From: Knellinger, Mark (KnelliMS@xxxxxxxxxxxxx)
Date: Tue Nov 20 2001 - 12:33:33 GMT-3
Just set the MTU on the interface manually on one side.
conf t
int to 0
mtu ?
Mark
-----Original Message-----
From: afiddler [mailto:afiddler@wi.rr.com]
Sent: Monday, November 19, 2001 4:53 PM
To: Ccielab (E-mail)
Subject: MTU mismatch on token-ring OSPF network
For you token-ring fans (all three of you):
I have a few older routers (2500's) and a one newer router (3640) with
token-ring interfaces. I have run across an OSPF MTU mismatch problem that
I consider to be a "gotcha" if you don't know about it. The old routers
with an MTU of 4464 on their token-ring interfaces will not neighbor up with
the newer router that has an MTU of 8136. I see debug messages as follows
on the older routers, which also reflects their ability to detect the MTU
mismatch:
01:57:32: OSPF: Nbr 200.0.0.1 has larger interface MTU
CCO has a tech note about this problem (URL is below). The tech note
indicates that MTU mismatch detection was introduced in 12.0(3) to comply
with the OSPF RFC. The "IP OSPF MTU-IGNORE" command was introduced in
12.1(3) to optionally "turn off" mismatch detection. Note that the v12.2
OSPF command documentation erroneously indicates the "IP OSPF MTU-IGNORE"
was introduced in 12.0(3). Instead, I believe there is a gap in the IOS
between the version that supports detection of MTU mismatch and the version
that provides the "MTU-IGNORE" command.
The tech note indicates that the IOS does not support changing MTU on a LAN
interface. With respect to the OSPF problem, that is not the case. In
fact, it's the only way I am able to get OSPF to neighbor up on a token-ring
network with my older routers running 12.0(18) code. I just change the MTU
on the router with the default MTU of 8136, down to 4464, and they neighbor
up immediately.
The tech note also indicates that the need for the IP OSPF MTU-IGNORE
statement is rare. I am not clear on their reasoning, particularly because
they also say that you can't change the MTU on a LAN interface. What else
can you do (besides not use token-ring anymore ;-)?
This problem is not specific to the mix of "older" and "newer" routers. I
ran into the same problem recently when I attended a training class, setting
up OSPF over token-ring between two seemingly identical 2600 routers!
It would have to be something to watch for anytime you set up OSPF over a
token-ring supporting more than one router and the routers are running
12.0(3) and up but less than 12.1(3). With these IOS versions, the routers
would detect MTU mismatches but would lack the OSPF parameter to ignore
them. The only other option is to change the MTU on the token-ring
interface of the larger MTU router to match the smaller MTU router using the
interface command "mtu nnnn", where the n's represent the lower MTU in
bytes.
If I am not mistaken, a similar issue exists with IS-IS routers on a
token-ring network. I tried the same test with ISIS defined instead of OSPF,
but was unable to resolve the MTU mismatch by reducing the MTU on my newer
router. CCO has a tech note on this issue as well (see URL below), but
their example involves two serial interfaces (also note that the link to
this URL from the Cisco > Service & Support > Technical Assistance Center >
Technologies > Integrated Intermediate System-to-Intermediate System (IS-IS)
> Implementation and Configuration menu points to a different URL). As
they do in the OSPF tech note, they suggest setting the MTU to the same
value on all ISIS routers on the network. This method did not work
successfully for me, even when manually setting MTU on both the old and new
routers. Their other suggestion was to use the "no isis hello padding" on
both routers. To test this, I upgraded my 2504 to v12.1(5)T10, since this
command is not supported in 12.0(18). I tried this separately and in
combination with setting the MTU on the 3640 to match the MTU on the 2504,
to no avail. Just to verify my results, I configured the two older routers
with ISIS to see if they neighbored up. They did immediately.
Does anyone know to circumvent the problem on routers running ISIS over
token-ring with mismatched MTU's?
http://www.cisco.com/warp/public/104/12.html
http://www.cisco.com/warp/public/97/isis_mtu.html
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