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Sam Tan
Beginner

Duplex Mismatch on 1921 -> 3650

Hi SME and Engineers,

I got a duplex mismatch scenario here and need your opinion.

Scenario:

Router:

c1900-universalk9-mz.SPA.154-3.M1.bin
interface GigabitEthernet0/1
 description LAN Port
 ip address 103.224.122.49 255.255.255.248
 ip tcp adjust-mss 1452
 duplex auto
 speed 100
 no keepalive proto-up

GigabitEthernet0/1 is up, line protocol is up
  Hardware is CN Gigabit Ethernet, address is b0aa.7764.47c1 (bia b0aa.7764.47c1)
  Description: LAN Port
  Internet address is 103.224.122.49/29
  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 not set
  Full Duplex, 100Mbps, media type is RJ45
  output flow-control is unsupported, input flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:07, output 00:00:00, output hang never
  Last clearing of "show interface" counters 1w5d
  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 207000 bits/sec, 74 packets/sec
  5 minute output rate 344000 bits/sec, 79 packets/sec
     126559062 packets input, 2781615399 bytes, 0 no buffer
     Received 35921 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 35008 multicast, 0 pause input
     133260637 packets output, 1942552617 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     35008 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output

Switch:

Switch details:
Switch version: 03.06.04.E

interface GigabitEthernet1/0/46
description Connection Internet Line
switchport access vlan 99
speed 100
no cdp enable


GigabitEthernet1/0/46 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 74a2.e6a9.3aae (bia 74a2.e6a9.3aae)
  Description: Connection Internet Line
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 2/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:38, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 3265536
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 965000 bits/sec, 104 packets/sec
  5 minute output rate 161000 bits/sec, 82 packets/sec
     3152236081 packets input, 2134901224894 bytes, 0 no buffer
     Received 1491675 broadcasts (407018 multicasts)
     0 runts, 0 giants, 0 throttles
     28337 input errors, 28337 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 407018 multicast, 0 pause input
     0 input packets with dribble condition detected
     2928757149 packets output, 1204608383637 bytes, 0 underruns
     3270906 output errors, 1802 collisions, 1 interface resets
     373476 unknown protocol drops
     0 babbles, 1458 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

The weirdest thing is when the router and switch sets to Full Duplex / 100 Speed, the interface will force a half duplex on both end causing packet lost.

If set to auto duplex, only it will register correctly like the above scenario.

Any ideas? I don't see any particular bugs on these two versions.

Regards,

ST

3 REPLIES 3
Mark Malone
VIP Mentor

Hi why are the set to speed 100 when there gig ports , can you hardcode them to speed 1000 and duplex full , what happes then ? it should work

Georg Pauwen
VIP Expert

Hello,

for links between Cisco devices, auto speed and auto duplex is recommended. 

I'll check for bugs...

paul driver
VIP Mentor

Hello

My understanding is cisco switches negotiate duplex , So if either side sets an hardcoded duplex the switch port will default to half duplex on 10/100 interfaces and full duplex on 1GB interfaces.

So when you set the duplex to auto, that why is works, and it doesn't when one side is hardcoded

res
Paul



kind regards
Paul

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