RFC 2105 (rfc2105) - Page 3 of 13

Cisco Systems' Tag Switching Architecture Overview

RFC 2105           Cisco's Tag Switching Architecture      February 1997


   accommodate new and emerging requirements.

   The rest of the document is organized as follows. Section 2
   introduces the main components of tag switching, forwarding and
   control. Section 3 describes the forwarding component.  Section 4
   describes the control component. Section 5 describes how tag
   switching could be used with ATM. Section 6 describes the use of tag
   switching to help provide a range of qualities of service.  Section 7
   briefly describes possible deployment scenarios. Section 8 summarizes
   the results.

2. Tag Switching components

   Tag switching consists of two components: forwarding and control.
   The forwarding component uses the tag information (tags) carried by
   packets and the tag forwarding information maintained by a tag switch
   to perform packet forwarding. The control component is responsible
   for maintaining correct tag forwarding information among a group of
   interconnected tag switches.

3. Forwarding component

   The fundamental forwarding paradigm employed by tag switching is
   based on the notion of label swapping. When a packet with a tag is
   received by a tag switch, the switch uses the tag as an index in its
   Tag Information Base (TIB). Each entry in the TIB consists of an
   incoming tag, and one or more sub-entries of the form (outgoing tag,
   outgoing interface, outgoing link level information). If the switch
   finds an entry with the incoming tag equal to the tag carried in the
   packet, then for each (outgoing tag, outgoing interface, outgoing
   link level information) in the entry the switch replaces the tag in
   the packet with the outgoing tag, replaces the link level information
   (e.g MAC address) in the packet with the outgoing link level
   information, and forwards the packet over the outgoing interface.

   From the above description of the forwarding component we can make
   several observations. First, the forwarding decision is based on the
   exact match algorithm using a fixed length, fairly short tag as an
   index. This enables a simplified forwarding procedure, relative to
   longest match forwarding traditionally used at the network layer.
   This in turn enables higher forwarding performance (higher packets
   per second). The forwarding procedure is simple enough to allow a
   straightforward hardware implementation.

   A second observation is that the forwarding decision is independent
   of the tag's forwarding granularity. For example, the same forwarding
   algorithm applies to both unicast and multicast - a unicast entry
   would just have a single (outgoing tag, outgoing interface, outgoing



Rekhter, et. al.             Informational