RE: serial interface and pinging [7:71391]

[Daniel Cotts]

Weird but useful. Great way to troubleshoot a serial line. Divide and
conquer. The equipment at each end can be taken off line (one end at a time)
and a loopback put in place. The other end pings its own address. The ping
packets traverse the line to the loopback and return. Various bit patterns
can be used. If the problem clears, there's a good indication the off-line
equipment is at fault.

 -Original Message-
 From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED]
 Sent: Wednesday, June 25, 2003 9:40 PM
 To: [EMAIL PROTECTED]
 Subject: RE: serial interface and pinging [7:71391]
 
 
 Yes, it's true that when you ping your own serial interface, the ping
 actually crosses the serial link! You can also see evidence of it by
 enabling debugging on the other side.
 
 When I first saw this documented on a Cisco page, I submitted a
 documentation bug report. :-)
 
 I guess it's the only way you'll get a response? It seems 
 awfully weird
 though...
 
 Priscilla




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RE: serial interface and pinging [7:71391]

[Priscilla Oppenheimer]

Yes, it's true that when you ping your own serial interface, the ping
actually crosses the serial link! You can also see evidence of it by
enabling debugging on the other side.

When I first saw this documented on a Cisco page, I submitted a
documentation bug report. :-)

I guess it's the only way you'll get a response? It seems awfully weird
though...

Priscilla

p b wrote:
 
 
 Found this a bit unusual... have a feel for why it works
 this way, but figured I'd float this to the list for
 thoughts...
 
 Got two routers connected via a serial interface. 
 
 R1 is assigned 192.168.2.1/30 on its serial
 R2 is assigned 192.168.2.2/30 on its serial
 
 On R1, do a debug ip icmp
 
 And then from R1, do a ping 192.168.2.1 (the IP on
 it's local serial interface).
 
 Interestingly we see the following:
 
 r2511#ping 192.168.2.1
 
 Type escape sequence to abort.
 Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2
 seconds:
 !
 Success rate is 100 percent (5/5), round-trip min/avg/max =
 68/73/84 ms
 r2511#
 01:35:35: ICMP: redirect rcvd from 192.168.2.2 -- for
 192.168.2.1 use gw 192.168.2.1
 01:35:35: ICMP: echo reply sent, src 192.168.2.1, dst
 192.168.2.1
 01:35:35: ICMP: echo reply rcvd, src 192.168.2.1, dst
 192.168.2.1
 01:35:35: ICMP: echo reply sent, src 192.168.2.1, dst
 192.168.2.1
 01:35:35: ICMP: echo reply rcvd, src 192.168.2.1, dst
 192.168.2.1
 01:35:36: ICMP: echo reply sent, src 192.168.2.1, dst
 192.168.2.1
 01:35:36: ICMP: echo reply rcvd, src 192.168.2.1, dst
 192.168.2.1
 01:35:36: ICMP: echo reply sent, src 192.168.2.1, dst
 192.168.2.1
 01:35:36: ICMP: echo reply rcvd, src 192.168.2.1, dst
 192.168.2.1
 01:35:36: ICMP: echo reply sent, src 192.168.2.1, dst
 192.168.2.1
 01:35:36: ICMP: echo reply rcvd, src 192.168.2.1, dst
 192.168.2.1
 
 Two items of interest:
 
 1) The router, when pinging it's local IP, actually transmits
 the packets onto the interface with source and destination being
 the interface's local IP address.  The packets aren't looped
 internally, as I would have expected, but are looped via the
 remote router.
 
 2) Router R2 sends an ICMP redirect suggesting a
 more efficient way to reach 192.168.2.1.
 
 
 Interesting behavior
 
 
 
 
 
 




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