MTU size

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Jan 24th, 2008
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Hi, my T1 to the internet is dragging. I have a lot of users on the one T1 but I suspect that there is something wrong with the config as well. The serial interface has a MTU set to 4470 and it is Point to Point T1 connection. I thought that 4470 MTU is used for ATM networks. Can someone explaing to me if this MTU is ok for the config? Here is the interface and sub interface config. Thanks for the help!!

interface Serial0/0/0.165 point-to-point

description Users

bandwidth 1536

ip address x.x.x.x255.255.255.252

ip access-group 104 in

ip nat outside

ip virtual-reassembly

no cdp enable

frame-relay interface-dlci 165 IETF

interface Serial0/0/0

mtu 4470

no ip address

encapsulation frame-relay IETF

ip route-cache flow

no ip mroute-cache

no fair-queue

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Overall Rating: 5 (2 ratings)
Edison Ortiz Thu, 01/24/2008 - 16:43
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I don't think a high value MTU would cause that kind of problem as opposed to a lower value MTU which may cause fragmented packets.

I believe the problem is due to an overloaded T1.

Do you have any traffic monitor such as MRTG and see if the interface is reaching close to its limit ?

BTW, I see you enabled netflow on the main interface. Do you see any information from the existing flows hammering the router ?



Joseph W. Doherty Thu, 01/24/2008 - 18:21
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I agee with Edison, more likely your T-1 is congested. If so, the "no fair-queue" will have the interface running with FIFO which usually causes erratic performance when bulk and interactive traffic are mixed. (What the other side of the link is doing for queue management is also often important.)

Besides total traffic monitoring, you can look at the interface rate when the link seems congested and see the inbound/outbound rates. Further, suggest you set the load interval to 30 seconds.

Another indication of congestion is seeing many recorded drops on the interace.

As for the MTU, being too large, the only negative effect, I believe, is suboptimal buffer management. You could set it down to 1500 if you know your packets never exceed that. (You don't, however, want to be smaller than you packets can be or you'll create packet fragmentation.)

I also see you're doing frame-relay. You might also have a situation where your CIR rate is much lower than the line rate and you're getting frame-relay drops.


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