[c-nsp] MTU on MPLS TE Tunnels w/PA-FE-TX?

[Vinny Abello]

Hello all in Cisco-NSP land,

I'm just experimenting with MPLS TE setting up a tunnel to make sure I can force traffic over it in the simplest scenario: One router connected directly to another. Nothing fancy. This is just a test. :)

In my testing though, despite the use of tag-switching mtu 1520 or mpls mtu 1520 on the interface on both sides, I seem to get reduced to an MTU of 1496 over the tunnel.

The hardware in question is a 7206VXR running 12.2(40) using a PA-FE-TX connected to a Catalyst 6506. The head end of the tunnel is the 7206VXR if it makes a difference. There is no switch in between. These devices are directly connected.

I've verified the LSP is up and TE tunnels show configured on the interfaces on both sides. I know it is working otherwise as I can advertise the path into my IGP and traffic will follow the tunnel. I just can't ping above 1496 successfully across it.

I found some things in the archives about people complaining the PA-FE-TX is limited in hardware to 1500 byte MTU, but then I found other people saying it should work fine with MPLS tagged packets as long as you specify the size with tag-switching mtu or mpls mtu, which I have done without success. I know the PA-FE-TX will refuse to be configured manually for any other size MTU larger than 1500, so I'm leaning towards it being a hardware limitation. The tunnel interface even shows an MTU of 1496 in the output of show mpls interfaces detail. It initially appears to come up as 1514 but then drops to 1496.

There isn't a whole lot of documentation on this other than use mpls mtu if you're using ethernet... Most MPLS documents assume you're using POS or Gig-e where you wouldn't really need to worry about it.

Can someone comment on this an indicate if there is a FastEthernet interface type on a 7200 that will support a larger MTU size for MPLS TE tunnels to pass 1500 byte packets or if there is a way to make the PA-FE-TX work and pass MPLS tagged packets larger than 1500 bytes? I know I could use the PA-GE but that quickly eats bandwidth points, or I could use NPE-G1 or NPE-G2 interfaces, but I don't have either engine presently and they are expensive propositions just to fix an MTU problem.


Thanks for any insight!