Nem számít, milyen jól állította be a hálózatot, készen kell állnia bizonyos problémák elhárítására. A hálózatok gyakran olyan eseményeknek vannak kitéve, amelyek állapotuk megváltozását okozhatják. Például egy interfész meghibásodhat, vagy a szolgáltató megszakad a kapcsolat. A hivatkozások túltelítettek lehetnek, vagy a rendszergazda rossz konfigurációt ad meg.

Ha a hálózatban változás történik, a kapcsolat megszakadhat. A hálózati rendszergazdák feladata a probléma pontos meghatározása és megoldása. A problémák megtalálásához és megoldásához a hálózati rendszergazdának ismernie kell az útválasztási problémák gyors elkülönítését segítő eszközöket.

• ping
• traceroute
• show ip route
• show ip interface brief
• show cdp neighbors detail

R1# ping 192.168.2.1 source 172.16.3.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.3.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/5 ms
R1#

R1# traceroute 192.168.2.1
Type escape sequence to abort.
Tracing the route to 192.168.2.1
VRF info: (vrf in name/id, vrf out name/id)
  1 172.16.2.2 1 msec 2 msec 1 msec
  2 192.168.1.1 2 msec 3 msec *
R1#

R1# show ip route | begin Gateway
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
S        172.16.1.0/24 [1/0] via 172.16.2.2
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.1/32 is directly connected, Serial0/1/0
C        172.16.3.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.3.1/32 is directly connected, GigabitEthernet0/0/0
S     192.168.1.0/24 [1/0] via 172.16.2.2
S     192.168.2.0/24 [1/0] via 172.16.2.2
R1#

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0/0   172.16.3.1      YES manual up                    up
GigabitEthernet0/0/1   unassigned      YES unset  up                    up
Serial0/1/0            172.16.2.1      YES manual up                    up
Serial0/1/1            unassigned      YES unset  up                    up
R1#

R1# show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
Switch           Gig 0/0/1         129              S I   WS-C3560- Fas 0/5
R2               Ser 0/1/0         156             R S I  ISR4221/K Ser 0/1/0
Total cdp entries displayed : 3
R1#

R1# ping 192.168.2.1 source g0/0/0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.3.1
.....
Success rate is 0 percent (0/5)

R1# ping 172.16.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

R1# ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms

R2# show ip route | begin Gateway
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.2/32 is directly connected, Serial0/1/0
S        172.16.3.0/24 [1/0] via 192.168.1.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, Serial0/1/1
L        192.168.1.2/32 is directly connected, Serial0/1/1
S     192.168.2.0/24 [1/0] via 192.168.1.1
R2#

R2# show running-config | include ip route
ip route 172.16.3.0 255.255.255.0 192.168.1.1
ip route 192.168.2.0 255.255.255.0 192.168.1.1
R2#
R2# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)# no ip route 172.16.3.0 255.255.255.0 192.168.1.1
R2(config)# ip route 172.16.3.0 255.255.255.0 172.16.2.1
R2(config)#

R2(config) # exit
R2#
*Sep 20 02:21:51.812: %SYS-5-CONFIG_I: Configured from console by console
R2# show ip route | begin Gateway
Gateway of last resort is not set
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C        172.16.1.0/24 is directly connected, GigabitEthernet0/0/0
L        172.16.1.1/32 is directly connected, GigabitEthernet0/0/0
C        172.16.2.0/24 is directly connected, Serial0/1/0
L        172.16.2.2/32 is directly connected, Serial0/1/0
S        172.16.3.0/24 [1/0] via 172.16.2.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, Serial0/1/1
L        192.168.1.2/32 is directly connected, Serial0/1/1
S     192.168.2.0/24 [1/0] via 192.168.1.1
R2#
<code>

<code>
R1# ping 192.168.2.1 source g0/0/0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.3.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms