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 30 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 30: MPLS LSP 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 30) must be smaller than the LSP’s MTU.
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Note: Frame size values do not include the frame checksum sequence (FCS) or delimiting flags. |
To determine the LSP MTU required to support IS-IS, use the following calculation:
- IS-IS MTU (minimum 1492, default 1497) + frame overhead
+ 4 (MPLS shim header) = Minimum LSP MTU
The framing overhead varies based on the encapsulation being used. The following lists the IS-IS encapsulation overhead values for various encapsulations:
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 Network Interfaces Configuration Guide.
To configure an LSP tunnel cross-connect, you must configure the following on the interdomain router (Router B in Figure 30):
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