MTU Issue on QinQ Eth link with MPLS

Unanswered Question
Dec 30th, 2007

Hi,

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.

I came across the following link: http://www.cisco.com/warp/public/473/175.html - Is increasing the system mtu something we must do?

Thanks in advance.

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cpubob Thu, 01/03/2008 - 12:33

on all the mpls enabled ethernet links in the mpls core, you should change the mtu or the mpls mtu so accomodate the size of the mpls encapsulated packets.

ip = 1500

2 mpls labels = 8

control word = 4

2 vlan headers = 8

ether II header = 14

= mtu 1534

or just go right to the top with 9194 and you'll never have to worry about it again.

I've seen this in the field where dns works, you can get to google but not much else.

johnelliot Thu, 01/03/2008 - 13:46

Hi,

Thanks for the response - Would I also need to adjust mtu on the entire path (i.e. The 2950 Trunk ports)?

The 2950's are C2950G-24-EI (Which support baby giants), and I assume I would need to set "system mtu" in global conf to 1530(Which is the max size) - I notice that one of the 2950's, which has a more recent IOS, only supports 1500 bytes?

Model number: WS-C2950G-24-EI

System serial number: FOC0833X3M8

System image file is "flash:c2950-i6q4l2-mz.121-22.EA7.bin"

No baby giant support:

gc-sw-08(config)#system mtu ?

<1500-1500> MTU size in bytes

Whereas:

Model number: WS-C2950G-24-EI

System serial number: FOC1012Z5BJ

System image file is "flash:/c2950-i6q4l2-mz.121-22.EA6.bin"

Baby giant support:

ESW01-BNE(config)#system mtu ?

<1500-1530> MTU size in bytes

Thanks for any comments/suggestions.

cpubob Fri, 01/04/2008 - 08:44

That is interesting because according to cisco, that model supports giants/baby giants as of 12.1(6)EA2, and since you are on 12.1(22) they should support it. It very well just may be a bug in the IOS, I would check with TAC just to be sure.

Yes, you should configure the system MTU on the switches, if they are in the mpls transit path, to accommodate the larger sized frames.

johnelliot Wed, 01/09/2008 - 13:18

Hi,

I now have the QinQ link connected to C2950G-24-EI's (Both supporting baby giants), and made the following changes:

Both 2950's:

system mtu 1530

7200's:

interface FastEthernet0/0.15

tag-switching mtu 1530

interface FastEthernet3/0.15

mpls mtu 1530

Running MPLS over the link, access to https(And other sites historically affected by mtu) was successful from "standard" Internet Tail, but VRF traffic was still affected by mtu(i.e. Could ping/resolve Internet Addresses, but could not visit majority of WebSites) - Switching back to our ATM link resolved the problem?

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