Where are the servers or mainframes that the Token Ring PCs are using? Do
they support a large frame size? What is the traffic flow? Does the traffic
from the PCs travel across an Ethernet segment or some other segment that
does not support a large frame size? Are you using bridging or routing?
The steps you should take include:
Analyze the physical topology.
Analyze the logical topology and traffic flow.
Define the problem with as much detail as possible. How else can you
characterize the "poor network performance?"
Gather facts, such as why was the frame size changed? Does it affect all
users? Is it reproducible across applications? What is the actual response
time? What are the error rates at the data-link layer, upper layers?
Analyze router configurations, determine if bridging or routing is
happening. What do show interface commands show?
Consider possibilities. Try to do this without bias and assumptions (such
as the assumption that performance problems are caused by frame size)
Create an action plan
Implement the action plan
Observe the results
If you have really done the initial steps and are ready to implement the
action plan of changing frame size, you are doing the right think to think
about the implications of making this change.
With very little data to go on, I think you can make the change without
much risk. Both routing and bridging supposedly handle various frame sizes.
Priscilla
At 04:38 PM 9/4/01, khramov wrote:
>Here is what I've got:
>We have a remote site where we run token ring and ethernet. Core router
>(Cisco 4500)
>has 1 tr interface and 2 ethernet interfaces. One ethernet interface goes
>out to the
>firewall and the other interface is just an ethernet subnet with
>workstations.
>Some time in the past, I am not sure but for whatever reason frame size on
>token ring
>interface was changed to 1500 from I believe the default 4500. Ever since
>users have
>been complaining about poor network performance. I think that if change MTU
>it might
>speed them up a little bit. However, all the PC are also set to 1500 frame
>size. So I
>am debating whether changing the frame size will help users and if I change
>it how long
>the network will be down or is it just a matter of rebooting PCs.
>So I guess my router does both routing and translation. What do you think I
>should do?
>
>Priscilla Oppenheimer wrote:
>
> > At 02:40 PM 9/4/01, khramov wrote:
> > >I need to change frame size on of my Token Ring subnets. If I change the
> > >Token Ring frame size on the router and it will be different (larger)
> > >than the frame size that is configured on a work station how is it going
> > >to affect the work station? And if I change the frame size on the
> > >workstation first would that crash the router?
> > >Thanks
> >
> > It shouldn't be a serious problem either way you do it. Users may have to
> > re-establish connections, however.
> >
> > Are the routers truly routers or are they source-routing bridges? With
> > source-route bridging, each bridge checks the "largest frame" field and
> > decrements it if the bridge is configured for something smaller than is
in
> > the frame. This happens during route discovery and session establishment.
> > Changing one of the bridges after sessions are established could cause
> > problems.
> >
> > If the routers are truly routers and are routing IP, there's a similar
> > situation. IP hosts can use IP maximum transmission unit (MTU) discovery
to
> > determine the largest possible frame size to use. This happens during
> > session establishment. Changing the MTU on one of the routers after
> > sessions are established could cause problems.
> >
> > But routers shouldn't crash and there shouldn't be any other dire
> > consequences. Also, it sounds like you are increasing MTU which is less
> > worrisome than decreasing MTU.
> >
> > Do you do any translational bridging between Ethernet and Token Ring
> > segments? Translational bridging can be difficult to get working,
> > especially if you use Token Ring frame sizes that are larger than
Ethernet
> > allows (1500 bytes). If you use routing instead of bridging, there will
be
> > fewer problems.
> >
> > If your routers are now going to need to fragment and reassemble IP
> > packets, in order to go between Token Ring which supports larger frames
and
> > Ethernet which supports 1500 byte frames, keep in mind that the routers
> > will be slower. This is additional work that they don't normally have to
> > do. I think it forces them to use process switching instead of the faster
> > switching methods.
> >
> > Priscilla
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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