MPLS Circuit Troubleshooting

In this scenario we are asked to validate a flapping interface.  

1] Find the serial interface and verify that it is up/up

 attga43c3#sho ip int br | include 10.112.210.45

 Serial9/1/1/20:0       10.112.210.45   YES manual up

2] Display the interface and note the errors

    attga43c3#sho int s9/1/1/20:0

        Serial9/1/1/20:0 is up, line protocol is up
          Hardware is cyBus 2CT3+
          Description: MNX | MYHOME SERVICES | MYHOUSE | GA | DHEC.123456..ATI | 23853 | 1305937 | 1364629 | USA | MIS |
          Internet address is 10.112.210.45/30
          MTU 1500 bytes, BW 1536 Kbit, DLY 20000 usec, rely 255/255, load 1/255
          Encapsulation PPP, crc 16, loopback not set
          Keepalive set (10 sec)
          LCP Open
          Listen: CDPCP
          Open: IPCP
          Last input 00:00:05, output 00:00:05, output hang never
          Last clearing of “show interface” counters never
          Input queue: 0/1000/0/0 (size/max/drops/flushes); Total output drops: 5
          Queueing strategy: VIP-based fair queuing
          5 minute input rate 0 bits/sec, 0 packets/sec
          5 minute output rate 0 bits/sec, 0 packets/sec
             6534487 packets input, 3035189030 bytes, 0 no buffer
             Received 0 broadcasts, 0 runts, 1 giants, 0 throttles
             116987 input errors, 7228 CRC, 51274 frame, 0 overrun, 0 ignored, 58484 abort
             5877595 packets output, 2144676741 bytes, 0 underruns
             0 output errors, 0 collisions, 170 interface resets
             0 output buffer failures, 0 output buffers swapped out
             166 carrier transitions no alarm present
          Timeslot(s) Used: 1-24, Transmitter delay is 0 flags, transmit queue length 5
          non-inverted data

3] Find the VRF and ping the VRF interface.

    attga43c3#sho ip vrf interface | include 10.112.210.45

        Serial9/1/1/20:0       12.112.210.45   1612 <— VRF

4] Run an extended Ping to the VRF

    Router3#ping vrf 1612   

        Protocol [ip]:
        Target IP address: 10.112.210.46
        Repeat count [5]: 5000
        Datagram size [100]: 1500
        Timeout in seconds [2]: 1
        Extended commands [n]:
        Sweep range of sizes [n]:
        Type escape sequence to abort.
        Sending 5000, 1500-byte ICMP Echos to 10.112.210.46, timeout is 1 seconds:
        !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

In the above ping I wanted to really hammer the heck out of circuit.  In this example we see that the circuit is clocking massive input errors and  no output. Instead of me writing what I did to correct this, I’m going to leave this post open for the reader to comment what they think I did.  Note the bold errors and circuit type.

2 thoughts on “MPLS Circuit Troubleshooting

  1. Since reliability is 255/255 I am ruling out cabling.

    All the errors are on the ‘input’ side of the fence.

    I would look at speed\duplex, one side is auto-negotiating and the other is not.

    Also MTU mismatch.

    It is a serial interface:
    A) Framing\coding. Check CSU\DSU
    B) Clocking.

    I am curious as to what is was.

    Thanks.

    • Speed and duplex would not matter in this case. The issue was a dirty circuit on the far end. The last mile.

      Also note the interface resets, this will cause BGP to flap which cause the routes to dampen, until it is cleared.

      Thanks for the comment.

      -RS

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