Unknown protocol drops reports the packets with unknown or un-configured protocol on a per interface basis.
If you cannot explain these errors, you can use a sniffer to identify the unknown protocol.
To reproduce a situation with unknown protocol drops two 2821 routers are back to back connected in the Cisco-faq lab (see the figure).
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With a default configuration as shown bellow no unknown drops occurs.
Router2#sh run int gi 0/1
Building configuration...
Current configuration : 97 bytes
!
interface GigabitEthernet0/1
ip address 99.99.99.1 255.255.255.0
duplex full
speed 100
end
Router2#sh run int gi 0/1
Building configuration...
Current configuration : 97 bytes
!
interface GigabitEthernet0/1
ip address 99.99.99.2 255.255.255.0
duplex full
speed 100
end
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Router1#sh int gi 0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is MV96340 Ethernet, address is 001e.1323.4c11 (bia 001e.1323.4c11)
Internet address is 99.99.99.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, media type is T
output flow-control is XON, input flow-control is XON
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:45, output 00:00:04, output hang never
Last clearing of "show interface" counters 02:04:20
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 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
125 packets input, 46064 bytes, 0 no buffer
Received 124 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
1338 packets output, 137992 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
Example of unknown protocol drops:
Disabling CDP on interface Gi 0/1 of Router1 will results in unknown protocol drops on that interface. The CDP packets will no longer be recognized and dropped.
Router1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#int gigabitEthernet 0/1
Router1(config-if)#no cdp enable
Router1(config-if)#^Z
Router1#
GigabitEthernet0/1 is up, line protocol is up
Hardware is MV96340 Ethernet, address is 001e.1323.4c11 (bia 001e.1323.4c11)
Internet address is 99.99.99.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, media type is T
output flow-control is XON, input flow-control is XON
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:28, output 00:00:01, output hang never
Last clearing of "show interface" counters 02:14:03
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 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
135 packets input, 49774 bytes, 0 no buffer
Received 134 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
1439 packets output, 147648 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
2 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
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