Table Of Contents
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.
Note
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:
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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.
Table B-1 lists the encapsulation protocols available for configuration through Cisco WAPMS.
Table B-1 Encapsulation Protocol and Network Layer Settings
Cisco (EtherType)
Ethernet frame format used by many routers as their proprietary, default encapsulation protocol. It has similar capabilities to bridged Ethernet or Token Ring, but is more efficient.1
Ethernet (bridged)
Ethernet frame format used by bridges and bridging routers. Can carry any network layer protocol supported by Ethernet.
Frame Relay (Auto)
(Frame Relay only) Either RFC1490 or EtherType encapsulation protocol based on the frames themselves. This is the best choice to use with Frame Relay because it works with both of the most common encapsulation techniques.
None - AppleTalk only
A network-layer protocol that directs the ASE to analyze data assuming it is AppleTalk protocol.2
None - IP only
A network-layer protocol that directs the ASE to analyze data assuming it is IP protocol.2
None - IPX only
A network-layer protocol that directs the ASE to analyze data assuming that it is IPX protocol.2
None - SNA only
A network-layer protocol that directs the ASE to analyze data assuming that it is SNA protocol.2
None - Vines only
A network-layer protocol that directs the ASE to analyze data assuming that it is Banyan VINES protocol.2
None - DECnet IV only
A network-layer protocol that directs the ASE to analyze data assuming it is DECnet IV protocol.2
Point-to-Point (PPP)
The standard point-to-point protocol used mainly on point-to-point links analyzed by the HDLC ASE, but can be used by Frame Relay ASEs.
RFC 1490 (IETF)
(Frame Relay only) Standard Frame Relay Multi protocol encapsulation. This is the most versatile and common encapsulation protocol used with Frame Relay.
Router (proprietary)
(HDLC only) An encapsulation protocol that decodes proprietary framing used by routers on point-to-point links.
Token Ring (bridged)
Token Ring frame format used by bridges and bridging routers. Can carry any network layer protocol supported by Token Ring networks.
Unknown or Proprietary
Encapsulation protocol is unknown. The ASE will not attempt to analyze data beyond the link layer. Do not use an encapsulation offset with this setting.
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.
