Specifications

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Cisco IGX 8400 Series Provisioning Guide, Release 9.3.3 and Later Releases
OL-1166-04
Chapter 4 Cisco IGX 8400 Series Trunks
Functional Overview
Virtual Trunking on the IGX
A virtual trunk is a trunk defined over a public ATM service. Virtual trunks provide customers with a
cost-effective way to build a private network over a public ATM network. This hybrid network
configuration allows private virtual trunks to use the mesh capabilities of the public network to
interconnect the nodes found in the private network.
To establish connectivity through a public ATM cloud, you allocate virtual trunks between the nodes on
the edges of the public ATM network. With a single trunk port from the private network attached to a
single ATM port within the public ATM network, the node uses virtual trunks to connect to multiple
destination nodes on the other side of the public ATM network. Functionally, the virtual trunk is
equivalent to a virtual path connection (VPC) provided by the public ATM network. By using a virtual
trunk number, you differentiate between the virtual trunks found within a physical port.
ATM equipment within the public ATM network must support virtual path switching and must move
incoming cells based on the virtual path ID (VPI) in the cell header. Within the public ATM network,
the virtual trunk is a VPC, and can support CBR, VBR and ABR traffic. Because the virtual trunk is
switched using the VPI value, the 16 virtual connection ID (VCI) bits defined in the ATM cell header
are passed transparently through to the destination node. The VPI must be provided by the public ATM
network administrator or your ATM service provider.
Congestion management (resource management) cells are also passed transparently through the
network. While Cisco-proprietary features such as Advanced CoS Management and Optimized
Bandwidth Management may not be supported within the public ATM network, the information can still
carried through the public ATM network into the private, destination node.
The node’s physical trunk interface to the public ATM network can be either a standard ATM UNI or
NNI interface, as specified by the public ATM network administrator or ATM service provider. If the
physical trunk interface is specified as NNI, an additional four bits of VPI addressing space become
available.
Note The virtual trunk cannot provide a clock for transport across the public ATM network.
VPI, VCI, and Cell Header Formats
The VPI value across the virtual trunk is identical for all cells on the virtual trunk. However, the VCI
will differ according to the final destination of the cell. Before the cell enters the public ATM network
on the virtual trunk, the cell header is translated to the user-configured VPI value for the trunk and a
unique VCI value is assigned to the cell by switch software. As cells are received from the public ATM
network by a BPX or IGX, these VPI and VCI values are mapped back to the appropriate VPI and VCI
addresses used by the node for cell forwarding.
The IGX supports only the ATM-NNI and ATM-UNI cell header formats. The ATM-NNI cell header
lacks the GFCI field found in the ATM-UNI cell header, so those four bits are added to the VPI to give
a 12-bit VPI on ATM-NNI virtual trunks.
See Table 4-4 for a summary of VPI and VCI values.
Table 4-4 Values Used in VPI and VCI Addressing
Address Type Value Range for UNI Value Range for NNI
VPI 1–255 1–4095
VCI 1–65535 1–65535