Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Time-to-Live uniform mode

Updated: July 31, 2026

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This topic describes Time-to-Live (TTL) uniform mode on the Cisco 8000 Series Router, a mechanism that synchronizes TTL values between inner and outer packet headers during encapsulation and decapsulation.

Time-to-Live (TTL) uniform mode is a mechanism that

  • ensures consistent TTL management by synchronizing the TTL values between inner and outer packet headers during encapsulation and decapsulation, allowing the receiving device to accurately interpret the packet's remaining lifespan

  • allows you to copy the TTL values from inner headers to outer headers during encapsulation and from outer headers to inner headers during decapsulation, and

  • ensures consistent TTL management across various network scenarios.

For more information on the various network scenarios, see Use cases for TTL uniform mode on a router.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

Copy TTL value to IP headers

Release 25.2.1

Introduced in this release on: Fixed Systems (8400 [ASIC: K100])(select variants only*)

*This feature is now supported on the Cisco 8404-SYS-D router.

Copy TTL value to IP headers

Release 25.1.1

Introduced in this release on: Fixed Systems (8200 [ASIC: Q200]; Centralized Systems (8600 [ASIC:Q200]); Modular Systems (8800 [LC ASIC: Q200])

Support for Time-to-Live (TTL) uniform mode is introduced. This mode ensures consistent TTL management by synchronizing the TTL values between inner and outer packet headers during encapsulation and decapsulation, allowing the receiving device to accurately interpret the packet's remaining lifespan. TTL uniform mode is enabled only for the pbr vrf-redirect mode in IP-in-IP tunnels.

Enabling TTL uniform mode offers these advantages:

  • Enhanced packet integrity and lifespan accuracy: TTL uniform mode ensures consistent TTL management by allowing the copying of TTL values between inner and outer headers during encapsulation and decapsulation. This consistency helps in accurately interpreting the packet's remaining lifespan at the receiving device.

  • Network diagnostics and troubleshooting: By controlling and monitoring the TTL values, you can better diagnose and troubleshoot network paths and performance issues.

Configuration guidelines for TTL uniform mode

These configuration guidelines apply to the TTL uniform mode:

  • Hardware and feature prerequisites: TTL uniform mode is enabled only on the Cisco Silicon One Q200 ASIC-based systems when the pbr vrf-redirect mode in the hw-module profile command is enabled.

  • Post-configuration requirements: You must reload the router by using the reload location all command for the configuration changes to take effect.

Use cases for TTL uniform mode on a router

This table details the various scenarios of encapsulation and decapsulation, and their corresponding TTL action that the router performs.

Table 2. Use cases for TTL uniform mode on a router

Use case if the TTL uniform mode is..

Then..

Example

encapsulation-only

The router decrements the TTL value and copies the value from the inner header to outer header.

Consider this example for the encapsulation-only use case for a packet:

  • Initial state: TTL value of the packet is 100.

  • Decrement: The router decrements the inner header TTL value by one, making it 99.

  • Copy: The router then copies the inner header TTL value to the outer header TTL value. The outer header TTL value becomes 99.

  • Result: The inner and outer header TTL becomes 99, making it uniform.

decapsulation-only

The router decrements the TTL value and copies the value from the outer header to inner header.

Consider this example for the decapsulation-only use case for a packet:

  • Initial state: The outer header TTL value is 77 and the inner header TTL value is 99.

  • Decrement: The router decreases the outer header TTL value by 1, making it 76.

  • Copy: The router then copies the outer header TTL value to the inner header TTL value.

  • Removal: The router removes the outer header TTL value.

  • Result: The final packet TTL value becomes 76.

decapsulation and encapsulation

The router copies the TTL value from the outer header and applies it to the new outer header. Decrements the TTL value by 1 and forwards the packet.

Consider this example for the encapsulation and decapsulation use case. Such scenarios apply when a packet travels through IP tunnels:

  • Encapsulation

    • Initial state: TTL value of the packet is 100.

    • Decrement: The router decrements the inner header TTL value by one, making it 99.

    • Copy: The router then copies the inner header TTL value to the outer header TTL value. The outer header TTL value becomes 99.

    • Result: The inner and outer header TTL becomes 99, making it uniform.

  • Decapsulation

    • Initial state: The outer header TTL value is 77 and the inner header TTL value is 99.

    • Decrement: The router decreases the outer header TTL value by 1, making it 76.

    • Copy: The router then copies the outer header TTL value to the inner header TTL value.

    • Removal: The router removes the outer header TTL value.

    • Result: The final packet TTL value becomes 76.

decapsulation and lookup

The router copies the TTL value from the outer header to the inner header. Decrements the TTL value by 1 and forwards the packet.

Consider this example for the decapsulation and lookup use case for a packet:

  • Initial state: The outer header TTL value is 77 and the inner header TTL value is 99.

  • Decrement: The router decreases the outer header TTL value by 1, making it 76.

  • Copy: The router copies the outer header TTL value to the inner header TTL value.

  • Removal: The router removes the outer header TTL value.

  • Lookup: The router then forwards the packet to the next hop.

repair

The router keeps the inner header TTL value unchanged and forwards the packet as usual.

The repair use case occurs after the encapsulation-only and decapsulation-encapsulation use cases.

Consider this example for the repair use case for a packet:

  • Initial state: The primary path for a packet on a tunnel is unavailable.

  • Initiate recycle: The router begins the recycling of the packet by re-entering the packet into the ingress pipeline from the egress pipeline.

  • Header update: The router updates the packet by replacing the old outer header with a new outer header.