[c-nsp] 3550<->3750 mtu miss-match, periodic giant errors on cooper

[Ruzhanskaya Olga]

Hello List!

In our network we have some places with common connection design, when two switches are cooper-connected via Gigabit Ethernet ports:
C3550-48, Gi0/1 <-> Gi1/0/1, C3750G

1) 3750G (E)
IOS version: 12.2(50)SE2
3750#sh system mtu 
System MTU size is 1998 bytes
System Jumbo MTU size is 9198 bytes
Routing MTU size is 1500 bytes
2) 3550
IOS version: 12.2(35)SE3
3550# sh system mtu 
System MTU size is 1546 bytes

On the side of 3550, there are some input errors, exactly - giants. They have periodic manner - 2-3 errors every 40 minutes.
As I understand, those errors are caused by mtu mismatch. But why are they so periodic? Is it some sort of keepalives (and I shouldn't worry about)?
Also, there is one location, where Catalysts are connected via fiber (all settings are the same as described ), but there are no such errors=giants. So, can media-type influence errors?  

Examples:
-----------------------------------------
1) Cooper:
-----------------------------------------
3550(c)#sh int gi0/2                                                                  
GigabitEthernet0/2 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 0012.4309.4b32 (bia 0012.4309.4b32)
  Description: ---# Gi trunk to CORE
  MTU 1546 bytes, BW 1000000 Kbit, DLY 10 usec, 
     reliability 255/255, txload 14/255, rxload 21/255
  Encapsulation ARPA, loopback not set
  Keepalive not set
  Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseTX
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  1000BaseT module (WS-G5483) in GBIC slot.
  Last input 00:00:27, output 00:00:24, output hang never
  Last clearing of "show interface" counters 4d00h
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 82605000 bits/sec, 11875 packets/sec
  30 second output rate 57005000 bits/sec, 6973 packets/sec
     2113350708 packets input, 1672028074 bytes, 0 no buffer
     Received 36984218 broadcasts (0 multicasts)
     0 runts, 478 giants, 0 throttles
     478 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 18983558 multicast, 0 pause input
     0 input packets with dribble condition detected
     1478563366 packets output, 2493416609 bytes, 10543 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     10543 output buffer failures, 0 output buffers swapped out
-----------------------------------------
2) Fiber:
-----------------------------------------
3550(f)#sh int gi0/1            
GigabitEthernet0/1 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 0011.20a3.8c31 (bia 0011.20a3.8c31)
  Description: ---# Gi trunk to CORE
  MTU 1546 bytes, BW 1000000 Kbit, DLY 10 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive not set
  Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseLX
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:05, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 602000 bits/sec, 227 packets/sec
  30 second output rate 455000 bits/sec, 197 packets/sec
     287412652 packets input, 2003280838 bytes, 0 no buffer
     Received 1865881462 broadcasts (0 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 1834251298 multicast, 0 pause input
     0 input packets with dribble condition detected
     296750587 packets output, 1987939750 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     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

Any suggestions are appreciated.
 
Best regards,
Olga