MTU size mismatch

[schwermie]

What issues can arise when you configure different MTU sizes on a metro ethernet link?

I have some doubts, but our development department says that it shouldn't be a problem.

What do you think is best practice.

Cheers, Marco

[mheusinger]

Hi,

MTU will be an issue on a link connecting two OSPF neighbors. The MTU value is part of the database description packet and no adjacency will form, if the values are not the same.

Also MTU related issues might arise, because a frame larger than the interface MTU will be discarded. In an IP environment there is fragmentation if DF not set, but Ethernet does not support fragmentation, so dropping is the only possible result.

I personally would not recommend such setups (not knowing the underlying reasons to suggest this setup).

Regards, Martin

[tmarlow]

ospf adjacencies will form if you tell the ospf config to ignore mtu size, of course that only matters if you are using ospf. I have this situation myself and unless you are configuring the MTU to be less than 1500 then you shouldn't run into a problem, since by default, hosts are configured with 1500 MTU.

[prashant.chitnis]

hi

same problem when u are running EIGRP as routing protocol. I would like know same change in MPLS , BGP environment .

bye.

[bjornarsb]

Hi,

For MPLS the core MTU should be greater than or equal to the total bytes of the items in the following equation:

Core MTU >= (Edge MTU + Transport header + AToM header + (MPLS label stack * MPLS label

size))

Edge MTU + Transport header + AToM header + (MPLS label stack * MPLS Label) = Core MTU

1500 + 18 + 0 + (2 * 4 ) = 1526

You must configure the P and PE routers in the core to accept packets of 1526 bytes.

Some interfaces (such as FastEthernet interfaces) require the mpls mtu command to change the MTU size.

Regards,

Bjornarsb

[dinesh.thathanath]

I am facing an issue where I am forced to give the "system jumbo mtu 1522" globally, mtu 1522 on l2 ports and mtu 1700 on the uplink l3 ports. Once I clear my ospf adjacancies, they never form again (gets stuck at loading). What could be the issue?. Once i remove system jumbo mtu..every thing is fine..

[mheusing]

Hi,

you might run into the issue mentioned in the posts above. Did you try to configure OSPF to ignore MTU?

interface FastEthernet1/0

ip ospf mtu-ignore

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801a807e.html#wp1109168

Be sure to configure this on all OSPF router interfaces where you have the adjacency issues. This should usually solve the adjacency issue.

Hope this helps!

Regards, Martin

[swaroop.potdar]

You may want to try and increase the routing mtu to 1700 using "system mtu routing 1700".

For the 3750: The size of frames that can be received by the switch CPU is limited to 1998 bytes, no matter what value was entered with the system mtu or system mtu jumbo commands. Although frames that are forwarded or routed typically are not received by the CPU, in some cases packets are sent to the CPU, such as traffic sent to control traffic, SNMP, or Telnet, or routing protocols.

Routed packets are subjected to MTU checks on the egress ports. The MTU value used for routed ports is derived from the system mtu configured value (not the system mtu jumbo value). That is, the routed MTU is never greater than the system MTU for any VLAN. The system MTU value is used by routing protocols when negotiating adjacencies and the MTU of the link. For example Open Shortest Path First (OSPF) protocol uses this MTU value before setting up an adjacency with a peer router.

Ref:

http://www.cisco.com/en/US/products/hw/switches/ps5532/products_configuration_guide_chapter09186a008081bb99.html

HTH-Cheers,

Swaroop

[dinesh.thathanath]

The problem " system mtu routing 1700" is not supported on 6524ME switches. What you mentioned is right about the issue. I sorted out the issue by giving ip mtu 1522 apart from mtu 1700 on MPLS interfaces.

Cheers!!