02-14-2011 03:41 AM - edited 03-06-2019 03:31 PM
Hello,
what does this command "system mtu routing" exatly do?
If I have one router with interfaces supported only MTU 1500 (c2600) and second router with MTU up to 9000 (c7200), how can I configure the switch c3560 in between, speaking OSPF with both of them? The command "ip ospf mtu-ignore" doesn help here.
Thank you!
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01-18-2014 06:38 AM
Hi,
maybe I'm a bit late to the party, but when searching the interwebs for "system mtu routing" this thread is what pops up, so I wanted to add a little more information:
Konstantin Dunaev wrote:
second, there is command "system mtu routing" which seems to be applyed only
on L3 phisycal ports (no switchport) and not on the SVI. (but I can't find any 100% clear
statement about it),. in my Lab "no switchports" L3 ports has taken over the
"system mtu jumbo" settings, in my case MTU 9000
The limitations of the platform in question would make no sense if system mtu routing would apply to routed ports, but at the same time would not apply to SVIs. The point here seems to be that the model line cannot fragment in hardware. So to avoid that, it just makes sure every L3 interface (routed port and SVI alike) has the same IP MTU, so there is never any need to fragment a packet.
I've tested this in the lab using two 3560E chassis, trivially set up with the default VLAN, connected through Gigabit (so system mtu jumbo will apply) and then pinging each other's SVIs. Now let's configure the following (using the maximum frame size the platform supports, there's not much sense in limiting it here):
system mtu jumbo 9198
After rebooting, all physical ports running 1000Base or better will allow payloads of up to 9198 bytes to be encapsulated. The routing mtu will still be 1500 at this point. Try to ping one switch from the other like
ping 10.1.1.2 size 1500 df-bit
and it will succeed, but increasing size to 1501 will fail.
Now let's add:
system mtu routing 9000
to both switches and try again (no reboots needed). You will see that
ping 10.1.1.2 size 6000 df-bit
will suddenly work now, and the interface counters will make clear that no fragmentation happens - it's really a single 6000 byte IP packet bouncing forth and back between the switches. That works up to 9000, and starts failing at 9001, exactly as you would expect.
Why is there a rumour that system mtu routing doesn't apply to SVIs? Probably because show interface of an SVI will show you an MTU of 1500 (or whatever your system mtu is), while the same command applied to a routed port will show 9000. This seems to be a glitch, as so often with interface MTU in show commands. More specifically, the show interface MTU is supposed to be the potential payload MTU of the underlying physical interface of that routed interface, and there are other cases where it displays rubbish. One should always compare to the IP MTU as given by show ip interface. Et voila: The IP MTU of our SVI (as given through show ip int vl1) displays as 9000. So the succeeding ping is not a mystery and system mtu routing does exactly what it intuitively states: Change the IP MTU of every L3 interface of the platform.
I know this won't help in cases like the one discussed here, where supposedly two L3 interfaces running at different MTUs are needed. In such cases, one should first reassure that what's needed is really that and there's no way to redesign the setup to avoid that (by placing L3 and L2 boundaries appropriately). If there is no way around that, the 3560 will likely have to go for something that has per-interface IP MTU, like the 49xx or 4500X platforms.
Discussions about MTU often mix up different problems and lead to chaos. IMO this is because two things are often not regarded to the necessary extent by the participants:
HTH,
Andre.
Message was edited by: Me Tried to remove the worst formatting issues introduced by this forum software. Can't fix the random position my answer was inserted into, instead of under the posting I replied to. Why everybody is trying to replace Usenet with something that isn't half as decent is beyond me (ranting mostly because I was logged out in the middle of writing that post and almost lost it).
02-14-2011 04:05 AM
Hi,
the "system mtu routing" command to configure the MTU size on routed ports.
And for your rest of the questions, please see the below link may hel you....
http://www.andovercg.com/datasheets/cisco-3560-catalyst-datasheet.pdf
Please rate the helpfull posts.
Regards,
Naidu.
02-14-2011 04:59 AM
Does this command really apply only to "routed ports" (I suppose the ports with "no switchport" command) ?
Does the SVI (interfece vlan XXX) affected by this command?
In PDF I couldn't find any usefull info about OSPF, it's only some kind of overview.
02-14-2011 05:09 AM
Hi,
This command can be applied only on layer 3 interfaces ie routed ports.
I checked in the lab the command is not supported on the SVI.
You cannot configure a routing MTU size that exceeds the system MTU size. If you change the system MTU size to a value smaller than the currently configured routing MTU size, the configuration change is accepted, but not applied until the next switch reset. When the configuration change takes effect, the routing MTU size automatically defaults to the new system MTU size.
HTH.
Regards,
Swati
Please rate helpful posts
02-14-2011 04:29 AM
Just found another link which might as well be helpful..
I would suggest you to go through this link it has a good expanation..
also on the 3560 switch you cannot specify the MTU on the interface basis, so when this command is applied globally on the switch it applies on all the 10/100 interfaces.. and not on gig interfaces.
Hope this helps..
Regards,
ranraju
02-14-2011 05:05 AM
thank you, but the same question has been arised, from the URL:
"You can use the system mtu routing command to configure the MTU size on routed ports."
does it mean that this command applys only on routed ports and doesn't affect SVI (interfca vlan) interface?
In my installytion I don't have any routed ports, they are all L2 ports and routing is done with help of SVI's .
Let sey "interface VLAN10" is connected to c2600 with MTU 1500 and
"interface VLAN20" is connected to c7200 with MTU 9000
If I increase "system mtu" on c3560 then OSPF connection with c2600 bacomes unstable, if I keep MTU 1500 then I get the problem on OSPF adjacency with c7200.
02-14-2011 05:13 AM
Yes, you need to give the "no switchport" to become routed ports.
And, a "routed port" on a 3750 is mostly just an SVI on a internal VLAN with only one port in it. So there isn't really all that much difference
Please rate the helpfull posts.
Regards,
Naidu.
02-14-2011 05:13 AM
To add to my above answer if you are changing MTU on one device you should change it on all the devices in path of the packet ,beacuse if one devices passes the packet with MTU with greater than 1500 , the other device will report packet as gaint and might drop it if the MTU configured on other switch is default at 1500.
Regards,
Swati
Please rate helpful posts
02-14-2011 05:24 AM
yes, it's true about, that MTU should be same on the whole path.
But let say c3560 and c7200 are building the backbone, all device in backbone support mtu 1600.
but the c2600 are connected as like "stub" network to c3560 and don't be used for forwarding of backbone traffic, c2600 are building more like "out of managment" access and they get only partly OSPF routing information from c3560.
02-14-2011 05:18 AM
Hi Konstantin,
"system mtu routing ###" affects SVIs as well. Please see below:
SW_1#sh system mtu
System MTU size is 1508 bytes <<------
System Jumbo MTU size is 1508 bytes
Routing MTU size is 1500 bytes
SW_1#sh int vlan 1
Vlan1 is up, line protocol is up
Hardware is EtherSVI, address is 0025.4553.7940 (bia 0025.4553.7940)
MTU 1508 bytes, BW 1000000 Kbit, DLY 10 usec, <<------
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive not supported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:01:36, output never, output hang never
Last clearing of "show interface" counters never
Cheers,
Shashank
Please rate helpful and correct posts.
02-14-2011 05:29 AM
yes i agree to the same.. it applies on for the SVIs on the switch aswell..
But you might want to make a note of this.. to change the MTU for the switch you could use the command "system mtu size" on the global config mode and it gets applied on all the interfaces including SVIs... but you dont get this flexibility using "system mtu routing" - which will default the mtu size to 1500 for all the layer 3 interfaces on 3560...
Hope this helps..
Regards,
ranraju
02-14-2011 05:33 AM
Hmm, do I see something different :
SW_1#sh system mtu
System MTU size is 1508 bytes <<------
System Jumbo MTU size is 1508 bytes
Routing MTU size is 1500 bytes
SW_1#sh int vlan 1
Vlan1 is up, line protocol is up
Hardware is EtherSVI, address is 0025.4553.7940 (bia 0025.4553.7940)
MTU 1508 bytes, BW 1000000 Kbit, DLY 10 usec, <<------
The "system mtu routing" in your case is still 1500, but "system mtu" is 1508 , and SVI has got the "system mtu" value, not the "system mtu routing"
02-14-2011 05:43 AM
Hi Konstantin,
Yes that is right. ALL ports including the SVIs get their MTU value from the "system mtu" command. "system mtu routing" value CANNOT have a greater value than the "sytem mtu" value.
This means that in case you have system mtu = 1516, you cannot set system routing as 1518 unless you increase the system mtu value to 1518 or higher.
Hope this helps,
Shashank
02-15-2011 01:09 AM
Shashank,
Pardon me but I have a feeling that you have not answered Konstantin's original query, and this has slightly confused me as well.
You have yourself stated that the system mtu routing setting affects all Layer3 interfaces including SVIs. However, Konstantin indicated that the SVI MTU follows the system mtu command, not the system mtu routing - so how does the system mtu routing apply to SVIs after all?
Best regards,
Peter
02-15-2011 03:13 AM
Hi Peter,
thank you, because I was confusing
But to get the things much more confusing, the command "system mtu routing" doesn't affect MTU on L3 (no switchport ) Gigabit interface at all.
It means that on c3560G this command does no changes nad make no difference.
But problem with OSPF is still there and I simply can't configure the router c2600 to use MTU 1546 or connect it to some other device.
Is there any possibility to force OSPF process to use a cirtain MTU value or let the OSPF packets fragmented?
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