From: Bob Rech (brech@kc.rr.com)
Date: Mon Oct 07 2002 - 12:29:29 GMT-3
I guess I'm kinda missing what you want to do.
Your right you can't reset the DF bit, you also can't set the MTU larger
then what's allowed by the media/protocol.
I was giving some info on the difference in using mtu vs ip mtu. Normally
you do not need to change the media mtu because it is set to the max for the
media. Under some conditions you may need to make the mtu smaller to work
around mtu mismatches in a path that can't be discovered by ICMP or PMTU.
----- Original Message -----
From: "Huy Luu" <hluu@veroxity.com>
To: "Robert Rech" <brech@kc.rr.com>
Cc: <ccielab@groupstudy.com>; "Donny MATEO"
<donny.mateo@sg.ca-indosuez.com>; <nobody@groupstudy.com>
Sent: Monday, October 07, 2002 8:59 AM
Subject: Re: Bridging 802.1q
>
> The problem here is thta you cannot reset the df bit or tune ip or
ethernet
> MTU. The router are configured as a bridge to bridge 802.1q and that's
all
> layer 2. I think the only way is to find equipment that would allow one
to
> modify the MTU of the ethernet interface to a bigger value than cisco's
> 2500. Does anyone know of any other vendor equipment that can supports
> this ?
>
> Huy Duc Luu
> Veroxity Technology Partners
> Fax 617-779-3099
> Cell# (617) 293-0940
> Email: hluu@veroxity.com
>
>
>
>
> "Robert Rech"
> <brech@kc.rr.c To: "Donny MATEO"
<donny.mateo@sg.ca-indosuez.com>,
> om> <ccielab@groupstudy.com>
> Sent by: cc:
> nobody@groupst Subject: Re: Bridging 802.1q
> udy.com
>
>
> 10/05/02 09:00
> AM
> Please respond
> to "Robert
> Rech"
>
>
>
>
>
>
> 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
> not.
> 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@sg.ca-indosuez.com>
> To: <ccielab@groupstudy.com>
> 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@frontrunnerne To:
> ccielab@groupstudy.com
> > tworks.com> cc:
> > Sent by: Subject: RE:
Bridging
> 802.1q
> > nobody@groupstudy.com
> >
> >
> > 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@veroxity.com]
> > Sent: Thursday, October 03, 2002 4:32 PM
> > To: ccielab@groupstudy.com
> > 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@veroxity.com
> >
> >
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