Serial interface showing up/up (looped) but can't ping

Unanswered Question
Feb 25th, 2013

What would cause an interface to show up/up (looped), but you still can't ping that loop?...It is on a Cisco 7613 with an Enhanced FlexWan (WS-X6582-2PA), with a Mx Serial PA, 8 ports (73-1580-10) and running IOS ver 12.2(33)SRE2 Here is some output: Walton#sh int s1/0/3 Serial1/0/3 is up, line protocol is up (looped)   Hardware is Serial   Internet address is   MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,     reliability 255/255, txload 1/255, rxload 1/255   Encapsulation HDLC, crc 16, loopback not set   Keepalive set (10 sec)   Last input 00:00:02, output 00:00:02, output hang never   Last clearing of "show interface" counters 00:32:22   Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0   Queueing strategy: fifo   Output queue: 0/40 (size/max)   5 minute input rate 0 bits/sec, 0 packets/sec   5 minute output rate 0 bits/sec, 0 packets/sec     210 packets input, 6240 bytes, 0 no buffer     Received 195 broadcasts (0 IP multicasts)     0 runts, 0 giants, 0 throttles     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort     210 packets output, 6240 bytes, 0 underruns     0 output errors, 0 collisions, 0 interface resets     0 unknown protocol drops     0 output buffer failures, 0 output buffers swapped out     0 carrier transitions     RTS up, CTS up, DTR up, DCD up, DSR up Walton#ping  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) Walton#sh run int s1/0/3 Building configuration... Current configuration : 100 bytes ! interface Serial1/0/3 ip address no fair-queue no cdp enable end

I have this problem too.
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jawad-mukhtar Tue, 02/26/2013 - 00:04

Serial x is up, line protocol is up (looped)  which means

Loop exists in the circuit. Contact your leased line provider or owner of remote router to remove loop.

Giuseppe Larosa Tue, 02/26/2013 - 00:57

Hello Horair,

the serial interface sees the loop and the ping to the serial interface IP address should work as the packet should be sent back to it.

You can check that HDLC keepalives are seen back with

debug serial interface

You could try to see the IP packets with

access-list 111 permit icmp host host

debug ip pack 111 detail

According to documentation if the serial interface does not ping itself when looped it is not working properly.

I would try to use a different serial port on the same port adapter.

You have a Flexwan on C7613 and this makes the scenario more complex.

It should be verified that flexwan is supported on the IOS image the system is running



according to release notes the hardware is supported from 12.2(33)SRA or better


note: what is exactly the port adapter part number?

Hope to help


horalr Tue, 02/26/2013 - 22:05


Thank you...Doing the debug serial interface, I can confirm that my loopback is good as the interface is showing up/up(looped) and that the HDLC keepalives are good:

1y40w: Serial1/0/3: HDLC myseq 8636, mineseen 8635*, yourseen 8636, line up (looped)

1y40w: Serial1/0/3: HDLC myseq 8637, mineseen 8636*, yourseen 8637, line up (looped)

1y40w: Serial1/0/3: HDLC myseq 8638, mineseen 8637*, yourseen 8638, line up (looped)

1y40w: Serial1/0/3: HDLC myseq 8639, mineseen 8638*, yourseen 8639, line up (looped)

1y40w: Serial1/0/3: HDLC myseq 8640, mineseen 8639*, yourseen 8640, line up (looped)

1y40w: Serial1/0/3: HDLC myseq 8641, mineseen 8640*, yourseen 8641, line up (looped)

My question is why can't I ping that interface (through the loop)?

My larger problem on this router is I have 48 serial ports and about 1/3 of them exhibit these same symptoms...I can't bring up any customers on these that I can't get a good ping (loop) on them....The funny thing is these bad ports are across the three flexwan cards and on different PICs...The PIC is a "PA-8T-V35"....There seems to be a disconnect between layer 2 and layer 3...I also performed the ACL test...What can you tell me about the output?

1y40w: IP: tableid=0, s= (local), d= (Serial1/0/3), routed via RIB

1y40w: IP: s= (local), d= (Serial1/0/3), len 100, sending

1y40w:    ICMP type=8, code=0

1y40w: IP: s= (local), d= (Serial1/0/3), len 100, output feature

1y40w:    ICMP type=8, code=0, Post-Ingress-NetFlow(51), rtype 1, forus FALSE, sendself FALSE, mtu 0

1y40w: IP: s= (local), d= (Serial1/0/3), len 100, output feature

1y40w:    ICMP type=8, code=0, HW Shortcut Installation(66), rtype 1, forus FALSE, sendself FALSE, mtu 0

1y40w: FIBipv4-packet-proc: route packet from (local) src dst

1y40w: FIBfwd-proc: Default: recieve entry

1y40w: FIBipv4-packet-proc: packet routing failed

Thanks for the help.....Bob

Giuseppe Larosa Wed, 02/27/2013 - 01:03

Hello Bob,

the debug serial interface shows the seq field is incrementing over time and this is good sign, if not the HW could be faulty.

This is good news.

The strange part comes from the debug ip packet detail output that ends with a packet routing failed.

It might be a problem with CEF switching, what happens if you disable it with:

interface ser1/0/3

no ip cef


no ip route-cache cef

But I 'm afraid you cannot disable CEF on a C7600.

>>My larger problem on this router is I have 48 serial ports and about 1/3 of them exhibit these same symptoms.

>> PA-8T-V35

this is supported in  release notes

There is any pattern in this? like the first ports in the PA, the odd ports or the even ports? Or the ports not working are randomly spread across the 6 port adapters?

Or the issues arises because you have so many port adapters of the same type in three different flexwan.

Hope to help


horalr Mon, 03/04/2013 - 18:34

There isn't a pattern that I can see...I have one PIC where all ports work (2/0), but the problem is spread across the other five PICs on random ports....I'm about ready to load an older IOS and see if that corrects the problem....


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