The MPLS-OPS Archive

[Robert Raszuk <raszuk@cisco.com>]

We overwrite the IPV4 TTL only if the TTL value on egress is less then
the TTL in the packet in the case you have just brought up.

R.

> Krzysztof Szarkowicz wrote:
> 
> Hi,
> 
> RFC 3032 says:
> 
> 2.4. Processing the Time to Live Field
> 2.4.1. Definitions
> (...)
>    The "outgoing TTL" of a labeled packet is defined to be the larger of:
>       a) one less than the incoming TTL,
>       b) zero.
> (...)
> 2.4.3. IP-dependent rules
> (...)
>    When an IP packet is first labeled, the TTL field of the label stack
>    entry MUST BE set to the value of the IP TTL field.  (If the IP TTL
>    field needs to be decremented, as part of the IP processing, it is
>    assumed that this has already been done.)
> 
>    When a label is popped, and the resulting label stack is empty, then
>    the value of the IP TTL field SHOULD BE replaced with the outgoing
>    TTL value, as defined above.  In IPv4 this also requires modification
>    of the IP header checksum.
> (...)
> 
> It is my understanding that in Cisco IOS you can disable thus behavior by disabling TTL propagation on ingress LSR. However, it is not immadietly clear to me, if disabling TTL propagation in IOS has any effect on copying of TTL value back from shim header into IPv4 header on penultimate router. Cisco IOS command guide mention only "from IPv4 into shim header" propagation process  not back, and I cannot explain myself the behavior that I have observed.
> 
> Could someone explain what is happening in more details?
> 
> Best regards,
> Krzysztof
> 
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