Yesterday, four of the eight systems (connected to same VLAN, identical ethernet adapter settings) were able to ping tweakers.net. Today, another system is not able to ping the internet anymore.

As the system site has been investigated extensively, the Cisco switch is perhaps suspected and has been analyzed as well (see Switch analysis section of the question), but the issue persists.

Current situation: this Q&A regarding packet sniffing is read

Question: Which further steps can be done to debug while some systems can ping the internet and others not while ethernet adapters are identical and these are connected to the same VLAN?

Switch Analysis


Switch>sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet            147   Y               ARPA   VlanNum

MAC-Address table

Switch>sh mac address-table
          Mac Address Table

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 Num    Y                 DYNAMIC     Gi0/Z


Switch>sh vl

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    
Num  VLAN0Num                         active    Gi0/Z

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1    enet  ...        1500  -      -      -        -    -        0      0
Num  enet  ...        1500  -      -      -        -    -        0      0


Primary Secondary Type              Ports
------- --------- ----------------- ------------------------------------------


Switch>sh run

interface GigabitEthernet0/Z
 switchport access vlan Num
 switchport mode access
 switchport nonegotiate

IP Interfaces

Switch>sh ip int br
Interface              IP-Address      OK? Method Status                Protocol
Vlan1                  X               YES NVRAM  up                    up
VlanNum            YES NVRAM  up                    up
GigabitEthernet0/Z     unassigned      YES unset  up                    up


Switch>show interface Gi0/Z
GigabitEthernet0/Z is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is MAC (bia MAC)
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 669
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 248000 bits/sec, 34 packets/sec
  5 minute output rate 516000 bits/sec, 51 packets/sec
     6069964 packets input, 2943921646 bytes, 0 no buffer
     Received 99 broadcasts (56 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 56 multicast, 88133 pause input
     0 input packets with dribble condition detected
     8562995 packets output, 4951152227 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out
  • Please clarify... do you want to sniff packets, or see the output of debug commands Aug 1, 2014 at 10:58
  • Question has been updated
    – 030
    Aug 1, 2014 at 12:36
  • If your question is merely about sniffing, the question is a duplicate Aug 1, 2014 at 12:47
  • Question has been updated.
    – 030
    Aug 1, 2014 at 12:51
  • 1
    I don't think sniffing packets will help you very much.Who is the default gateway? Routing setup properly? Any access lists? correct netmask everywhere? What about a firewall?
    – user2084
    Aug 1, 2014 at 13:08

2 Answers 2


if you want to capture packages and analyze them with a packet sniffer (e.g. wireshark) the correct search term for Cisco IOS is SPAN (Switched Port Analyzer). See this documentation from Cisco.

Regarding your problem: I don't think that you'll find anything in the logs.

I would start checking if the port System1 is connected to is still active an really still accessing the right vlan. (show run)

sh ip int brief and show interface would be my first choices. Check if there are any error counters > 0.

There may also be some issues on the system itself. Check ip settings, proxy, DNS, ... (again). Ping the default gateway and system 2 and check if arp resolution is working.

  • Question has been updated. I will now check the documentation regarding SPAN.
    – 030
    Aug 1, 2014 at 12:36
  • i meant arp -n (or whatever the syntax is) on your client.
    – user2084
    Aug 1, 2014 at 13:04
  • Output of arp -n is identical on all systems
    – 030
    Aug 1, 2014 at 13:06
  • The ping hangs on the default gateway, while pinging the DG works.
    – 030
    Aug 1, 2014 at 13:10
  • and there is something wrong on the default gateway.
    – user2084
    Aug 1, 2014 at 13:33

If you are able to ping the default gateway from the computer then the switch has done it's job and is forwarding packets. I think you should move on to troubleshoot the configuration of the default gateway router instead of the switch.

Check the routers NAT configuration, more specifically the access-list which determines which source IP addresses to NAT. I've seen a similar issue in the past caused by a NAT access-list being:

ip access-list extended ACL-NAT-HOSTS
 permit ip 0.0.0.**15** any

When it should have been configured:

ip access-list extended ACL-NAT-HOSTS
 permit ip 0.0.0.**255** any

And thus only allowing the first 15 servers (with the ip addresses - from the network to access the internet.

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