MTU and GRE Tunnel

[Erico Verissimo]

Hi all,

I have two interfaces tunnel and the connection between them are established by physical ethernet. If I want to incriease the packets up1500 bytes I need only to change the MTU of the Tunnel or the physical interface as well?

example:

tunnel gre -----ethernet-----<router>-----ethernet-----<router>-----ethernet--------tunnel gre

 1528                 1500                                     1500                                   1500                   1528

I appreciate any comments..         

[luis_cordova]

Hi @erico Verissim,

Check this link:

https://community.cisco.com/t5/switching/gre-tunnel-default-mtu/td-p/905193

I hope can help you.

Regards

[Erico Verissimo]

Thank you Luis.

[Georg Pauwen]

Hello,

the tunnel interface always 'deducts' at least 24 bytes (for a simple GRE) from the ip mtu size set for the physical interface, so if you want the tunnel mtu to be 1500, the physical interface mtu needs to be 1524. If you try to configure a higher value on the tunnel interface, you will get an error message such as the one below:

R1(config-if)#ip mtu 1476
%Warning: IP MTU value set 1476 is greater than the current transport value 1452, fragmentation may occur
R1(config-if)#
*Feb 12 18:49:01.667: %TUN-4-MTUCONFIGEXCEEDSTRMTU_IPV4: Tunnel0 IPv4 MTU configured 1476 exceeds tunnel transport MTU 1452

[Erico Verissimo]

Georg,

If this tunnel was mpls over gre...? how can I configure the gre interface ..

|--------MTU1500--------|
<host>------<sdn router>------<CE router>---------<ISP>------------<CE router>----------<sdn router>--------<host>
|---bgp vpnv4---||mpls over gre|----------------------|mpls over gre||--bgp vpnv4--|

[Joseph W. Doherty]

The real "MTU" is tied to the physical interface, so you need to change that. To support a GRE tunnel to carry standard full size Ethernet packets, you would enable jumbo Ethernet on all transit interfaces carrying the tunnel packets. (This assumes such will support jumbo Ethernet.)

BTW, you later ask about MPLS. For MPLS, why not use a MPLS tunnel?

[Erico Verissimo]

Is it possible I confiure a mpls tunnel over ISP?
My ISP connection is a dedicated link and I need to tranfer packet to 1500 MTU df-bit set 1.

[Joseph W. Doherty]

Over an ISP, not likely.

Over a MPLS SP, possibly.

[Erico Verissimo]

Thank you

[Vishnu Vardhan S]

Nice question , Could you please tell me how the MTU is impacting your service and how you checked it. I have never faced an MTU issue even in ospf for extart state.

[Erico Verissimo]

Impact in performace cause fragmentation.