EoMPLS MTU size

[Erik Boon]

Hi All,

I'm configuring EoMPLS in our LAN and ran into the mtu size. Best practice says to change the mtu size to 1504 (end tot end) due to the MPLS label. The physical link is a trunk and routing is done by OSPF trough a transit vlan. Should I change the mtu size of the svi to 1504 as well or should that be a different size? Some examples showed a mtu size of 1538, which confused me even more. Don't know how to calculate that number. Maybe frame overhead plus payload (whithout tagging). The platform is 6500 based. I hope someone can help me sorting this out.

Thanks in advance.

Erik

                

[Erik Boon]

Someone who can help me?

[Juan Mendoza]

Hello Erik

MTU issues arise as far as you add extra header because protocols need it.

1.- Considering you´re running EoMPLS for VLAN o for  a whole port, so for the worst case you need to know ythe following

MTU Network = MTU Access port/VLAN + Extra Header

2.- The MTU between the Calayts is running MPLS, mus have the MTU Network

Asume you try to transport EoMPLS for a VLAN, so you have a 1500 + VLAN Tag Header (4 Bytes) + Ethernet Header (14bytes)

besides MPLS HEader require 4 more bytes and MPLS L2VPN requiered extra 4 Bytes also.

So at least your Network MTU = 1500 +4 + 14 + 4+ 4  = 1526

3.- So All the Path for MTU should be at least 1526, so I recommend you as a Policy to sert up the MTU for Networks port Purpose as big as it support, actually is common to use 9192 bytes

Hope this helps

[dominic.caron]

On the 6500, you need to change to "system jumbomtu" value and the mtu on L2 and L3 interface(physical and SVI). Since you are in ospf, disable the MTU check before starting to make change or it's going to be a bad day.

I agree with Juan, max out the mtu to 9198 or 9216 depending on the platform.

[Erik Boon]

Thanks Juan and Dominic!

I agree with max out the the MTU to 9216. Nevertheless I wanted to know the exact size to understand it more.

I did some lab testing and indeed the MTU size was 1526. Although during testing I monitored a header PW ethernet control word (4 bytes). I know this is optional for EoMPLS, but how can I verify on the 6500 if it's actually "on". When this is the case the MTU size would be 1530 or am I thinking the wrong way?

Thanks for the OSPF MTU check tip, almost forgot. One more question about this. Since I use a transit vlan for OSPF. Would it affect the adjacency when I change the MTU size of the physical interface en leave the MTU of the SVI 1500?

[dominic.caron]

OSPF will not be impacted if you change the MTU of the physical interface.

For the control word, it's optional but Cisco switch use it by default. 1530 is the good value for transporting ethernet frame with the vlan tag. If you do MPLS TE, add 4 more bytes.