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Is Cisco 871 now the bottleneck?

I've been using a Cisco 871 for 8 years now, and I found it to be an excellent router for my home. Recently I upgraded my Internet connection from 25 Mbps/5 Mbps to 50 Mbps/11 Mbps. When I connect my MacBook Pro directly to the cable modem with a CAT5e cable, I get 55 Mbps down and 11 Mbps up. When I connect my MacBook Pro directly to an interface on the Cisco 871, the most I get is 27.5 Mbps down and 11 Mbps up. So, the download bandwidth is clearly reduced with the Cisco 871 being between the cable modem and my MacBook Pro.

So, I cleared the WAN interface on my Cisco 871 (FastEthernet 4) and did a "show interface fast 4" command. The output is here:

WAN_VPN#show int fast 4

FastEthernet4 is up, line protocol is up

  Hardware is PQUICC_FEC, address is "X"

  Description: WAN INTERFACE

  Internet address is "Y"

  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 set (10 sec)

  Full-duplex, 100Mb/s, 100BaseTX/FX

  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 00:00:20

  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 122000 bits/sec, 16 packets/sec

  30 second output rate 7000 bits/sec, 5 packets/sec

     176 packets input, 13865 bytes

     Received 155 broadcasts (0 IP multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog

     0 input packets with dribble condition detected

     46 packets output, 8658 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 unknown protocol drops

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier

     0 output buffer failures, 0 output buffers swapped out

This looked fine. Then, I did a speed test from speedtest.net. As soon as the download test completed (and the upload speed test had not yet commenced), I issued another "show interface fast 4" command, and here what I saw:

WAN_VPN#show int fast 4

FastEthernet4 is up, line protocol is up

  Hardware is PQUICC_FEC, address is "X"

  Description: WAN INTERFACE

  Internet address is "Y"

  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

     reliability 255/255, txload 1/255, rxload 33/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Full-duplex, 100Mb/s, 100BaseTX/FX

  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 00:00:51

  Input queue: 0/75/29/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  30 second input rate 13110000 bits/sec, 1093 packets/sec

  30 second output rate 165000 bits/sec, 302 packets/sec

     43181 packets input, 64567366 bytes

     Received 395 broadcasts (0 IP multicasts)

     0 runts, 0 giants, 55 throttles

     58 input errors, 0 CRC, 0 frame, 0 overrun, 58 ignored

     0 watchdog

     0 input packets with dribble condition detected

     11883 packets output, 814636 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 unknown protocol drops

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier

     0 output buffer failures, 0 output buffers swapped out

This router's WAN interface (FastEthernet 4) is a 10/100 Mbps interface. I've been running it in auto/auto, but once I saw the input errors and the throttles, I tried to hard set it to 100/full. The result was that I saw throttles, input errors, but also CRC errors and frame errors. So, I now set it back to auto/auto.

As soon as I run a speed test from www.speedtest.net, I get the throttles and the input errors.

I cannot control what the cable modem's interface is set to - it's configuraiton page does not allow one to change settings on the interface. The cable modem is Motorola Surfboard SB6121 (DOCSIS 3) modem.

So, can someone explain what the throttles are and why they are present? Are input errors caused by the throttles or vice versa? Why am I getting these erorrs? The physical interface speed is 100 Mbps, but the most I'm able to pull from my cable modem is perhaps 60 Mbps, so why the throttles?  Finally, is it possible that I've exceeded the capacity of this router to handle traffic (in Mbps or Packets per second)?

I also have an ASA5505 (still sitting in a box). If the problem is that my ISP speeds now exceed the capabilities of the Cisco 871, would replacing it with the ASA5505 resolve this issue? Is ASA5505 capable of handling ~50 Mbps down and ~10 Mbps up?

Thank you!

2 REPLIES

Is Cisco 871 now the bottleneck?

Each router has routing performance limitations.  So even though if the port is a 100Meg port it doesn't mean that it will give you 100Meg and router will be able to handle all that traffic.  Here are couple of links to understand this better:

http://www.cisco.com/en/US/prod/collateral/routers/ps10538/aag_c45_556315.pdf

http://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf

Hall of Fame Super Gold

Is Cisco 871 now the bottleneck?

Input errors can sometimes mean Duplex Mismatch errors.

What's the interface configuration?  I am seeing the speed/duplex setting being hardcoded, instead of auto negotiate.  Can you kindly confirm?

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