Hop count - what will happen in this simple case

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?

user39966

Posted 2010-10-29T06:20:38.397

Reputation: 389

hmm... three different answers to a universal standard situation, ammazed – user39966 – 2010-10-29T15:03:50.237

Answers

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.

Ignacio Vazquez-Abrams

Posted 2010-10-29T06:20:38.397

Reputation: 100 516

so what if D has to forward this packet, can it?? – user39966 – 2010-10-29T06:37:45.233

IF the TTL is 1 at D it will not forward it. – dbasnett – 2010-10-29T11:47:03.383

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

LawrenceC

Posted 2010-10-29T06:20:38.397

Reputation: 63 487

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.

dbasnett

Posted 2010-10-29T06:20:38.397

Reputation: 459

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 http://www.cisco.com/web/about/security/intelligence/ttl-expiry.html

– dbasnett – 2010-10-29T12:15:45.237