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New Member

PPP Padding

We are using Cisco Routers to connect our equipment to a Frame Relay network for sending and receiving UDP messages between central server and number of clients.

The equipment (client side) is connected to the router via serial port using PPP protocol. The other port of the router is connected to Frame Relay equipment.

According to the TCP/IP protocol, the UDP packet that the client sends or receives should be encapsulated like this:

[PPP header][IP Header][UDP datagramm][PPP footer]

However we noticed that in some zones over the US the messages transferred by the router to our equipment are padded at the PPP level like this:

[PPP header][IP Header][UDP datagramm] 0x55 0x55 ... 0x55 0x55 [PPP footer]

Does the router perform this padding? Or does it depend on the Frame relay equipment?

Does it depend on the router model or a WIC card model?

Is it possible to remove the padding by appropriate router configuration?

1 REPLY
New Member

Re: PPP Padding

I believe the padding is done by the router to get the frame to at least the minimum size frame that can be forwarded out that interface. This may be directly due to a manually configured MTU size on the router for that particular frame type.

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