[c-nsp] MTU Issue on QinQ Eth link with MPLS

[Matt Carter]

> We have recently had a QinQ link provisioned, but have noticed mtu
> issues when testing.
> 
> The link is terminated on 2950's(Trunk Ports) at each end, with the
> 2950's then connected(Also trunk ports) to 7204VXRs, and the various
> vlans broken out into dot1q Ints.
> 
> Without MPLS(Including MPLS L3 VPNs) running on the link, we can access
> all WebSites without issue - As soon as MPLS is enabled, some sites are
> not accessible(https sites, www.smh.com.au...)
> 
> If we switch back to our primary link(ATM), we also do not experience
> any mtu issues.
> 

Howdy,

ATM MTU = 4470
Cat 2950 MTU = 1500

Obviously the latter isn't going to allow for MPLS label stacking without
running into the same issues as described in this URL in relation to GRE
overheads.

Why Can't I Browse the Internet when Using a GRE Tunnel?
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a0080
093f1f.shtml

Catalyst 2950/2955 Series
http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration
_example09186a008010edab.shtml#c5

You can classify the Catalyst 2950/2955 Series switches into two major
groups, where one supports baby giants (up to 1530 bytes), but the other
does not. However, this refers to traffic that flows through the switch.
Packets destined to the management (VLAN) interface can support only 1500
bytes.

These models of 2950 switches support only 1500 bytes:
      WS-C2950-12
      WS-C2950-24
      WS-C2950-48
      WS-C2950C-24
      WS-C2950T-24

These models of 2950/2955 switches support up to 1530 bytes:
      WS-C2950G-12-EI
      WS-C2950G-24-EI
      WS-C2950G-24-EI-DC
      WS-C2950G-48
      WS-C2950G-48-EI
      All models of 2950 LRE Series switches
      All models of 2955 Series switches

--matt