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
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=18478&t=18466
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