Slow LAN performance over 3G cellular connection

Posted on 2011-10-14
Last Modified: 2012-05-12
I have 18 remote sites connected to my datacenter over a MPLS network. I’m utilizing a wireless 3G cellular modem with Verizon (connected to my MPLS network) as a backup in the event I lose the T1.

The problem is when I fail-over, the internal network performance is terribly slow. However outbound to the internet is fine. If I download 3mb from the public internet it will take under a minute depending on the speed of the cellular site. The same file from a server on the inside takes over 2 -3 minutes. All 18 remote sites perform the same way. I do not have any problems when using the T1 circuits over the same hardware.

Remote site hardware
Cisco 2911 routers
HP procurve switch

DataCenter hardware
Cisco 3845 router
Cisco 3845 stacked switches (3 vlans)
ASA5510 to the internet

BGP is used to update the MPLS network

When I connect the cellular failover it first will establishes a GRE tunnel with the datacenter. The datacenter is also the default route for everyone. The connections travels the same route out to the internet as it would to reach my internal server. Except when it hits my switch it’s separates per vlan. This is confirmed with a traceroute. Failover and connectivity is not a problem, just the performance is slow.

I don’t see why it would be slower internally when it takes the same path then out to the public internet, however the distance internally is shorter.

Any suggestion on where the bottle next would be?

Path with terrible performance
Server---Switch----router---- (GRE tunnel) ---router---switch (internal vlan) –server

Path with expected performance
Server---switch----router---- (GRE tunnel) ---router---switch (external vlan)--ASA---public internet

All help is greatly appreciated
Question by:Gallo915
    LVL 10

    Expert Comment

    what redundancy protocol do you use on your routers connected to your T1 and your 3G?

    Is the switch connected to the ASA doing inter-vlan routing?

    Author Comment

    The cellular interface is configured as a backup interface to the serial interface. this handles the fail-over

    No routing is done on the switch
    LVL 6

    Accepted Solution

    check for MTU issues ...i c as MTU issue ...

    try to configure below MTU on GRE interfaces...

    interface Tunnel11
     ip mtu 1420
     ip tcp adjust-mss 1380

    Author Comment

    Thanks for the suggestions Sanjeevloke

    I need to schedule a time to bring down the remote site to give this a try. I will follow-up with the results later this week.
    LVL 6

    Expert Comment

    no need of downtime wont disconnect it u from just as adjusting packet size ..

    Author Comment

    Correct, my downtime is to fail-over to the cellular circuit. I'm using this as a backup to my MPLS circuit and can only test during certain hours. I already applied your suggestions, now I need to test if they worked.

    Author Comment

    Thanks for the suggestion Sanjeevloke, I tested your suggest on 3 of my 1 sites and all shown great improvement. 50 -75% quicker downloads. 1 site improved from a 2 minute download down to a 24 second download.

    Your help is very much appreciated.

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