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I would just like to verify one thing. When a router receives a packet with a TTL of 1, the router can still process and accept it if its destined for itself, correct? Because for example, eBGP messages use a TTL of 1 by default and they’re not dropped by the destination BGP neighbor.

However, if the router receives a packet with a TTL of 1 that needs to be forwarded, it will just drop the packet and send back an ICMP Time Exceeded message, right?

And one more thing related to this, when exactly is the TTL decremented? The moment the router receives the packet or the moment it realizes it has to forward it somewhere, so it decrements it and ends with 0.

Mitrixsen
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    https://networkengineering.stackexchange.com/questions/10929/when-is-an-ipv4-ttl-decremented-and-what-is-the-drop-condition – manish ma Nov 23 '23 at 17:21

1 Answers1

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When a router receives a packet with a TTL of 1, the router can still process and accept it if its destined for itself, correct?

Yes.

Because for example, eBGP messages use a TTL of 1 by default and they’re not dropped by the destination BGP neighbor.

That is by design: receive and process but rule out any intermediate hop.

However, if the router receives a packet with a TTL of 1 that needs to be forwarded, it will just drop the packet and send back an ICMP Time Exceeded message, right?

Yes.

And one more thing related to this, when exactly is the TTL decremented? The moment the router receives the packet or the moment it realizes it has to forward it somewhere, so it decrements it and ends with 0.

That doesn't really matter. A simple implementation might decrement the TTL on a packet right at reception and then disable forwarding with TTL==0, but as long as the rules are kept it's all the same.

Zac67
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