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Knowledge Base > WAN Cards (T1/E1 and V.35) > Why are T1 forwarding tests failing with large packet sizes?

Why are T1 forwarding tests failing with large packet sizes?

The default MTU for the T1/E1 interface is 1500 bytes. However, this includes the IP packet and all data link headers for FrameRelay or PPP. This results in large IP packets getting dropped during any forwarding tests.

To correct this, you should configure the MTU of your WANPIPE device to 1522 under the Hardware setup for your WANPIPE T1/E1 card. You can also modify the MTU setting of your wanpipe1.conf configuration file.

In /etc/wanpipe/wanpipe1.conf: 

  MTU             = 1522