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Re: Bridging 802.1q posted 10/05/2002
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A couple of other things about Interface MTU vs. ip mtu settings
1.) The interface MTU setting does apply to layer to on the physical
interface and requires the interface be reset to take effect. ip mtu does
2.) The mtu setting can only be applied to a physical interface it cannot be
used on a sub-if or tunnel interface. The ip mtu can.
3.) Where I have had to use this is on end-to-end paths that traverse a
tunnel of some sort usually GRE.
If there is an MTU mismatch were the traffic with the DF bit set enters a
tunnel on an interface with a large MTU say 4472 like most optical links
then hits a link with a smaller MTU like a T1 the icmp message goes back to
the ingress of the tunnel since that is the source IP within the tunnel, not
to the sender. This causes traffic to get dropped. Setting the ip MTU on the
ingress to the tunnel causes the ICMP message to go back to the sender now
and everyone is happy.

----- Original Message -----
From: "Donny MATEO" <donny.mateo@xxxxxxxxxxxxxxxxxx>
To: <ccielab@xxxxxxxxxxxxxx>
Sent: Friday, October 04, 2002 10:08 PM
Subject: RE: Bridging 802.1q

> Correct me if I'm wrong
> ip mtu and mtu are different. In a way for IP MTU is the packet(L3) size
that is check and then
> depending on whether fragmentation is allowed or not packet could still be
sent over the link. If
> you enable fragmentation, the L3 packet that exceed the specified ip mtu
would just be chop down
> into serveral packet and will still be sent by the route.
> mtu on the interface is to set the maximum size of the frame (L2), meaning
it guarantee the size of
> the frame that is sent over the L2 media.
> Donny
>                       "Pylko, Eric"
>                       <EPylko@xxxxxxxxxxxxx        To:
>             >                  cc:
>                       Sent by:                     Subject:  RE: Bridging
>                       nobody@xxxxxxxxxxxxxx
>                       05-10-2002 04:28
>                       Please respond to
>                       "Pylko, Eric"
> Why not increase the MTU size on the interface?  Isn't it "ip mtu ..."
> instead of "mtu ..."?
> -Eric
> -----Original Message-----
> From: Huy Luu [mailto:hluu@xxxxxxxxxxxx]
> Sent: Thursday, October 03, 2002 4:32 PM
> To: ccielab@xxxxxxxxxxxxxx
> Subject: OT:Bridging 802.1q
> Hello gents,
> Can you guys help me?
> here's the scenario:
> SwitchA--------802.1q-------Router-----------T1------------Router-------
> 802.1q--------SwitchB
> I tried to configure the router to bridge 802.1q accross from swtich A
> to switch B.  The routers ethernet interface are hardcoded with 1500
> MTU, so when lardge packets are passed through, the 802.1q header makes
> increase
> the size of the packet over 1500 bytes and the packet is dropped.
> Seems
> like the only cisco equipment that allows changing the MTU on ethernet
> is a GSR, which is too expensive to carry T1.
> Is there any other version of the IOS or hardware that allows MTU size
> to be manipulated over the 1500 bytes limit on ethernet interface on
> routers?
> Is there another vendor with a router that supports both a T1 interface
> and allow MTU to be changed?
> thanks in advance.
> Huy Duc Luu
> Veroxity Technology Partners
> Fax 617-779-3099
> Cell# (617) 293-0940
> Email: hluu@xxxxxxxxxxxx
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