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I have questions about the TTL value.

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  1. whether the Router3 Gig0/0/0 to Gig0/0/1, there will cost one TTL?

  2. if the Router2 Gig0/0/0 to Router3's Gig0/0/0, will there cost one TTL? or do not cost?

  3. if Router2 loopback0 to Router2's Gig0/0/0, will cost one TTL?

  4. if Router2 Gig0/0/0 to Router6's Gig0/0/0, there only cost one TTL?

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A router always decrements TTL when it forwards a packet. It's part of the fundamentals of IP, and it happens when the router receives a packet on an interface and decides to send it out of a (usually different) interface.

In your example, consider a packet (perhaps ping) from R2 to R6, there are two networks you could view the packet on, left and right. The right one will see a packet with a lower TTL, because it was changed by R3.

In your example about a loopback interface, the question is whether it is being forwarded. The algorithm is:

  1. receive packet
  2. check integrity, discard if no good
  3. is it for me (any of my IP addresses?)
  4. if yes, process it appropriately
  5. if no, decrement TTL
  6. if TTL is zero, discard it
  7. if not, see if can forward to a next hop

If you were, for example, doing ping from R2 with source address from lo0, and next-hop R3, then no decrement happens because the packet is not being forwarded, it is being originated, on R2.

The only confusing case is a packet through a tunnel: the outer, transport, packet always has TTL decremented, but the inner, payload, packet usually does not -- it's just payload. Because this is a tricky situation which can make loops, some systems will adjust the inner payload packet too.

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  • You did not mention Router2 lo0 to its Gig0/0/0. whether there will cost TTL. – three-blocks Nov 26 '18 at 12:27
  • The packet isn't being forwarded, so the TTL is not decremented. I added a bit to the answer. – jonathanjo Nov 26 '18 at 12:37
  • Even if it is forwarded from one interface to another in the same router, the TTL is decremented, and that is per the RFC. I tested this in a Cisco ISR. – Ron Maupin Nov 26 '18 at 14:34
  • I tried it too, and all the Cisco devices I tried responded the same for a given TTL on an incoming packet whether addressed to nearside or farside interface. – jonathanjo Nov 26 '18 at 16:39
  • The fact that our experiments gave different answers to this made me do some experiments, the subject of networkengineering.stackexchange.com/questions/54998 – jonathanjo Nov 26 '18 at 20:18
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The TTL is decremented by the router, so in your case as it passes between interfaces on Router 3.

To answer your question though:

  1. Yes
  2. No
  3. No
  4. Yes
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  • Why you think the Router2's loopback0 to its Gig0/0/0 do not cost TTL? – three-blocks Nov 26 '18 at 12:25
  • because I wasn't sure, and just tested it in the lab – Benjamin Dale Nov 27 '18 at 5:33

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