[c-nsp] 7200 MPLS MTU Issues

[Kenny Stoltz]

Yes this is what I have been dealing with for some time...

I have both 7200s and 2900XLs. All my 2900s are running 12.0(5.2)
which is the first IOS to support bigger MTUs (up to 2018), and I have
all links in question at that size (I have also had them at 1600 and
1524).

my interface on the 7200 looks like this:

interface FastEthernet1/0
ip address ip2 secondary
ip address ip1
load-interval 30
duplex full
tag-switching mtu 1526
tag-switching ip
end

my switch ports look like:

interface FastEthernet1/2
mtu 2018
switchport access vlan 112

end

and my other router looks like:

interface Vlan112
ip address ip2 secondary
ip address ip1
no ip redirects
tag-switching mtu 1526
tag-switching ip
end

(my other router is a 7600, the link into it is an ISL trunk)

and with tag-switching on, I cannot ping above 1496, with it off I can
ping to 1500. I plan on hooking a 7200 with a PA-FE-TX directly into
my 7600 tomorrow to absolutely eliminate the switches, but they are
the appropriate version to do this correctly.


On Tue, 15 Mar 2005 23:43:11 -0500 (EST), Jon Lewis <jlewis at lewis.org> wrote:
> On Tue, 15 Mar 2005, John Osmon wrote:
> 
> > On Tue, Mar 15, 2005 at 09:44:22PM -0500, Kenny Stoltz wrote:
> > [...]
> > > Ironically, cisco claims that the 7200 can /do/ MPLS... This is almost
> > > laughable if it can't exceed 1500 bytes.
> >
> > If you can find way to migrate the links to GigE and/or serial ports,
> > the routers should work fine.  If you're stuck with ether/fastE, you may
> > need to find another workaround.
> 
> Huh?  The 7200 FEs (both IO/FE and PA-FE) handle the 'oversized' MPLS VPN
> tagged packets just fine.  You have to configure tag-switching and a
> tag-switching MTU on any interface that will be passing tagged packets.
> i.e. Try adding
> tag-switching mtu 1520
> tag-switching ip
> to your FE interfaces that need to pass MPLS VPN tagged packets.
> 
> The next problem you'll run into is switches not wanting to deal with
> oversize (giant) packets.  Older software revs on the 2900XL series
> wouldn't do it at all.  Fortunately, cisco came up with a fix in later
> software revs allowing you to set a larger than 1500 MTU on 2900XL switch
> ports.  The packets are still counted as giant errors, but at least they
> pass.
> 
> ----------------------------------------------------------------------
> Jon Lewis                   |  I route
> Senior Network Engineer     |  therefore you are
> Atlantic Net                |
> _________ http://www.lewis.org/~jlewis/pgp for PGP public key_________
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/
>