Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

MTU size

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

Hall of Fame Super Bronze

Re: MTU size

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 ?



Super Bronze

Re: MTU size

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.