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New Member

Pinging Serial Int returns surprising round trip times

Hi all,

I have notice that while on a router here at work (using telnet) over the Fa Int. If I ping anyone of the IP addresses configured on S0.{blah} (FR local sub interface) the echo reply times are around 44ms?? Shouldn't a local interface reply much faster than this? Or is this normal?

Any help would be much appreciated.

6 REPLIES
Blue

Re: Pinging Serial Int returns surprising round trip times

how busy is your router? verify the cpu usage while seeing the symptoms.

also, you could need to verify any ACLs or other hops that could induce latency between the two endpoints.

New Member

Re: Pinging Serial Int returns surprising round trip times

The router is not very busy at all...

3222111112222111111112112111111112211111211211222222112111212122222333

100

90

80

70

60

50

40

30

20

10

0....5....1....1....2....2....3....3....4....4....5....5....6....6....7.

0 5 0 5 0 5 0 5 0 5 0 5 0

CPU% per hour (last 72 hours)

* = maximum CPU% # = average CPU%

There are no ACL's configured on the router.

The interface I'm pinging is a local serial int.

Hall of Fame Super Silver

Re: Pinging Serial Int returns surprising round trip times

Terryn

What you are seeing is probably normal. I believe that you are assuming that pinging a local interface should be fast because the interface would generate a local response. But that is not the way that Cisco does ping on serial interfaces. If you ping the address of your local serial interface the router sends a ping packet out the serial interface, it reaches the router on the other end which forwards it back to you (this is the ping request). When you receive the ping request you generate a ping response and send it out the serial interface. It reaches the router on the other end who forwards it back to you.

So pinging your own serial interface address will actually generate two round trips over the serial interface.

Perhaps an interesting experiment to validate this is to ping the address of the other end of the serial and then ping your own end. The time to ping your own end should be just about double the time to ping the remote end.

HTH

Rick

New Member

Re: Pinging Serial Int returns surprising round trip times

Thats fantastic Rick. Exactly the answer I was hoping to hear. So pinging your own interface also confirms that the link to the other end is up also.

I really appreciate your response, so much to learn isn't there :)

Terryn.

Hall of Fame Super Silver

Re: Pinging Serial Int returns surprising round trip times

Terryn

I am glad that my answers was helpful to you.

Yes for point to point serial interfaces (including frame relay point to point subinterfaces) pinging your own interface address does confirm the link to the other end.

HTH

Rick

New Member

Re: Pinging Serial Int returns surprising round trip times

Awesome thanks again.

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