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9-4 Cisco BPX 8600 Series Reference
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 9-2 shows 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