Understanding Interface Encapsulation on SONET/SDH Interfaces
Point-to-Point Protocol (PPP) encapsulation is the default encapsulation type for physical interfaces. You need not configure encapsulation for any physical interfaces that support PPP encapsulation. Note that if you do not configure encapsulation, PPP is used by default.
For physical interfaces that do not support PPP encapsulation, you must configure an encapsulation to use for packets transmitted on the interface. You can optionally configure an encapsulation on a logical interface, which is the encapsulation used within certain packet types.
When you configure a point-to-point encapsulation (such as PPP or Cisco HDLC) on a physical interface, the physical interface can have only one logical interface (that is, only one unit statement) associated with it. When you configure a multipoint encapsulation (such as Frame Relay), the physical interface can have multiple logical units, and the units can be either point to point or multipoint. Use PPP if you are running Cisco IOS Release 12.0 or later. If you need to run Cisco HDLC, the Junos OS automatically configures an ISO family MTU of 4469 in the router. This is due to an extra byte of padding used by Cisco.
When the encapsulation type is set to Cisco-compatible Frame Relay encapsulation, ensure that the LMI type is set to ANSI or Q933-A.
For more information about physical interface encapsulation, see Configuring Interface Encapsulation on Physical Interfaces.
Understanding Encapsulation on a Physical SONET/SDH Interface
For SONET/SDH interfaces, the physical interface encapsulation can be one of the following:
Point-to-Point Protocol (PPP)
PPP encapsulation is defined in RFC 1661, The Point-to-Point Protocol (PPP) for the Transmission of Multiprotocol Datagrams over Point-to-Point Links. PPP is the default encapsulation type for physical interfaces. Two related versions are supported:
Circuit cross-connect (CCC) version (ppp-ccc)—The logical interfaces do not require an encapsulation statement. When you use this encapsulation type, you can configure the ccc family only.
Translational cross-connect (TCC) version (ppp-tcc)—Similar to CCC and has the same configuration restrictions, but used for circuits with different media on either side of the connection.
Cisco HDLC
E1, E3, SONET/SDH, T1, and T3 interfaces can use Cisco HDLC encapsulation. Two related versions are supported:
CCC version (cisco-hdlc-ccc)—The logical interfaces do not require an encapsulation statement. When you use this encapsulation type, you can configure the ccc family only.
TCC version (cisco-hdlc-tcc)—Similar to CCC and has the same configuration restrictions, but used for circuits with different media on either side of the connection.
Frame Relay
Defined in RFC 1490, Multiprotocol Interconnect over Frame Relay. E1, E3, SONET/SDH, T1, and T3 interfaces can use Frame Relay encapsulation. Two related versions are supported:
CCC version (frame-relay-ccc)—The same as standard Frame Relay for DLCIs 0 through 511. DLCIs 512 through 1022 are dedicated to CCC. This numbering restriction does not apply to IQ and IQE interfaces. The logical interface must also have frame-relay-ccc encapsulation.
TCC version (frame-relay-tcc)—Similar to Frame Relay CCC and has the same configuration restrictions, but used for circuits with different media on either side of the connection.
Frame Relay Ether Type
Physical interfaces can use Frame Relay ether type (frame-relay-ether-type encapsulation for compatibility with Cisco Frame Relay. IETF Frame Relay encapsulation identifies the payload format using NLPID and SNAP formats. Cisco-compatible Frame Relay encapsulation uses the Ethernet type to identify the type of payload. Two related versions are supported:
TCC version (frame-relay-ether-type-tcc)—Cisco-compatible Frame Relay for DLCIs 0 through 511. DLCIs 512 through 1022 are dedicated to TCC. This numbering restriction does not apply to IQ and IQE interfaces. This encapsulation is used for circuits with different media on either side of the connection.
Extended TCC version (extended-frame-relay-ether-type-tcc)—This encapsulation allows you to dedicate Cisco-compatible Frame Relay TCC for DLCIs 1 through 1022. This encapsulation is used for circuits with different media on either side of the connection. All ether type TCC encapsulation is supported on the same PICs as non-ether type Frame Relay TCC encapsulation.
Understanding Encapsulation on a Logical SONET/SDH Interface
Generally, you configure an interface’s encapsulation at the [edit interfaces interface-name] hierarchy level. However, for Frame Relay encapsulation, you can also configure the encapsulation type that is used inside the Frame Relay packet itself. To configure an encapsulation on a logical SONET/SDH interface, see Configuring Interface Encapsulation on SONET/SDH Interfaces.
With the atm-nlpid, atm-cisco-nlpid, and atm-vc-mux encapsulations, you can configure the inet family only.
With the circuit cross-connect (CCC) encapsulations, you cannot configure a family on the logical interface.
A logical interface cannot have frame-relay-ccc encapsulation unless the physical device also has frame-relay-ccc encapsulation. A logical interface cannot have frame-relay-tcc encapsulation unless the physical device also has frame-relay-tcc encapsulation.
You must assign this logical interface a DLCI from 512 through 1022. This numbering restriction does not apply to IQ and IQE interfaces. You must configure the logical interface as point-to-point.
The ATM encapsulations are defined in RFC 2684, Multiprotocol Encapsulation over ATM Adaptation Layer 5.
For more information about logical interface encapsulation, see Configuring Interface Encapsulation on Logical Interfaces.