Intermitent WAN issues

Posted on 2012-08-29
Last Modified: 2012-08-30
We have a managed MPLS VPN between our office in the UK and our office in China. All was working fine until the weekend where something changed and it became very unreliable

Ping and traceroute between sites is working fine with no apparent routing issues. There is no or very little packet loss. The problem is with any other traffic. Windows Networking, RDP, VNC and Http traffic all fails to connect 99% of the time. Every so often it will work but not for long. Each service has been tested locally and they work fine. Generally connecting from UK to China is a bit more reliable than China to UK for example in the UK I can view the windows shares on the China servers but not the other way around. RDP wont connect at all in either connection. Http will connect from China to UK around 40% of the time and to one server only

It's like there is a firewall blocking traffic but there isnt. The routing path is pretty much source --> default gateway --> ISP router --> WAN --> ISP Router --> destination

We're not aware of any network changes but then I'm note sure of what network changes would cause this especially when Ping/routing is fine.

Does anyone have any suggestions on what tests I can try to isolate the problem? The ISP is also investigating but they dont have an answer yet
Question by:nexusinds
    LVL 10

    Accepted Solution

    Wow ... A tough one . I have a similar setup in my environment . What do you see on interfaces ? Any errors ? I mean MPLS router interfaces . Have you involved your MPLS provider to look into the health of their devices and links ? They should be able to tell you if there are errors and they can also see the source that generates those errors. I'd start this troubleshooting from vendor .
    LVL 10

    Expert Comment

    Also vendor can start some debug logs to see why your interesting traffic (e.g. Https) fails .
    Do you see constant amber color on MPLS interfaces ? May want to connect it to a different port on the switch .
    LVL 7

    Expert Comment

    Are you perhaps running into MTU mismatches?  Assuming windows based hosts, try the following command:- 'ping -l 1472 -f x.x.x.x' and post the results.

    A ping payload of 1472 bytes should result in a layer 2 payload of 1500 bytes, which most networks and carriers should support.  If so, you will see 'normal' ping responses.

    If you don't see this, divide and conquer until you find the largest payload that succeeds.  If 1472 does not work, there maybe some tweaks you can use in your network to encourage hosts not to exceed the end-to-end MTU.

    Author Closing Comment

    Thanks. We did report to our provider however I was being imaptient because I dont like big problems being out of my hands. The provider has reported they are having some BGP and routing issues with our link and have detailed the steps they are taking to resolve this so I guess there's nothing I need to do for now

    Thank you all for your responses

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