How far to adjust MTU?

  • Thread starter Thread starter Tom Del Rosso
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Tom Del Rosso

How low can the MTU setting go before it is not expected to solve anything
and it's time to try something else?

My router needs to be reset every day for almost the past year. Lowering
the MTU never made much difference (replacing Linksys with Netgear helped a
lot though, as with Linksys I had to reset it every hour), and I have
lowered it incrementally down to about 700.

(As soon as I get the current loss of connection fixed I'll check for
firmware updates again.)
 
Tom Del Rosso said:
How low can the MTU setting go before it is not expected to solve anything
and it's time to try something else?

My router needs to be reset every day for almost the past year. Lowering
the MTU never made much difference (replacing Linksys with Netgear helped a
lot though, as with Linksys I had to reset it every hour), and I have
lowered it incrementally down to about 700.

(As soon as I get the current loss of connection fixed I'll check for
firmware updates again.)
Click to expand...

I never mess with it.

It would be very odd if your Lysys or Netgear
required this.
 
Tom Del Rosso said:
How low can the MTU setting go before it is not expected to solve anything
and it's time to try something else?

My router needs to be reset every day for almost the past year. Lowering
the MTU never made much difference (replacing Linksys with Netgear helped
a
lot though, as with Linksys I had to reset it every hour), and I have
lowered it incrementally down to about 700.

(As soon as I get the current loss of connection fixed I'll check for
firmware updates again.)
Click to expand...


Determining the Proper MTU size
http://tinyurl.com/4th5k
 
How low can the MTU setting go before it is not expected to solve
anything and it's time to try something else?

My router needs to be reset every day for almost the past year.
Lowering the MTU never made much difference (replacing Linksys
with Netgear helped a lot though, as with Linksys I had to reset
it every hour), and I have lowered it incrementally down to about
700.

(As soon as I get the current loss of connection fixed I'll check
for firmware updates again.)
Click to expand...

The MTU shouldn't have anything to do with your problem.

Your problem sounds like an IP lease that is not getting properly
renewed. The initial IP lease is obtained using low level ethernet
packets because an IP address to use is not available. Renewing the
lease requires the use of ports 67 & 68. Improperly configured
firewalls can block this renewal process and cause the lease to expire
and the connection to be lost. Different routers have different
default lease times which may explain your Linksys/Netgear difference.
When you reboot your router, check the lease times on both the WAN
side (by looking at your router status page) and the LAN sides (by
doing "ipconfig /all" on your PCs) of the router. The lease should
automatically renew when it has half expired. If it doesn't, that's
your problem.

HTH,
John
 
Herb Martin said:
I never mess with it.

It would be very odd if your Lysys or Netgear
required this.
Click to expand...

It certainly is odd, since nobody I know of with a cable modem in this
general area has a problem like this. I had the Linksys router for 2+ years
with only 1 lock-up. Then it suddenly happened all the time. Changing the
MTU was the only actual suggestion that came out of the cable company or
Linksys or Netgear support, so it must be the kind of issue a firmware
update would help, and I just downloaded one. (I just wish the router's
status page would show the number of the current version because I might
have had it already.)
 
Tom Del Rosso said:
It certainly is odd, since nobody I know of with a cable modem in this
general area has a problem like this. I had the Linksys router for 2+ years
with only 1 lock-up. Then it suddenly happened all the time. Changing the
MTU was the only actual suggestion that came out of the cable company or
Linksys or Netgear support, so it must be the kind of issue a firmware
update would help, and I just downloaded one.
Click to expand...

That would be my first move.

My cable company let's me to a counter "swap"
and that would be my second move (and they
are only a mile away.)
(I just wish the router's
status page would show the number of the current version because I might
have had it already.)
Click to expand...

Weird too that it doesn't show the version.
 
John Wunderlich said:
The MTU shouldn't have anything to do with your problem.

Your problem sounds like an IP lease that is not getting properly
renewed. The initial IP lease is obtained using low level ethernet
packets because an IP address to use is not available. Renewing the
lease requires the use of ports 67 & 68. Improperly configured
firewalls can block this renewal process and cause the lease to expire
and the connection to be lost. Different routers have different
default lease times which may explain your Linksys/Netgear difference.
When you reboot your router, check the lease times on both the WAN
side (by looking at your router status page) and the LAN sides (by
doing "ipconfig /all" on your PCs) of the router. The lease should
automatically renew when it has half expired. If it doesn't, that's
your problem.
Click to expand...

Thats the first logical explanation I've seen, thanks. It can't be the
local IPs because the problem affects both of my PCs simultaneously, and
their leases don't expire for 33 years according to ipconfig. Unfortunately
the Netgear router doesn't show the lease time of its WAN address, and the
Scientific Atlanta modem hardly gives any information.

It may be that the routers are locking up and the latest firmware will fix
it. I've updated them frequently but the latest claims to fix some
lock-ups. Or maybe the cable segment has somebody playing around?

Here are my notes taken from 10 to 6 months ago (as I recall it was perfect
the day before):

Motorola Surfboard SB3100
Linksys BEFSR41

SYMPTOM: must restart modem every 10 minutes (can usually be done through
the config page)

ACTION: updated router firmware (Linksys BEFSR41 1.45.7, Jul 31 2003)

SYMPTOM: must restart router every day; can't ping router at those times

ACTION: changed router gateway IP from default 192.168.1.1 to 192.168.10.1

SYMPTOM: back to "must restart modem every 10 minutes"

ACTION: reset all defaults on modem

SYMPTOM: must restart modem even more often

ACTION: replace Motorola modem with Scientific Atlanta

SYMPTOM: must restart router at least every day; can't ping router at those
times

ACTION: insert hub between cable modem and router

SYMPTOM: no change; removed hub

ACTION: progressively decrease MTU, now down to 900

SYMPTOM: must restart router, but a little less often than every day

ACTION: Linksys power supply looks steady on oscilloscope; replace Linksys
with Netgear RP614v2 router

SYMPTOM: good for 1 week, then can ping modem but not net; reset modem no
effect; must restart router

......... good for a couple months after that last entry, then back to
resetting router every other day
 
Herb Martin said:
That would be my first move.
Click to expand...

Looking back at my notes, it was my first move too. (See my reply to John
Wunderlich.)

My cable company let's me to a counter "swap"
and that would be my second move (and they
are only a mile away.)
Click to expand...

Hey, that was my second move too! Two great minds....

Weird too that it doesn't show the version.
Click to expand...

As I recall, Linksys didn't show the firmware version either. Netgear says
RP614v2 but v2 refers to hardware version, and there are different firmware
download pages for v1, v2, and v3 models.
 
New cables -- believe it or not, cables
are responsible for far more problems
than most people would even believe.
 
Herb Martin said:
New cables -- believe it or not, cables
are responsible for far more problems
than most people would even believe.
Click to expand...

That's a good idea. I have wired quite a few offices and punched many 48
port panels, so maybe I was too confident of my cables, but they could have
been damaged.
 
That's a good idea. I have wired quite a few offices and punched many 48
port panels, so maybe I was too confident of my cables, but they could have
been damaged.
Click to expand...

And many are just marginal to begin
with.

$5 cables are so cheap it isn't cost effective
to spend time testing them.
 
Thats the first logical explanation I've seen, thanks. It can't
be the local IPs because the problem affects both of my PCs
simultaneously, and their leases don't expire for 33 years
according to ipconfig. Unfortunately the Netgear router doesn't
show the lease time of its WAN address, and the Scientific Atlanta
modem hardly gives any information.

It may be that the routers are locking up and the latest firmware
will fix it. I've updated them frequently but the latest claims
to fix some lock-ups. Or maybe the cable segment has somebody
playing around?
Click to expand...
[snip]

I have a Linksys BEFSR41 router that has been updated to firmware
revision 1.46.02, Aug 03 2004. It will display the remaining time of
lease on status page. My ISP has configured a 3-hour lease time.
Clicking on the "renew" button on the status page will bring the "DHCP
Remaining Time" back to 3 hours. If you can do this (or similar), then
it probably is not a lease renewal problem on the WAN side.

On the LAN side, it's a little scary that your PCs report a 33 year
DHCP lease. My BEFSR41 router will give a default 2-day lease (by
putting a '0' in the client lease time box). If your PC thinks it has
a 33 year lease, then it will not attempt to renew the lease for 15
years. Meanwhile, the router has timed-out the lease after 2 days.
This could manifest itself as a lockup condition.

HTH,
John
 
John Wunderlich said:
I have a Linksys BEFSR41 router that has been updated to firmware
revision 1.46.02, Aug 03 2004. It will display the remaining time of
lease on status page. My ISP has configured a 3-hour lease time.
Clicking on the "renew" button on the status page will bring the "DHCP
Remaining Time" back to 3 hours. If you can do this (or similar), then
it probably is not a lease renewal problem on the WAN side.
Click to expand...

The Netgear's configuration is laid out in a way that doesn't always make
sense to me, and I'm not used to it, so it took me forever to find these
things. I found the firmware version on a page I didn't expect but I
updated it before I looked, so I might have had the same version before.

A second-level linked page has the WAN DHCP lease date, but it gives the day
of the week and not the date, so I renewed it to see what day it thinks it
is, and now it says renewed Saturday but today is Monday. The router has
the correct date and time, so can this be coming from the ISP? Oh damn, I
renewed it again and even the day of the week disappeared, showing only
time. Netgear must be a bit flaky. Ok, renewed it again after a few
minutes and now it says renewed Wednesday!

In any case it indicates an 84 hour lease time, so I will see if it next
goes out at the corrected time, and if it renews in 42 hours. Thanks for
the tip. On the other hand, I can't even ping it internally when this
happens, so if it is the loss of the WAN lease then it must cause a complete
lock-up.

I just found another interesting thing on a second-level page that I hadn't
seen before. A table of statistics, and it says "Error!" on the WAN side
where it says 100M (I assume that means 100 base T) on the LAN side. So
this thing could be having trouble even negotiating its speed. Now I know
which cable to try replacing!

Port Status TxPkts RxPkts Collisions Tx B/s Rx B/s Up Time
WAN Error! 762687 3296044 0 8650519 138185 2 days 00:27:13


Port Status TxPkts RxPkts Collisions Tx B/s Rx B/s Up Time
LAN 100M 835630 776142 0 6896 4771 2 days 00:27:13


On the LAN side, it's a little scary that your PCs report a 33 year
DHCP lease. My BEFSR41 router will give a default 2-day lease (by
putting a '0' in the client lease time box). If your PC thinks it has
a 33 year lease, then it will not attempt to renew the lease for 15
years. Meanwhile, the router has timed-out the lease after 2 days.
This could manifest itself as a lockup condition.
Click to expand...

Sorry I forgot to mention that I have DHCP set to reserve the addresses.
Before I did that, the leases were a few days.
 
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