MTU issue

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May 7th, 2009
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hi, i have DSL connection from my service provider, he sets mtu 1492 on its dsl modem, My LAN has cisco 2960 switches which is by default 1500 mtu size. The problem is when i connect dsl modem to my LAN switches it stops browsing and when i connect it directly to system it start browsing. what might be the issue and how to resolve it.


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Laurent Aubert Fri, 05/08/2009 - 05:50
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When the modem is connected to the switch, is it acting as a DHCP server ?

How are configured your hosts: static IP or DHCP client ?

The 1492 MTU configured on the DSL interface is normal as you have 8Bytes of overhead due to PPPoE encapsulation.



lamav Fri, 05/08/2009 - 14:35
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"The problem is when i connect dsl modem to my LAN switches it stops browsing and when i connect it directly to system it start browsing."

This sentence makes no sense. Can you clarify.

In short, your MTU should be the same as your providers, if not even a bit smaller to prevent fragmentation.



paolo bevilacqua Fri, 05/08/2009 - 14:47
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Do you have a cisco router in network ?

If yes, something can be done about that.

If not, you need to seek solution from dsl modem vendor.

jjchen978 Sat, 05/09/2009 - 19:13
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1. you might want to check your switch configuration? (i.e. vlan configure?)

2. what is the mtu size on your pc?

pkurdziel Sat, 05/09/2009 - 19:32
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I used to see this a lot a few years ago when I used DSL. The problem is that the dls modem is looking for your PC's MAC address. With linksys, dlink, etc.. to resolve this you would clone your pc's MAC address to the router.

davegibelli Sun, 05/10/2009 - 01:16
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You need to define the problem better.

MTU path discovery maybe broken if the DSL router is stopping ICMP packets.

Your PC sends a packet with size 1500, the DSL router should drop the packet and send an ICMP packet back to your PC to get the PC to reduce the packet size to 1492, is this working?

itdsmartnet Sun, 05/10/2009 - 23:52
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Thanks all of you for your reply, actually i figure out the problem, My vlans and the DSL Lan ips are with the same network. i.e my vlan management ips are with network and my DSL LAN ip is also

All of my vlans are assigned ip range but my DSL LAN is don't know why it is causing problem. Any suggestion


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