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Re: ATM Question

  • From: Jeff Bartig
  • Date: Fri Oct 06 12:30:31 2000

On Fri, Oct 06, 2000 at 04:50:10AM -0700, Richard Inhand wrote:
> I'm currently looking into a weird problem at an ATM
> peering point on the West Coast, when I ping from my
> peer router to any of my peers I get some but not
> total packet loss, when I ping from one router back or
> use the loopback interface as source things are fine.
> Whereas, with all my other ATM peering point routers,
> for example at MAE-EAST I can ping my peers cleanly
> from the same router. This isn't (to my knowledge)
> affecting traffic just wondering why this strange
> phenomenon should occur only in the one place.

Have you verified the return path of the packets from your peers?
Are they getting to you via that peering connection or taking a
different path back to you?

If you ping without specifying a source address to use, the router
will most likely be using the IP address of the interface that
connects to the peering point.  The router on the other end, due
to the connected route, should respond directly back over that
peering connection.

If you are pinging from a different router or using a different
interface off your peering router, then your peer's router is not
directly connected and BGP or other routing protocols may be
directing the reply packet back to you via a different path.

You could verify this with traceroute's loose source route option,
if you and your peers have not disabled this useful feature.  Here
is an example testing that one of my AADS peers is sending traffic
back to me via our peering session.     =  my loopback0 interface, adv via BGP to peers as 140.189/16  =  my interface connecting to AADS NAP  =  my peer's interface connecting to AADS NAP

Protocol [ip]: 
Target IP address:           <--- my loopback
Source address: 
Numeric display [n]: Y
Timeout in seconds [3]: 
Probe count [3]: 
Minimum Time to Live [1]: 
Maximum Time to Live [30]: 
Port Number [33434]: 
Loose, Strict, Record, Timestamp, Verbose[none]: L
Source route:                         <--- my peer's interface
Loose, Strict, Record, Timestamp, Verbose[LV]: V      <--- toggle off verbose
Loose, Strict, Record, Timestamp, Verbose[L]: 
Type escape sequence to abort.
Tracing the route to

  1 12 msec 4 msec 4 msec
  2 20 msec *  20 msec

If my peer was preferring a different path back to my network, I would
see a lot more hops.

This may not be your problem, but it is quite a useful tool when you
don't have access to your peer's network to do troubleshooting on what
they are doing with your routes.  It is also useful for testing if your
peer has pointed default at your network by using a destination address
that isn't part of your network and seeing if the peer sends the packet
back to you as the next hop.

I request that all my peers have source routing enabled for
troubleshooting purposes, at least on their router I peer with.
I have only had one very small network refuse.  


Jeff Bartig                  |  University of Wisconsin - Madison
1210 W Dayton, Rm B263       |  Division of Information Technology
Work Phone: (608) 262-8336   |  Network Engineering Technology
E-Mail: [email protected]  |