Ping Failure Troubleshooting

Network Topology

Physical Network Topology

Figure 1-1 Network where a ping failure occurs

Fault Description

SwitchA and SwitchC could not ping each other.

Configuration Files

l   SwitchA

         interface Vlanif20
             ip address 1.1.1.1 255.255.255.0
        interface Ethernet0/0/0
             ip address 10.220.7.128 255.255.254.0
        interface GigabitEthernet1/1/23
             port link-type access
             port default vlan 20                             
       ip route-static 0.0.0.0 0.0.0.0 10.220.6.1

l   SwitchB

         vlan batch 20 30 103 220
       acl number 3000
             rule 5 permit icmp source 2.1.1.1 0 destination 1.1.1.1 0
             rule 10 permit icmp source 1.1.1.1 0 destination 2.1.1.1 0
       interface Vlanif20
             ip address 1.1.1.2 255.255.255.0
       interface Vlanif30
             ip address 2.1.1.2 255.255.255.0
       interface Vlanif220
             ip address 10.220.7.121 255.255.254.0
       interface Ethernet0/0/2
             port link-type access
             port default vlan 103
       interface Ethernet0/0/23
             port link-type access
             port default vlan 20
       interface Ethernet0/0/24
             port link-type access
             port default vlan 30
       interface GigabitEthernet0/0/2
             port link-type trunk
             port trunk pvid vlan 220
             undo port trunk allow-pass vlan 1
             port trunk allow-pass vlan 220           
       ip route-static 0.0.0.0 0.0.0.0 10.220.6.1

l   SwitchC

       vlan batch 30 10
       interface Vlanif30                         
             ip address 2.1.1.1 255.255.255.0
       interface MEth0/0/1
             ip address 10.220.7.122 255.255.254.0
        interface GigabitEthernet0/0/2
             port link-type access
             port default vlan 103
       interface GigabitEthernet0/0/24
             port link-type access
            port default vlan 30
       ip route-static 0.0.0.0 0.0.0.0 10.220.6.1
       ip route-static 1.1.1.0 255.255.255.0 2.1.1.2

Troubleshooting Location

If SwitchA cannot ping IP address 2.1.1.1, it is difficult to locate the fault cause. In this situation, reduce the fault range. Check the IP routing table and ping SwitchB, SwitchC from SwitchA, ping SwitchCfrom SwitchB, and so on. Then you can determine which network segment has the problem.

Step 1      Check IP routing table on each switch.

Run the display ip routing-table command in any view of the device to check whether the ip routing table is correct.

<SwitchA> display ip routing-table
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 7        Routes : 7        
 
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface
 
        0.0.0.0/0   Static  60   0          RD   10.220.6.1      Ethernet0/0/0/0
        1.1.1.0/24  Direct  0    0           D   1.1.1.1         Vlanif20
        1.1.1.1/32  Direct  0    0           D   127.0.0.1       Vlanif20
     10.220.6.0/23  Direct  0    0           D   10.220.7.128    Ethernet0/0/0/0
   10.220.7.128/32  Direct  0    0           D   127.0.0.1       Ethernet0/0/0/0
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0

<SwitchB> display ip routing-table
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 9        Routes : 9        
 
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface
 
        0.0.0.0/0   Static  60   0          RD   10.220.6.1      Vlanif220
        1.1.1.0/24  Direct  0    0           D   1.1.1.2         Vlanif20
        1.1.1.2/32  Direct  0    0           D   127.0.0.1       InLoopBack0
        2.1.1.0/24  Direct  0    0           D   2.1.1.2         Vlanif30
        2.1.1.2/32  Direct  0    0           D   127.0.0.1       InLoopBack0
     10.220.6.0/23  Direct  0    0           D   10.220.7.121     Vlanif220
   10.220.7.121/32  Direct  0    0           D   127.0.0.1       InLoopBack0 
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0

<SwitchC> display ip routing-table
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 6        Routes : 6        
 
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface
 
        0.0.0.0/0   Static  60   0          RD   10.220.6.1      MEth0/0/1
        1.1.1.0/24  Static  60   0          RD   2.1.1.2         Vlanif30
        2.1.1.0/24  Direct  0    0           D   2.1.1.1         Vlanif30
        2.1.1.1/32  Direct  0    0           D   127.0.0.1       Vlanif30
     10.220.6.0/23  Direct  0    0           D   10.220.7.122    MEth0/0/1
   10.220.7.122/32  Direct  0    0           D   127.0.0.1       MEth0/0/1
      127.0.0.0/8   direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0

By checking the routing table for SwitchA, SwitchB, and SwitchC, you can find that the routing table on the SwitchA device does not have a table entry to the 2.1.1.0/24 network segment.

Solution：Configure a static route to SwitchC on teh SwitchA.On SwitchA, configure a static route to SwitchC.

[SwitchA] ip route-static 2.1.1.0 255.255.255.0 1.1.1.2

After the configuration modification,  you can find that SwitchA still failed to ping the SwitchC, go to Step 2.

<SwitchA> ping 2.1.1.1  
PING 2.1.1.1: 56  data bytes, press CTRL_C to break
    Request time out
    Request time out
    Request time out
    Request time out
    Request time out
 
  --- 2.1.1.1 ping statistics ---
    5 packet(s) transmitted
    0 packet(s) received
    100.00% packet loss

Step 2      Check the ping test on per segment.

Run the ping command on the device to determine which network segment has the problem.

#Check whether the host at 2.1.1.2 is reachable on SwitchA.

<SwitchA> ping 2.1.1.2
  PING 2.1.1.2: 56  data bytes, press CTRL_C to break
    Reply from 2.1.1.2: bytes=56 Sequence=1 ttl=254 time=2 ms
    Reply from 2.1.1.2: bytes=56 Sequence=2 ttl=254 time=2 ms
    Reply from 2.1.1.2: bytes=56 Sequence=3 ttl=254 time=2 ms
    Reply from 2.1.1.2: bytes=56 Sequence=4 ttl=254 time=2 ms
    Reply from 2.1.1.2: bytes=56 Sequence=5 ttl=254 time=2 ms
  --- 2.1.1.2 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 2/2/2 ms

#Check whether the host at 2.1.1.2 is reachable on SwitchC.

<Switchc> ping 2.1.1.2  
PING 2.1.1.2: 56  data bytes, press CTRL_C to break
    Request time out
    Request time out
    Request time out
    Request time out
    Request time out
 
  --- 2.1.1.2 ping statistics ---
    5 packet(s) transmitted
    0 packet(s) received
    100.00% packet loss

The fault exists between SwitchB and SwitchC,go to Step 3.

Step 3      Check the STP status.

Run the display stp brief command to view the STP status. In the STP State column, FORWARDING indicates forwarding and DISCARDING indicates blocked.

<SwitchC> display stp brief 
 MSTID  Port                     Role  STP State     Protection 
    0   GigabitEthernet0/0/2     ROOT  FORWARDING      NONE
    0   GigabitEthernet0/0/24    ALTE  DISCARDING      NONE

It is found that GE0/0/24 of SwitchC was blocked.

Solution: Shutdown the other interface connectd to the SwitchB OR Add the two interfaces connecting SwitchB and SwitchC to the same Eth-Trunk.

Root Cause

1.         No static route to SwitchC is configured on the SwitchA.

2.         The port is blocked by the STP protocol because of the loop.