2620 with cable modem

Posted on 2006-11-28
Medium Priority
Last Modified: 2009-09-02
I have a 2620 with a cable modem.  I am recieving the ip through dhcp, but with a couple problem.  

For one it is very hard to get an ip assigned to this router.  I reload the router, power cycle the modem, shutdown and no shutdown the interface and eventually whether it takes a minute or 30 minutes of fooling around I will recieve an ip somewhere along the way.  no set process will give me an ip address.

Second, pages are not as snappy as if I hooked up a little linksys router.  Speed tests come out the same on either, but the cisco router seems to be waiting for page, in most cases a second or two longer.  Sometimes pages will stop loading half way through.  The overall expeirence is just not as quick.

Does anyone have an thoughts?
Question by:thur6165
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Assisted Solution

lrmoore earned 1000 total points
ID: 18031315
Been there, done that. 2611, 3640, both with same results you're seeing.
PIX 506 is awesome and runs circles around any router, but absolutely will not pick up an IP address via DHCP.
Linksys WRV54G is close 2nd in performance to the PIX

Try manually assigning the IP address that you get on the Linksys to the 2620 and see if the performance is better.
Cable IP's almost never change. I've had 3 different IP's in over 6 years of service from my same cable provider.

Expert Comment

ID: 18031570
did you try:

ip address dhcp

To acquire an IP address on an Ethernet interface from the Dynamic Host Configuration Protocol (DHCP), use the ip address dhcp interface configuration command. To deconfigure any address that was acquired, use the no form of this command.

ip address dhcp [client-id interface-name] [hostname host-name]

no ip address dhcp [client-id interface-name] [hostname host-name]

see http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipras_r/1rfdhcp.htm#wp1049289

Expert Comment

ID: 18032279
Some other things that may need to be done are :
1. Check your interface for speed/duplex (check for errors)
   Sometimes this can cause big problems, especially in a large broadcast domain (i.e. cable modem)
2. Try lowering your MTU and/or MSS sizes.
   Depending on how your ISP handles their cable modem platform, your router may be having problems with larger packets (causing web page slowdowns)  This is especially true if your router uses PPPoE to connect to the ISP.  Try reducing the MTU to a value below 1400.  Then if that works, then you can slowly increase the value to maximize the performance of the interface.

You can change these settings on the interface with the following commands:
speed [10/100]
duplex [half/full]
ip mtu [value]
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Expert Comment

ID: 18033815
In my experience with 2600's, 1700's, etc...getting an ip address via dhcp on cable networks will cause you to go insane.  What I would do is this, if your ip rarely changes.  Use your linksys  to pull an ip, or use your router if it will grab one - note the ip address.  Then, if you used the linksys, assign the mac address of your linksys to the outside interface of your router, and statically assign the interface the dhcp info you received.  If you are able to pull dhcp with just the router, statically assign the interface the dhcp'd info.  

I would recommend that you spend the extra money to get what most cable companies call a "sticky" or "persistent" ip address.  It is basically dhcp reservation, but for some reason they still won't guarantee that you will keep that address.  Especially during periods in which they change their core equipment.  But, the great thing is, you can statically assign the ip that is dhcpd'd to you, and you won't have to worry about their dhcp server giving your ip to someone else because it is reserved for the mac of your router....most times.  

Author Comment

ID: 18062210
I did manually assign the ip that the linksys was getting, but still having the performance issues.  At least I don't have to worry about the dhcp issue anymore.

I checked for errors, none what so ever.  I could not change the mtu setting, router would not let me. I did set mss to 1400 with no effect.

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, 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 never
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 6000 bits/sec, 13 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
   36063 packets input, 3825865 bytes
   Received 32015 broadcasts, 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
   3841 packets output, 1889451 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 output buffer failures, 0 output buffers swapped out

Alot of broadcasts, guess thats just cable company.

The error I get when trying to set the mtu on an interface is this
% Interface FastEthernet0/0 does not support user settable mtu

and when I try to set the mtu on a sub interface I get
% Non-TRISL encapsulated sub-interface FastEthernet0/0.10 does not support user settable mtu.

I can use ip tcp adjust-mss 1400 on both

Author Comment

ID: 18062258
Although I did just manually set the linksys to 1500MTU and its working great.  

Expert Comment

ID: 18065564
Are you using for your default route statement....
ip route fa 0/0


ip route x.x.x.x

Where x.x.x.x is the actual ip of your gateway?

Author Comment

ID: 18065693
I'm using
ip route fa0/0.10

which is the subinterface to the WAN connection.

Accepted Solution

gmooney7 earned 1000 total points
ID: 18066531
Try changing your default gateway to actually point at the correct ip of the gateway, vs using the interface.  I've had a lot of trouble in the past doing that on cable networks, because it is not a "point to point" connection.  It works really well with serial interfaces, or any interface where there is only 1 other endpoint.  See what it does for you.
LVL 79

Expert Comment

ID: 18067886
Agree w/gmooney7
It is not recommended to use a broadcast interface (Ethernet) as a gateway. It will severely impact performance because the router has to arp for a next hop for every foreigh ip address.

Expert Comment

ID: 18070236
Also, I've seen some cases where you may get an IP address, but still can't connect to the internet.  If you remove the route all together, that may help.
This only works when you are receiving information via DHCP (which will include the default gateway address)


Author Comment

ID: 18073414
You were right on, I needed to specify the gateway in the route. works great now, thanks.

Author Comment

ID: 18082151
Strangely, my ip has changed 3 times in the last 2 days with this setup.

Expert Comment

ID: 18085591
You might have to go for the sticky ip option then.  Call your cable company for pricing.  

Unfortunately, I have not heard of a cable company yet offereing actual static ip's.  But, I've talked to a couple that have it in the works to actually make static ip's available to customers.  Thank God!

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