Tag Switching 9-3
Elements in a Tag Switching Network
Elements in a Tag Switching Network
The basic elements in a tag switching network are tag edge routers, tag switches, and a tag
distribution protocol as defined in the following:
• Tag edge routers
Tag edge routers are located at the boundaries of a network, performing value-added network
layer services and applying tags to packets. These devices can be either routers, such as the
Cisco 7500, or multilayer LAN switches, such as the Cisco Catalyst 5000.
• Tag switches
These devices switch tagged packets or cells based on the tags. Tag switches may also support
full Layer 3 routing or Layer 2 switching in addition to tag switching. Examples of tag switches
include the Cisco LightStream 1010, Cisco BPX 8650, Cisco 7500, and future gigabit router
systems from Cisco.
• Tag distribution protocol
The tag distribution protocol (TDP) is used in conjunction with standard network layer routing
protocols to distribute tag information between devices in a tag switched network.
Tag Switching Operation at Layer 3
Tag switching operation comprises two major components:
• Forwarding
• Control
Forwarding
The forwarding component is based on label swapping. When a tag switch (or router in a packet
context) receives a packet with a tag, the tag is used as an index in a Tag Forwarding Information
Base (TFIB). Each entry in the TFIB consists of an incoming tag and one or more sub-entries of the
form
<outgoing tag, outgoing interface, outgoing link level information>
For each sub-entry, the tag switch replaces the incoming tag with the outgoing tag and sends the
packet on its way over the outgoing interface with the corresponding link level information.
Figure 9-1 shows an example of tag switching. It shows an untagged IP packet with destination
128.89.25.4 arriving at Router A (RTA). RTA checks its TFIB and matches the destination with
prefix 128.89.0.0/16. (The /16 denotes 16 network masking bits per the Classless Interdomain
Routing (CIDR) standard.) The packet is tagged with an outgoing tag of 4 and sent toward its next
hop RTB. RTB receives the packet with an incoming tag of 4 that it uses as an index to the TFIB.
The incoming tag of 4 is swapped with outgoing tag 9, and the packet is sent out over interface 0
with the appropriate layer 2 information (e.g., MAC address) according to the TFIB. RTB did not
have to do any prefix IP lookup based on the destination as was done by RTA. Instead, RTB used the
tag information to do the tag forwarding. When the packet arrives at RTC, it removes the tag from
the packet and forwards it as an untagged IP packet.
