Cisco Systems 8600 Series Elements in a Tag Switching Network, Tag Switching Operation at Layer

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 9-1shows an example of tag switching. It shows an untagged IP packet with destination

128.89.25.4arriving 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.

Tag Switching 9-3