TCP/IP configuration - MTU and MSS

Posted on 2010-01-10
Medium Priority
Last Modified: 2012-05-08
I have two 6509s that operate in HSRP mode to do routing for 40 vlans on my network.
I do NOT have any routed ports on these switches.  One of my vlans shares a subnet with the WAN router and that is how my network gets out into the ATM network and internet.
See sketch  attached and requirement from new ISP.

We are changing ISPs and ultimately, the WAN router.  The new provider requires changing of the MTU setting from its default  1500 and I need an experts opinion on where to apply the setting given my network. Will provide more information as it is necessary to understand my  network.

Question by:Bokis
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LVL 34

Expert Comment

by:Istvan Kalmar
ID: 26282136
You use VPN, or MPLS, or extra overheaded protocolls to outside you need to use smaller MTU, for the best traffic distribution
LVL 17

Assisted Solution

rochey2009 earned 500 total points
ID: 26282444

You'd normally set the mtu size on the WAN interface of the WAN router.
LVL 50

Assisted Solution

by:Don Johnston
Don Johnston earned 1100 total points
ID: 26282545
The most efficient place to set the MTU is at the source of the traffic. In this case, the host PCs. That way, the packets never have to be fragmented (and re-assembled).

However, that can be a bit of a pain as you have to configure each PC.  

The MTU can be set at any layer 3 interface. It looks like your only option there is either the 6509's or the ATM router. Since you've probably got some internal traffic that is not subject to these MTU restrictions, I would probably set the MTU at the ATM router.
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Assisted Solution

bsohn417 earned 400 total points
ID: 26283044
i agree with (rochey2009). on cisco 6509 u can change it to only interface thats attached wan. or on rtr thats attached your internet and lan attached your network. u don't need to change global mtu on sw.

Author Comment

ID: 26288098
Thanks guys, for your input.
I am ready to change it on the interface that attaches to the WAN router. However, My 6509 is not even physically attached to the WAN router.  It is attached to my server farm switch and I do NOT have a routed port on the 6509. i.e I just have one of my vlans sharing the same subnet with the WAN router such that the ARP entries for this vlan actually show up on the WAN router and I do NOT manage this router. I just need to make sure that I conform with their MTU settings on my 6509 and right now I do not see how I would apply such a config....on the SVI that shares the same subnet with the WAN router??  Please review the labels on my diagram. I am looking at a situation where I may have to reconfigure the 6509 different than it is currently configured. I will provide an except of my routing table if needed.
LVL 50

Accepted Solution

Don Johnston earned 1100 total points
ID: 26288821
If the closest layer 3 interface to the ATM router is the SVI on the 6509, that's where you would set the MTU.

Author Comment

ID: 26289140
Thank you Donjohnston, I will check this setting out tomorrow while at the office and report back in this forum.

Author Comment

ID: 26312431
The thing worked like a charm! Thanks Guys, for all your help. I will close the case when I get back here.

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