Strange result while ping serial interface
Hi nanog, Strange thing happens everyday. When I were troubleshooting yestoday, I found a ping to my local serial interface (no matter it's 155M POS, or 2M Serial interface) takes nearly twice delay compared with a ping to the remote side of the serial interrface ! When I trace to my local serial interface, I got the first hop to the remote end, then second hop back to my local interface ! For example, 202.97.2.1 is local serial interface, while 2.2 is remote side. A trace on 2.1 to 2.1 first go to 2.2, then back to 2.1. And this situation can be verified on both side. Have you ever seen this situation ? Is it due to our misconfiguration? or it's sheerly a mis-display of result in Cisco box (can't believe it) The background is: we're using ISIS as IGP, and utilized level-1/2 link. thanks for any input. regards, Yu Ning ------------------------------------------- (Mr.) Yu(2) Ning(2) ChinaNet Backbone Operation Networking Dep.,Datacom Bureau China Telecom.,Beijing(100088),P.R.C +86-10-66418121/66418122/66418123(fax) -------------------------------------------
to the remote side of the serial interrface ! When I trace to my local serial interface, I got the first hop to the remote end, then second hop back to my local interface !
IBM AIX 4.3.3 on an RS/6000 does the same thing. I'm told it has to do with the inability of some PPP stacks to recognize a chance to short-circuit packets for its own address - If it comes down from the stack, it's going out the serial interface, and isn't checked if the IP address is our own and should just hand it back up the stack as a "received" packet.. I'm sure the protocol police will write me a citation if I botched this. ;) The annoying part is when some program binds to the interface, and you watch all the packet traffic take a loop out and back. It's *amazing* how slow an X11 program can be if it's invoked as 'xfoo -display myhostname:0', if it loops all the X traffic over the 14.4 modem - twice ;) The "fix" that I used was '/etc/route add ip.of.interface 127.0.0.1'. Ugly, but it worked. Just remember that you did it if you start adding packet filters (as packets to yourself will be on the loopback interface, with non 127.0.0.1 addresses).... Valdis Kletnieks Operating Systems Analyst Virginia Tech
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Valdis.Kletnieks＠vt.edu -
Yu Ning