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I have a Cisco 3825 with redundant upstream connections to the datacenter border router. I noticed that there are a large number of "collisions" on the two interfaces. I've read elsewhere that that can be caused by having the interface set to Half-Duplex, but they are both definitely set to Full-Duplex. I'm also certain that the upstream connection is also 1Gbps. There are no issues reaching outside networks, but there are frequent bouts of unusual network slowness where the router is only pushing 30Mbps or so, but everything is crawling. Any idea on resolving these collisions?

c3825# sh run
interface GigabitEthernet0/0
 description Link to Border10
 ip address XX.XX.239.118 255.255.255.252
 ip flow ingress
 ip flow egress
 duplex full
 speed 1000
 media-type rj45
 no mop enabled
!
interface GigabitEthernet0/1
 description Link to Border11
 ip address XX.XX.239.122 255.255.255.252
 ip flow ingress
 ip flow egress
 duplex full
 speed 1000
 media-type rj45
!

c3825#sh  int GigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up 
  Hardware is BCM1125 Internal MAC, address is 001d.7039.1140 (bia 001d.7039.1140)
  Description: Link to Border10
  Internet address is XX.XX.239.118/30
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec, 
     reliability 255/255, txload 5/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full Duplex, 1Gbps, media type is RJ45
  output flow-control is XON, input flow-control is XON
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/127 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 7225000 bits/sec, 2287 packets/sec
  5 minute output rate 21278000 bits/sec, 2918 packets/sec
     2031411499 packets input, 281305862 bytes, 0 no buffer
     Received 4 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     1789 input errors, 0 CRC, 0 frame, 1789 overrun, 0 ignored
     0 watchdog, 4294868991 multicast, 0 pause input
     2351846464 packets output, 3281062849 bytes, 0 underruns
     308 output errors, 4286578687 collisions, 3 interface resets
     0 unknown protocol drops
     0 babbles, 4294950911 late collision, 0 deferred
     310 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
c3825#sh int GigabitEthernet0/1
GigabitEthernet0/1 is up, line protocol is up 
  Hardware is BCM1125 Internal MAC, address is 001d.7039.1141 (bia 001d.7039.1141)
  Description: Link to Internap Border11
  Internet address is XX.XX.239.122/30
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full Duplex, 1Gbps, media type is RJ45
  output flow-control is XON, input flow-control is XON
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:19, output 00:00:01, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/2 (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
     188601 packets input, 34900616 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 watchdog, 2650799615 multicast, 0 pause input
     135889 packets output, 13672869 bytes, 0 underruns
     0 output errors, 4293852927 collisions, 3 interface resets
     0 unknown protocol drops
     0 babbles, 4150263791 late collision, 0 deferred
     2 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
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you provided only the output on your side, what about the other side ? how could you be sure it is also duplex full and speed 1000 ? if you are 1000/full and the other side is auto, there might be mismatch. –  aseaudi Jan 6 at 9:28

1 Answer 1

up vote 2 down vote accepted

Any idea on resolving these collisions?

The simple answer, remove the manual speed / duplex configurations from the interfaces affected by the collisions. Auto negotiation is required by Gigabit Ethernet in the IEEE spec.

Almost all ethernet interfaces default to autonegotiation. If you have configured manual speed / duplex on your router interfaces, then autonegotiation on the other side will fail (usually falling back to half duplex). The mismatched duplex results in collisions (and thus packet loss)...

There are very few reasons to hard-code speed / duplex these days. In the early days of autoneg, it was not well-standardized and did not work well; however, those days are long gone. Use auto-negotiation everywhere... and tell your friends.

share|improve this answer
    
Simple rule of thumb: Unless you have a very good reason to do otherwise, do not ever force the speed or duplex on an Ethernet device unless you are certain that the device on the other end has the speed and duplex both forced to precisely the same settings you are forcing your end to. And never, ever force the speed or duplex on Gigabit devices if the connection is copper. –  David Schwartz Jan 6 at 7:33

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