Configuring MPLS LSP Tunnel Cross-Connects Using CCC
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. The topology in Figure 1 illustrates an MPLS LSP tunnel cross-connect. In this topology, two separate networks, in this case ATM access networks, are connected through an IP backbone. CCC allows you to establish an LSP tunnel between the two domains. With LSP tunneling, you tunnel the ATM traffic from one network across a SONET backbone to the second network by using an MPLS LSP.
Figure 1: MPLS Tunnel Cross-Connect
When traffic from Router A (VC 234) reaches Router B, it is encapsulated and placed into an LSP, which is sent through the backbone to Router C. At Router C, the label is removed, and the packets are placed onto the ATM permanent virtual circuit (PVC) (VC 591) and sent to Router D. Similarly, traffic from Router D (VC 591) is sent over an LSP to Router B, then placed on VC 234 to Router A.
You can configure LSP tunnel cross-connect 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 to support IS-IS, you must ensure that the LSP’s maximum transmission unit (MTU) can, at a minimum, accommodate a 1492-octet IS-IS protocol data unit (PDU) in addition to the link-level overhead associated with the technology being connected.
For the tunnel cross-connects to work, the IS-IS frame size on the edge routers (Routers A and D in Figure 2) must be smaller than the LSP’s MTU.
 | Note: Frame size values do not include the frame check sequence (FCS) or delimiting flags. |
To determine the LSP MTU required to support IS-IS, use the following calculation:
The framing overhead varies based on the encapsulation being used. The following lists the IS-IS encapsulation overhead values for various encapsulations:
- ATM
- AAL5 multiplex—8 bytes (RFC 1483)
- VC multiplex—0 bytes
- Frame Relay
- Multiprotocol—2 bytes (RFCs 1490 and 2427)
- VC multiplex—0 bytes
- HDLC—4 bytes
- PPP—4 bytes
- VLAN—21 bytes (802.3/LLC)
For IS-IS to work over VLAN-CCC, the LSP’s MTU must be at least 1513 bytes (or 1518 for 1497-byte PDUs). If you increase the size of a Fast Ethernet MTU above the default of 1500 bytes, you might need to explicitly configure jumbo frames on intervening equipment.
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. For more information about setting the MTU, see the Junos® OS Network Interfaces.
To configure an LSP tunnel cross-connect, you must configure the following on the interdomain router (Router B in Figure 2):
Configuring the CCC Encapsulation for LSP Tunnel Cross-Connects
To configure LSP tunnel cross-connects, you must configure the CCC encapsulation on the ingress and egress routers (Router B and Router C, respectively, in Figure 2).
| Note: 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, include the encapsulation statement to configure the entire physical device. For these circuits to work, you must configure logical unit 0 on the interface.
You can include these statements at the following hierarchy levels:
- [edit interfaces]
- [edit logical-systems logical-system-name interfaces]
For ATM circuits, specify the encapsulation when configuring the VC by including the following statements. For each VC, you configure whether it is a circuit or a regular logical interface.
You can include these statements at the following hierarchy levels:
- [edit interfaces]
- [edit logical-systems logical-system-name interfaces]
For Frame Relay circuits, include the following statements to 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 1022.
You can include these statements at the following hierarchy levels:
- [edit interfaces]
- [edit logical-systems logical-system-name interfaces]
For more information about the encapsulation statement, see the Junos® OS Network Interfaces.
Configuring the CCC Connection for LSP Tunnel Cross-Connects
To configure LSP tunnel cross-connects, include the remote-interface-switch statement to define the connection between the two circuits on the ingress and egress routers (Router B and Router C, respectively, in Figure 2). 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.
You can include these statements at the following hierarchy levels:
- [edit protocols connections]
- [edit logical-systems logical-system-name protocols connections]
Example: Configuring an LSP Tunnel Cross-Connect
Configure a full-duplex MPLS LSP tunnel cross-connect from Router A to Router D, passing through Router B and Router C. See the topology in Figure 2.
Figure 2: Example Topology of MPLS LSP Tunnel Cross-Connect
On Router B:
On Router C:
