Circuit Cross-Connect | Juniper Networks Field Guide and Reference

Circuit cross-connect (CCC) allows you to configure transparent connections between two circuits, where a circuit can be a Frame Relay DLCI, an ATM VC, a PPP interface, a Cisco HDLC interface, or an MPLS LSP. When CCC is enabled, packets from the source circuit are delivered to the destination circuit with, at most, the Layer 2 address being changed. No other processing ”such as header checksums, TTL decrementing , or protocol processing ”is done.

CCC circuits fall into two categories: logical interfaces, which include DLCIs, VCs, VLAN IDs, PPP, and Cisco HDLC interfaces; and LSPs. The two circuit categories provide three types of cross-connect:

Layer 2 switching ”Cross-connects between logical interfaces provide what is essentially Layer 2 switching. The interfaces that you connect must be of the same type.
MPLS tunneling ”Cross-connects between interfaces and LSPs allow you to connect two distant interface circuits of the same type by creating MPLS tunnels that use LSPs as the conduit.
LSP stitching ”Cross-connects between LSPs provide a way to " stitch " together two label-switched paths, including paths that fall in two different TED areas.

For Layer 2 switching and MPLS tunneling, the cross-connect is bidirectional, so packets received on the first interface are transmitted out the second interface, and those received on the second interface are transmitted out the first. For LSP stitching, the cross-connect is unidirectional.

For CCC connections that connect interfaces, the interfaces must be of the same type; that is, ATM to ATM, Frame Relay to Frame Relay, PPP to PPP, or Cisco HDLC to Cisco HDLC.

Configuring Layer 2 Switching Cross-Connects

Layer 2 switching cross-connects join logical interfaces to form what is essentially Layer 2 switching. The interfaces that you connect must be of the same type. You can configure Layer 2 switching cross-connects on PPP, Cisco HDLC, Frame Relay, Ethernet, and ATM circuits. In a single cross-connect, only like interfaces can be connected.

To configure Layer 2 switching cross-connects, you must define the CCC encapsulation for Layer 2 cross-connects, define the connection between the two circuits, and configure MPLS.

To configure the CCC encapsulation on the router for PPP or Cisco HDLC circuits, specify the encapsulation in the encapsulation statement. This statement configures the entire physical device. For these circuits to work, you must configure a logical interface unit 0.

 [edit]  interfaces {  type  -  fpc  /  pic  /  port  {     encapsulation (ppp-ccc  cisco-hdlc-ccc);     unit 0;   } }

For Ethernet circuits, specify the encapsulation in the encapsulation statement. This statement configures the entire physical device. For these circuits to work, you must configure a logical interface ”unit 0. Note that only the four-port Fast Ethernet PICs support the ethernet-ccc encapsulation type.

 [edit]  interfaces {   fe-  fpc  /  pic  /  port  {     encapsulation ethernet-ccc;     unit 0;   } }

For ATM circuits, specify the encapsulation when configuring the VC. Configure each VC as a circuit or a regular logical interface.

 [edit]  interfaces {   at-  fpc  /  pic  /  port  {     atm-options {       vpi  vpi-identifier  maximum-vcs  maximum-vcs;  }     unit  logical-unit-number  {       point-to-point;       encapsulation atm-ccc-vc-mux;       vci  vpi-identifier.   vci-identifier  ;     }   } }

For Frame Relay circuits, specify the encapsulation when configuring the DLCI. Configure each DLCI as a circuit or a regular logical interface. The DLCI for regular interfaces must be in the range 1 through 511. For CCC interfaces, it must be in the range 512 through 1,022.

 [edit]  interfaces {   interface-switch frame-relay-ccc;  type  -  fpc  /  pic  /  port  {     unit  logical-unit-number  {       point-to-point;       encapsulation frame-relay-ccc;       dlci  dlci-identifier  ;     }   } }

To configure Layer 2 switching cross-connects, define the connection between the two circuits. The connection joins the interface that comes from the circuit's source to the interface that leads to the circuit's destination. When you specify the interface names , include the logical portion of the name , which corresponds to the logical unit number. The cross-connect is bidirectional, so packets received on the first interface are transmitted out the second interface, and those received on the second interface are transmitted out the first.

 [edit protocols]  connections {   interface-switch  connection-name  {     interface  interface-name.unit-number  ;     interface  interface-name.unit-number  ;   } }

For Layer 2 switching cross-connects to work, you must configure MPLS. The following is a minimal MPLS configuration:

 [edit protocols]  mpls {   interface (  interface-name  all); }

Configuring MPLS LSP Tunnel Cross-Connects

MPLS tunnel cross-connects between interfaces and LSPs allow you to connect two distant interface circuits of the same type by creating MPLS tunnels that use LSPs as the conduit. You can configure LSP tunnel cross-connects on PPP, Cisco HDLC, Frame Relay, and ATM circuits. In a single cross-connect, only like interfaces can be connected.

When you use MPLS tunnel cross-connects, if you use the default MTU size, IS-IS does not form adjacencies across the tunnel. For the tunnel cross-connects to work, the MTU size on the edge routers must be smaller than the LSP's MTU. Use the following calculation to determine the maximum IS-IS MTU size:

IS-IS MTU = MPLS MTU “ 4 bytes “ link-layer overhead

The link-layer overhead varies, depending on the encapsulation:

ATM ”8 bytes
Frame Relay ”2 bytes
HDLC ”4 bytes
PPP ”4 bytes
VLAN ”4 bytes

See Chapter 6, "Interfaces and Class of Service," on page 185.

Juniper Networks recommend that you simply set the MTU to 1,497 bytes, which is small enough so that IS-IS works properly.

To modify the MTU, include the mtu statement when configuring the logical interface family, at the [edit interfaces interface-name unit logical-unit-number encapsulation family ] hierarchy level.

To configure LSP tunnel cross-connects, you must configure the CCC encapsulation on the ingress and egress routers. You cannot configure families on CCC interfaces; that is, you cannot include the family statement at the [edit interfaces interface-name unit logical-unit-number ] hierarchy level.

For PPP or Cisco HDLC circuits, specify the encapsulation in the encapsulation statement. This statement configures the entire physical device. For these circuits to work, you must configure a logical interface unit 0.

 [edit]  interfaces {  type  -  fpc  /  pic  /  port  {     encapsulation (ppp-ccc  cisco-hdlc-ccc);     unit 0;   } }

For ATM circuits, specify the encapsulation when configuring the VC. For each VC, you configure whether it is a circuit or a regular logical interface.

 [edit]  interfaces {   at-  fpc  /  pic  /  port  {     atm-options {       vpi  vpi-identifier  maximum-vcs  maximum-vcs;  }     unit  logical-unit-number  {       point-to-point;       encapsulation atm-ccc-vc-mux;       vci  vpi-identifier.   vci-identifier  ;     }   } }

For Frame Relay circuits, specify the encapsulation when configuring the DLCI. For each DLCI, you configure whether it is a circuit or a regular logical interface. The DLCI for regular interfaces must be in the range 1 through 511. For CCC interfaces, it must be in the range 512 through 1,022.

 [edit} interfaces {   interface-switch frame-relay-ccc;  type  -  fpc  /  pic  /  port  {     unit  logical-unit-number  {       point-to-point;       encapsulation frame-relay-ccc;       dlci  dlci-identifier  ;     }   } }

To configure LSP tunnel cross-connects, define the connection between the two circuits on the ingress and egress routers. The connection joins the interface or LSP that comes from the circuit's source to the interface or LSP that leads to the circuit's destination. When you specify the interface name, include the logical portion of the name, which corresponds to the logical unit number. For the cross-connect to be bidirectional, you must configure cross-connects on two routers.

 [edit protocols]  connections {   remote-interface-switch  connection-name  {     interface  interface-name.unit-number  ;     transmit-lsp  label-switched-path  ;     receive-lsp  label-switched-path  ;   } }

Configuring LSP Stitching Cross-Connects

LSP stitching cross-connects "stitch" together LSPs to join two LSPs. For example, they stitch together LSPs that fall in two different TED areas. You can use LSP stitching to create a seamless LSP for LSPs carrying any kind of traffic.

To configure LSP stitching cross-connects, you configure the two LSPs that you are stitching together on the two ingress routers. Then, on the interdomain router, you define the connection between the two LSPs. The connection joins the LSP that comes from the connection's source to the LSP that leads to the connection's destination.

 [edit protocols]  connections {   lsp-switch  connection-name  {     transmit-lsp  label-switched-path;  receive-lsp  label-switched-path;  } }