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I need some advise on the TTL. Currently my application vendor sets TTL to be with value 1 and he should increase it. He is thinking about value of 10, but I'm not sure that this is enough:

Assuming I have a network working in its domain when the connection between the 2 entities is based on LAN / single router.

Now I'm appending a WAN between them. Meaning I'm appending a routers at each side running GRE tunnel between them (between the routers and the common network there is also an encryptor).

  • what are the TTL considerations when adding the WAN in between the 2 entities
  • what is the GRE tunnel shall have its own TTL or it is uses the TTL from the original IP packet?
  • if there are 2 routings in the router from packet input to packet output, will the TTL be decrised twice?

Thank you :)

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 8:55
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Inside the tunnel, the payload (inner) IP packet remains untouched, including the TTL. Hops are counted on the wrapper (outer) IP packet instead.

The outer packet either inherits its TTL from the inner packet or is initialized by a default, depending on your tunnel ends.

That said, a TTL minimum of 16 is highly advisable with 64, 128 or 255 being very common. The TTL is primarily a safeguard against routing loops and should not used for anything else. Unvoluntary packet leakage should be controlled by precise filtering.

  • Thank you! Do you know what are the default Cisco configuration for GRE TTL? Is it uses the original packet TTL or some default? – user1977050 Jun 19 '18 at 8:20
  • I'm not into Cisco, but tunnel ttlshould configure the inner packet TTL. I can't find anything for the outer/GRE packet, so I guess it uses the global default (255). – Zac67 Jun 19 '18 at 10:02
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Believe the default for servers, is to inherit TTL from the inner IP packet to the GRE header IP. In Cisco, default is 255 Tunnel TTL (value) is the command to change it manually.

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