9-2 Cisco BPX 8600 Series Reference
Tag Switching Benefits
Tag Switching Benefits
For multi-service networks, tag switching enables the BPX switch to provide ATM, frame relay, and
IP Internet service all on a single platform in a highly scalable way. Support of all these services on
a common platform provides operational cost savings and simplifies provisioning for multi-service
providers.
For internet service providers (ISPs) using ATM switches at the core of their networks, tag switching
enables the Cisco BPX 8600 series and the Lightstream 1010 ATM switches to provide a more
scalable and manageable networking solution than just overlaying IP over an ATM network. Tag
switching avoids the scalability problem of too many router peers and provides support for a
hierarchical structure within an ISPs network, improving scalability and manageability.
By integrating the switching and routing functions, tag switching combines the reachability
information provided by the router function with the traffic engineering optimizing capabilities of
the switches.
When integrated with ATM switches, tag switching takes advantage of switch hardware that is
optimized to take advantage of the fixed length of ATM cells, and to switch these cells at wire speeds.
Tag Switching Overview
Tag switching is a high-performance, packet (frame) forwarding technology. It integrates the
performance and traffic management capabilities of data link layer 2 with the scalability and
flexibility of network layer 3 routing.
Tag switching enables switch networks to perform IP forwarding. It is applicable to networks using
any layer 2 switching, but has particular advantages when applied to ATM networks. It integrates IP
routing with ATM switching to offer scalable IP-over-ATM networks.
Tag switching is based on the concept of label switching, in which packets or cells are assigned short,
fixed length labels. Switching entities perform table lookups based on these simple labels to
determine where data should be forwarded.
In conventional layer 3 forwarding, as a packet traverses the network, each router extracts all the
information relevant to forwarding from the layer 3 header. This information is then used as an index
for a routing table lookup to determine the packet’s next hop. This is repeated at each router across
a network.
In the most common case, the only relevant field in the header is the destination field. However, as
other fields could be relevant, a complex header analysis must be done at each router through which
the packet travels.
In tag switching the complete analysis of the layer 3 header is performed just once, at the tag edge
router at each edge of the network. It is here that the layer 3 header is mapped into a fixed length
label, called a tag.
At each router across the network, only the tag needs to be examined in the incoming cell or packet
in order to send the cell or packet on its way across the network. At the other end of the network, a
tag edge router swaps the label out for the appropriate header data linked to that label.
