Frame Relay Encapsulation

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Frame Relay is a data link protocol and the customer router encapsulates each Layer 3 packet inside a Frame Relay frame comprising a header and trailer before it is sent out the access link. The header and trailer used is actually defined by the Link Access Procedure Frame Bearer Services (LAPF) specification, ITU Q.922-A. That was quite a mouthful but the LAPF framing, shown in Fiure 12-9, provides important functionality including error detection with the FCS in the trailer and a DLCI field along with a few oher fields in the header.

Figure 12-10 LAPF Framing

The standard LAPF header is too simplistic and does not provide all the fields needed by Frame Relay routers. More specifically, there is no Protocol Type field in LAPF. Each data link layer needs such a field to define the type of Layer 3 packet carried by the data link frame. If Frame Relay uses only LAPF header, routers cannot support multiprotocol traffic because there is no way to identify the type of Layer 3 protocol.

The simple LAPF header was extended to compensate for the absence of a Protocol Type field:

◈ Cisco created a proprietary additional header, which appears between the LAPF header and the Layer 3 packet shown in Figure 12-10. It includes a separate 2-byte Protocol Type field with values exactly matching the ones used in the same field Cisco uses for HDLC, as discussed earlier in the chapter.

◈ Internet Engineering Task Force (IETF) defined the second solution via RFC standards 1490 and later 2427. This solution is known as Multiprotocol Interconnect over Frame Relay and it defines a header similar to the Cisco propreitary solution placed between the LAPF header and Layer 3 packet. The additional header includes a Protocol Type field as well as several other options.

Key Concept: Frame Relay encapsulation has two types: Cisco which is proprietary and the default on Cisco routers, and IETF which is standards based. Cisco encapsulation can be used when all routers are Cisco while IETF can be used in a multi-vendor environment.

You should keep in mind that Frame Relay encapsulation should match on the routers at the two ends of a VC. If you fail to match the Frame Relay encapsulation (both sides cisco or both ietf) on the two routers, the connection does not come up. However, if you have Cisco routers at both ends of the connection (a likely scenario), and you don’t explicitly configure Frame Relay encapsulation, both routers default to cisco and the connection does get established.  Frame Relay switches do not care about the Frame Relay encapsulation. In Cisco IOS Software configuration, the Cisco proprietarty encapsulation is called cisco while the other one is called ietf. 

Figure 12-11 Cisco and IETF Framing

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