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Possible Layer 2 or Layer 3 Problem

Conwell Less
Level 1
Level 1

I am trying to resolve a problem and do not know what could be wrong.

I have a 2 cisco switches that are at different sites one is a Cisco CAT4500 and other 3560. The connection between the switches is a layer 2 fibre link using Neotel as the provider. CAT4500 and 3560 both have VLAN interface ip addresses in order for me to connect to the switches.

So the scenario is this:

I have PC A connected to CAT4500 and PC B connecting to 3560. All these devices are the same VLAN and in the same subnet. I do have trunk links all the way though allowing this vlan as we run vlans. From PC A, I can ping CAT4500, 3560 and even PC B. This tells me everything is fine regarding Layer 2 & 3.

The problem:

As soon as I move PC B and connect it to the other switch CAT4500 or move the other PC vice versa (this happens in both directions). Both switches learn that the mac address has moved locations and updates its mac address table accordingly. So when I do a show mac address table and show arp, everything has learned and moved fine. However when I try to ping any other device from PC B I cannot. So the 3560 switch PC B was originally connected to, I can no longer ping, the switch cannot see the pc also. It is as if PC B has not learned anything from the switch and cannot respond or reply to any icmp.

Then if I leave if for let’s say an hour, then randomly it starts to work again. Where do i start troubleshooting? Is there any debug commands I can run to see what happens on Layer 2 and Layer 3 once this mac is moved or what can i try to resolve this.

All suggestions are welcome.

5 Replies 5

johnlloyd_13
Level 9
Level 9

hi,

for starters, probably you'll need to force update your PC's cache.

arp -d -a

i would like to find out what is causing the problem, this does not only happen on PC's, it happens with laptops, servers, etc. we done a VM migration from site A to Site B for failover purposes. after migration the VM is not contactable.

I am thinking it is something to do with the switches because i have tried different end devices. Same thing

hi,

could you provide a brief network topology, config from both L3 switches and also post show ip interface brief, show interface status and show vlan brief output?

kindly remove sensitive info.

CAT4500

SHOW RUN INT GIG4/47

interface GigabitEthernet4/47

description ###Link to Neotal BC Trunk###

switchport trunk allowed vlan 103,114,124,900

switchport mode trunk

switchport nonegotiate

mtu 9198

speed 1000

duplex full

!

SHOW RUN INT VLAN103

interface Vlan103

mtu 9198

ip address 192.168.103.117 255.255.254.0

ip directed-broadcast

!

SHOW IP INT BRIEF

GigabitEthernet4/47    unassigned      YES unset  up                    up     

Vlan103                192.168.103.117 YES manual up                    up     

!

SHOW VLAN BRIEF

VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

103  Office                           active    Gi2/13, Gi2/32

show int 4/47

GigabitEthernet4/47 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet Port, address is a44c.1105.78b6 (bia a44c.1105.78b6)

  Description: ###Link to Neotal BC Trunk###

  MTU 9198 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

     reliability 255/255, txload 3/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Full-duplex, 1000Mb/s, link type is auto, media type is 10/100/1000-TX

  input flow-control is off, output flow-control is off

  Auto-MDIX on (operational: on)

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:02, output never, output hang never

  Last clearing of "show interface" counters 1w0d

  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  5 minute input rate 1047000 bits/sec, 1241 packets/sec

  5 minute output rate 14222000 bits/sec, 1972 packets/sec

     2065268438 packets input, 181612237107 bytes, 0 no buffer

     Received 1008466 broadcasts (508661 multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 input packets with dribble condition detected

     3778257107 packets output, 4658965716557 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 unknown protocol drops

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier

     0 output buffer failures, 0 output buffers swapped out

!

SHOW INT TRUNK

Port        Mode             Encapsulation  Status        Native vlan

Gi4/47      on               802.1q         trunking      1

Port        Vlans allowed on trunk

Gi4/47      103,114,124,900

Port        Vlans allowed and active in management domain

Gi4/47      103,114,124,900

Port        Vlans in spanning tree forwarding state and not pruned

Gi4/47      103,114,124,900

3560

SHOW RUN INT GIG0/24

interface GigabitEthernet0/24

description Uplink-eCs

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 101,103,114,124,900

switchport mode trunk

switchport nonegotiate

speed 1000

duplex full

!

SHOW RUN INT VLAN103

interface Vlan103

ip address 192.168.103.246 255.255.254.0

ip accounting output-packets

!

SHOW IP INT BRIEF

Vlan103                192.168.103.246 YES NVRAM  up                    up     

GigabitEthernet0/24    unassigned      YES unset  up                    up     

!

SHOW VLAN BRIEF

VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

103  Office                           active    Gi0/6, Gi0/15

!

SHOW INT GIG0/24

GigabitEthernet0/24 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet, address is d4a0.2a71.c398 (bia d4a0.2a71.c398)

  Description: Uplink-eCs

  MTU 9000 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

     reliability 255/255, txload 1/255, rxload 3/255

  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 00:00:00, output 00:00:00, output hang never

  Last clearing of "show interface" counters 1w0d

  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 13403000 bits/sec, 1875 packets/sec

  5 minute output rate 979000 bits/sec, 1169 packets/sec

     3780327511 packets input, 4658836404155 bytes, 0 no buffer

     Received 12825482 broadcasts (4809749 multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog, 4809749 multicast, 0 pause input

     0 input packets with dribble condition detected

     2065695617 packets output, 181659821059 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 unknown protocol drops

     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

!

SHOW INT TRUNK

Port        Mode             Encapsulation  Status        Native vlan

Gi0/24      on               802.1q         trunking      1

Port        Vlans allowed on trunk

Gi0/24      101,103,114,124,900

Port        Vlans allowed and active in management domain

Gi0/24      101,103,114,124,900

Port        Vlans in spanning tree forwarding state and not pruned

Gi0/24      103,114,124

johnlloyd_13
Level 9
Level 9

Hi,

I'm not sure what you're trying to achieve here since no topology map is given. But based from your info, both switches are running L2 and L3.

You'll need to decide which switch does L2 and L3 (collapsed core) or have both as flat network trunked together and running L2 only.

Sent from Cisco Technical Support iPhone App

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