MTU Question.

Question

Can some one please explain the two different behaviour of MTU as per below output :
In the first output why we dont see the packet loss although the packet size is bigger than the MTU size.
where as in the output 2 we notice the packet loss where as the packet size it 1481 and MTU size is 1480.
=== OutPut 1 ===
ROU#sh int t3
Tunnel3 is up, line protocol is up
Hardware is Tunnel
Description: ***Connect to Ro_03 Tunnel1 Fe0/0/0***
Internet address is 21.233.41.21/30
MTU 17920 bytes, BW 4096 Kbit/sec, DLY 50000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
Encapsulation TUNNEL, loopback not set
Keepalive set (10 sec), retries 3
Tunnel source 21.233.7.22 (GigabitEthernet0/0/4), destination
21.233.41.246
   Tunnel Subblocks:
     src-track:
         Tunnel3 source tracking subblock associated with
GigabitEthernet0/0/4
         Set of tunnels with source GigabitEthernet0/0/4, 10 members (includes iterators), on interface <OK>
Tunnel protocol/transport IP/IP
Tunnel TTL 255, Fast tunneling enabled
Tunnel transport MTU 1480 bytes
Tunnel transmit bandwidth 8000 (kbps)
Tunnel receive bandwidth 8000 (kbps)
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 12000 bits/sec, 1 packets/sec
5 minute output rate 13000 bits/sec, 1 packets/sec
     16274329 packets input, 3173533969 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     18686934 packets output, 8984626725 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out
ITC#ping
Protocol [ip]:
Target IP address: 21.233.41.22
Repeat count [5]: 1000
Datagram size [100]: 2048
Timeout in seconds [2]:
Extended commands [n]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 1000, 2048-byte ICMP Echos to 21.233.41.22, timeout is 2 seconds:
Success rate is 100 percent (1000/1000), round-trip min/avg/max = 33/33/76 ms
=== OutPut 2==
ROU#ping
Protocol [ip]:
Target IP address: 21.233.179.241
Repeat count [5]: 1000
Datagram size [100]: 1481
Timeout in seconds [2]:
Extended commands [n]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 1000, 1481-byte ICMP Echos to 21.233.179.241, timeout is 2 seconds:
Success rate is 98 percent (984/1000), round-trip min/avg/max = 69/70/175 ms
===
Best Regards,

Answer 1

Hi,
I think you should move this post to the appropriate section because I don't see any relationship with IPv6 here.
Hi if you had an MTU problem all your packets should be dropped and you would have to set the DF-bit in the extended ping to test because by default if the DF bit is not set the routers will fragment the packets.
Regards.
Alain.