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Consider ABCD

router A wants to send to D

It keeps hop count 3

What will be the HOP COUNT when the packet reaches D

Will it be zero or one?

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hmm... three different answers to a universal standard situation, ammazed – user39966 Oct 29 '10 at 15:03
up vote 1 down vote accepted

B will decrement it from 3 to 2, and C will decrement it from 2 to 1. Therefore it will be 1 when it reaches D.

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so what if D has to forward this packet, can it?? – user39966 Oct 29 '10 at 6:37
    
IF the TTL is 1 at D it will not forward it. – dbasnett Oct 29 '10 at 11:47

D will forward it. The node on the receiving end, say E, will drop it if it can't process it.

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If I understand the question correctly, a ping is being sent from router A with a TTL of 3. In that case it will not reach D. (I could be wrong, but it depends on how the router OS defines traversing)

a       b       c       d   
3   2   2   1   1 not forwarded by c

If a were a PC then

a       b       c           d   
3   3   3   2   2   1       1

edit - To determine how Router A defines "traversing" ping Router B from A with a TTL of 1. My home routers do not correctly decrement the TTL so I can't test here.

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I found this, "As insurance against this outcome, the TTL value of an IP datagram is decremented by a value of one each time the packet is forwarded by a network device. Thus, an IP packet can never be forwarded more than 254 times, preventing the infinite packet loop problem." here cisco.com/web/about/security/intelligence/ttl-expiry.html – dbasnett Oct 29 '10 at 12:15

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