Cisco Systems 8600 Series manual Tag Switching in an ATM WAN, Control

Tag Switching in an ATM WAN

Control

The control component consists of tag allocation and maintenance procedures. The control component is responsible for creating tag bindings between a tag and IP routes, and then distributing these tag bindings to the tag switches.

The tag distribution protocol (TDP) is a major part of the control component. TDP establishes peer sessions between tag switches and exchanges the tags needed by the forwarding function.

Figure 9-1 Tag Forwarding Information Base (TFIB) in an IP Packet Environment

Tag Switching in an ATM WAN

With tag switching over an ATM network, the forwarding and control components can be described as follows:

•Forwarding: In an ATM environment, the tag switching forwarding function is carried out identically to normal switching. The tag information needed for tag switching can be carried in the VCI field within one or a small number of VPs. The tags are actually the VCIs.

•Control: For the control component over ATM networks, a tag distribution protocol is used to bind VCIs to IP routes. The switch also has to participate in IP routing protocols such as OSPF, BGP, and RSVP.

Forwarding

Figure 9-2shows the forwarding operation of an ATM switch in which the tags are designated VCIs. In Figure 9-2,an untagged IP packet with destination 128.89.25.4 arrives at router A (RTA). RTA checks its TFIB and matches the destination with prefix 128.89.0.0/16. RTA converts the AAL5 frame to cells, and sends the frame out as a sequence of cells on VCI 40. RTB, which is an ATM Tag Switch Router (TSR) controlled by a routing engine, performs a normal switching operation by switching incoming cells on interface 2/VCI 40 to interface 0/VCI 50

9-4Cisco BPX 8600 Series Reference