Table Of Contents
Encapsulation Protocols
Encapsulation Protocols
Encapsulation protocols connect the layer- 2 (link layer) protocol with the layer-3 (network layer) protocol. For an ASE to analyze network-layer protocols, it must know how to access them above the link layer. Toward this end, you can set the ASE's encapsulation protocol.
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Encapsulation protocols are not supported for ATM ASEs.
An encapsulation protocol is not always necessary on a communications link, however, typically, it is used to carry more than one layer-3 protocol over a link. For example, you can configure a Frame Relay link to send only IP packets, in which case no encapsulation protocol is required. However, if the link were configured to also send IPX, an EtherType or RFC1490-type encapsulation protocol must be used.
The encapsulation protocol selections (such as IP) are not all proper encapsulation protocols. Instead, they tell the ASE what to look for above the link layer. IP means IP immediately follows the Frame Relay header and no encapsulation protocol is used.
The encapsulation offset in the protocol-definition window (titled Beginning n octets after the frame header) works in conjunction with the encapsulation protocol. This mechanism instructs the ASE to skip a certain number of octets after the link-layer headers before it attempts to decode the packet further using the encapsulation protocol:
•for Frame Relay, the encapsulation offset represents the number of octets to skip after the Frame Relay header
•for HDLC, the encapsulation offset is the number of octets to skip after the opening flag.
You also can use the offset for data in an encapsulation protocol that is not supported in Cisco WAPMS. The offset then instructs the ASE to ignore the specified number of octets because the ASE would not be able to recognize the encapsulation protocol. For example, you may know that on your line the IP packet begins ten octets after the link-layer header. Therefore, setting an encapsulation of IP with an offset of ten would work—the ASE would simply ignore the ten octets between the header and the IP protocol.
TableB-1 lists the encapsulation protocols available for configuration through Cisco WAPMS.
1 This is a common setting for Frame Relay with an offset of zero. However, you can use it when an unsupported encapsulation protocol is in use but the user knows the line is carrying EtherType formatted frames. In this case, configure the offset with the appropriate number of octets to skip between the link-layer header and the beginning of the EtherType frame.
2 Use this setting when no encapsulation protocol in use and the specified network address is the only one in use on the link. Or, you can use this setting when an unsupported encapsulation protocol is in use, in which case you must determine the offset from the link-layer header to reach the specified protocol. The ASE would therefore skip the analysis of the encapsulation protocol and jump to the network-layer protocol.
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