9-6 Cisco BPX 8600 Series Reference
Tag Switching in an ATM WAN
Figure 9-3 shows an example of conservative allocation. ATM edge TSR RTA is an IP routing peer
to ATM-TSR RTB. In turn, ATM-TSR RTB is an IP routing peer to ATM-TSR-RTC. IP routing
updates are exchanged over VPI/VCI 0/32 between RTA-RTB and RTB-RTC. For example:
1 RTA sends a tag binding request toward RTB in order to bind prefix 128.89.0.0/16 to a specific
VCI.
2 RTB allocates VCI 40 and creates an entry in its TFIB with VCI 40 as the incoming tag.
3 RTB then sends a bind request toward RTC.
4 RTC issues VCI 50 as a tag.
5 RTC sends a reply to RTB with the binding between prefix 128.89.0.0/16 and the VSI 50 tag.
6 RTB sets the outgoing tag to VCI 50.
7 RTB sends a reply to RTA with the binding between prefix 128.89.0.0/16 and the VCI 40 tag.
8 RTA then creates an entry in its TFIB and sets the outgoing tag to VCI 40.
Optimistic mode operation is similar to that shown in Figure 9-3, except that the events labeled 7 and
8 in the figure may occur concurrently with event 3.
Figure 9-3 Downstream on Demand Tag Allocation, Conservative Mode Shown
