The layer 2 encapsulation of the link. The use of ppp(1) as described in RFC 1661 is the preferred encapsulation for point-to-point links. The encapsulation is set to x25(2) for X.25-only links and to x25-ppp(3) for concurrent use of X.25 and PPP.

Mpr-lapb(7) and mpr-hdlc(8) are popular proprietary encapsulations. They both use the ethertype number for protocol multiplexing. The former is used when error correction or data compression (v42bis) is desired. The latter (mpr-hdlc) is compatible to Cisco's HDLC encapsulation.

On IP-only links it is also possible to use the ISO LAPB protocol (ip-lapb(4)), also known as X.75, or raw hdlc framing (ip-hdlc(6)).

The x75-ppp encapsulation provides a proprietary solution for using PPP encapsulation over X.75 (LAPB) framing, x75btx-ppp provides PPP over T.70 and LAPB framing including a BTX protocol header to access BTX services over ISDN links.

The x25-nosig(13) encapsulation is used to establish X.25 connections over dialup links without specific signalling on the D-channel (pure data call).

The x25-ppp-opt encapsulation provides a special kind of the x25-ppp encapsulation. Dialin partner will be authenticated either outband to establish an X.25 connection via ISDN or optional inband by using the point-to-point protocol (PPP). Concurrent use of X.25 and PPP encapsulation is excluded.

The x25-noconfig(16) and x25-noconfig-nosig(17) encapsulations provide a solution fr establishing X.25 connections via ISDN. The dial number will be derived from X.25 destination address by using special rules.

V42bis data compression can be enabled on LAPB links only, because v42bis requires an error free connection.

Dialup links can be removed by setting this field to delete(5). This has no effect on permanent links.