I got a Cisco 1841 router with HWIC-4ESW module that's connected to a Catalyst 2960 switch through a trunk link. Both connected interfaces have equal settings (duplex full and speed 100, MTU set as default).
On the switch: "sh interfaces fastEthernet 0/1 counters errors" showing increase in the numbers of undersized frames received by switch:
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize
Fa0/1 0 0 0 0 520
On the switch :"sh interfaces fastEthernet 0/1 controller"
FastEthernet0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0017.0ef8.9c83 (bia 0017.0ef8.9c83)
Description: trunk to 1841 Fa0/0
MTU 1500 bytes, BW 100000 Kbit, 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 10/100BaseTX
input flow-control is off, output flow-control is unsupported
Transmit FastEthernet0/1 Receive
47325749 Bytes 118224174 Bytes
192797 Unicast frames 115621 Unicast frames
2025 Multicast frames 58934 Multicast frames
2045 Broadcast frames 3309 Broadcast frames
0 Too old frames 111149330 Unicast bytes
0 Deferred frames 4081197 Multicast bytes
0 MTU exceeded frames 211776 Broadcast bytes
0 1 collision frames 0 Alignment errors
0 2 collision frames 0 FCS errors
0 3 collision frames 0 Oversize frames
0 4 collision frames 33710 Undersize frames
0 5 collision frames 0 Collision fragments
0 6 collision frames
0 7 collision frames 12566 Minimum size frames
0 8 collision frames 60372 65 to 127 byte frames
0 9 collision frames 27747 128 to 255 byte frames
0 10 collision frames 6032 256 to 511 byte frames
0 11 collision frames 4243 512 to 1023 byte frames
0 12 collision frames 66904 1024 to 1518 byte frames
0 13 collision frames 0 Overrun frames
0 14 collision frames 0 Pause frames
0 15 collision frames
0 Excessive collisions 0 Symbol error frames
0 Late collisions 0 Invalid frames, too large
0 VLAN discard frames 0 Valid frames, too large
0 Excess defer frames 0 Invalid frames, too small
62571 64 byte frames 33710 Valid frames, too small
100777 127 byte frames
On the router some commands run without any errors.
I test both interfaces on half-duplex (and _ or ) 10Mbps but unsuccessful. I try to connect switch to other interface on HWIC but undersized frames counter still increase in the number. And, of course, I try to use other UTP path. All unsuccessful.
I am to draw attention to LED on my C2960, but LED on C2960 newer blinking amber. And IOS version on my device (12.2(25)FX) top-of-the IOS version specified in your link.
I think it's a problem of HWIC-4ESW installed on 1841, but i do not have one more HWIC for check up. And what strange occurrence, that is all other interfaces on HWIC newer have errors while work with hosts directly connected to interface.
Check whether it is possible to upgrade IOS on 1841. It can help. According to the document in my previous answer only short dot1q encapsulated frames are counted as undersized, which explains why you never got this error with hosts directly connected to the interface.
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