TTL field

[katab]

Does TTL field related to the number of hopes in routing table ?

[Harold Ritter]

The main purpose of this field is to prevent a packet being forwarded forever if there is a routing loop in the network topology. The TTL field is set by the device originating IP packets and is decremented every time this packet crosses an IP router. When the TTL reaches 0, the pachet is dropped and an ICMP Time Exceeded message is sent to notify the originator of this condition. The initial TTL value differs depending on the TCP/IP stack implementation. The maximum value is 255 due to the TTL field being one octet. The TTL value is usually 1 for packets that have to be kept on the local subnet such as routing protocol advertisements.

Hope this helps,

[katab]

Thanks Hritter ,

-Does TTL differ from one routing protocol to another?

-What do you mean by "The TTL value is usually 1 for packets that have to be kept on the local subnet such as routing protocol advertisements" ? I understand it that in LAN TTL equal 1 but router goning to change it if it is going to route it out.

-Is it enough to have TTL=255 to send packets around the world in Intetrnet .

Regards,

[Harold Ritter · Accepted solution]

- Do you mean the TTL use to propagate routing updates. If so yes. BGP can propagate routing updates to a router that is not directly connected an therefore needs to set the TTL to a value greater than 1 in such a scenario.

- Routing updates are destined to the router itself and will not be forwarded.

- TTL of 255 is plenty to go around the galaxy ;o)

Let me know if this answers your questions,