nat/pat

[amms68958]

Hi,

Just want to know how an ICMP ping behaves when we are using PAT ?How the

echo reply can be directly sent to the source ?

Thanks in advance,

Thomas.

[slaurin]

Hi,

Generally speaking, ICMP echo and echo-reply packets(types 8 and 0) will create a PAT xlate using port "0". The source IP of your ping request will be translated to the "global" or "PAT overload" address of your Cisco device and then the reply will be sent back to that address where it will be simply "denatted" by your device to the real IP address of the host where the ping request originated.

Hope that answers your question, if yes please rate,

Regards

Simon Laurin

[amms68958]

hi simon,

thanks for your reply.But imagine a scenario where 5 hosts tries pinging yahoo server at the

same time.So once patted these 5 source addresses will be a.b.c.d port 0 . And when the echo replies of the 5 hosts comes ,how it will distinguish the echo replies, because all the echo replies will have a destination address of a.b.c.d (pat overloa address).

Thanks

Thomas.

[pkhatri]

AFAIK, each ping packet will be identified by the "Identifier" field of the ICMP Echo packet. Therefore, you will have translations such as 10.1.1.1:100, 10.1.1.1:101. Therefore, when you receive the ICMP Reply message, it will be de-mutliplexed based on the Identifer field within the packet.

Hope that helps,

Paresh

PS. Pls do rate posts.

[ansley.verzosa]

Hi Thomas,

Please refer to the link below.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_q_and_a_item09186a00800e523b.shtml#Q14

HTH,

Ansley