Cisco Systems 8600 Series manual Tag Switching and the BPX

Tag Switching and the BPX 8650

Tag Switching and the BPX 8650

With tag switching the router function can be accomplished by either integrating the routing engine into the switch or by using a separate routing controller (associated router). The BPX 8650 tag switch combines a BPX switch with a separate router controller (Cisco Series 7200 or 7500 router). This has the advantage of separating the various services (e.g., AutoRoute, SVCs and tag switching) into separate logical spaces that do not interfere with one another. Figure 9-4shows two scenarios, one in which the IP packets are applied to the network via the edge routers (either part of the BPX 8650 Tag Switches or independent 7500 Tag Edge Routers), and the other where IP packets are routed via a BPX 8620 to a BPX 8650 via Frame Relay permanent virtual circuits (PVCs).

Example 1: An IP packet is applied to the network via BPX 8650s on the edge of the network and then tag switching is used to forward the packet across the network via BPX 8650s. In this example the shortest path is not used, but rather the tag switch connection is routed across BPX 8650 TS-A, BPX 8650 TS-B, BPX 8650 TS-C, BPX 8650 TS-D, and 7500 TER-S. This particular routing path might, for example, have been selected with administrative weights set by the network operator. The designated tags for the cells transmitted across the network in this example are shown as 40, 60, 70, and 50, respectively. The router component of the tag switches that are located at the boundaries of the network (BPX 8650 TS-A, BPX 8650 TS-C, BPX 8650 TS-H), perform edge-routing network layer services including the application of tags to incoming packets. The tag edge routers, 7500 TER-S, 7500 TER-T, and 7500 TER-U, perform the same edge-routing network layer services in this example.

Example 2: An IP packet is routed to BPX 8650 TS-H at the interior of the network via BPX 8620 switch-F, using a Frame Relay PVC. The BPX switch interface for a Frame Relay PVC might be an MGX 8220 as shown. The applicable Frame Relay interface for BPX 8650 TS-H is connected via cable to a Frame Relay interface on its TSC where tag switching is performed on the incoming IP packet. The designated tag switching cells are shown with an tag designation of 12. These tag switching cells are then forwarded to BPX 8650 TS-D where they are converted back to an IP packet and routed to the CPE at the edge of the network as a Frame Relay PVC via an MGX 8220.

Tag Edge Router functionality is necessary to add and remove tags from IP packets, but not to switch tagged packets. Figure 9-4shows 3 stand-alone Tag Edge Routers (TERs).These would typically be co-located with BPX 8650 Tag Switches in Points of Presence. However the Tag Switch Controller in a BPX 8650 can also act as a TER if required.

In Figure 9-4,Tag Switches A, C, D and H use this combined Tag Switch/Tag Edge Router functionality. Only Tag Switch B acts purely as a Tag Switch. Note also that the Tag Edge Router performance of a BPX 8650 Tag Switch is significantly lower than its Tag Switching performance. Typically there will be several Tag Edge Routers (or combined TSC/TERs) for each BPX Tag Switch.

Tag Switching 9-7