Workaround

Further Problem Description

Links

Top Cisco Defects by Risk Score

Ready to prevent the next vendor outage?

BugZero Risk Score
6.1 Medium