How to's of mixing token ring, Ethernet

Whether the result of a corporate merger or a simple need for speed, melding token-ring and Ethernet networks can present a variety of coexistence and migration challenges.

However, use of the Inter-Switch Link (ISL) protocol can minimize the complexity and inconvenience of a gradual token-ring-to-Ethernet migration. The end result is a media-independent corporate backbone capable of carrying both protocols at switched fabric speeds right down to the server. ISL deployment can also open the door to virtual LANs and helps with the amortization of token-ring equipment.

ISL is a method of encapsulating tagged LAN frames and transporting them over a full-duplex, point-to-point Ethernet link. The encapsulated frames may be token-ring or Fast Ethernet, and are carried unchanged from transmitter to receiver.

Because ISL carries data hop-by-hop over point-to-point links, neither the Fast Ethernet nor Gigabit Ethernet constraints of 1,500-byte data frame size are applicable. Therefore, ISL can be used to transport the far-larger 18K-byte token-ring frames (or alternatively, 100-byte packets). And because ISL is based on Ethernet technology, Fast Ethernet Category 5 copper cabling or fiber-optic cabling can be used as a transport medium, allowing speeds of 100M bit/sec or even gigabit speeds between switches and to servers.

An ISL frame consists of three primary fields: header, the original packet and a frame checksum field. The header is further divided into fields that identify the type of frame being encapsulated, the VLAN to which the frame belongs and process management information.

As the name implies, Inter-Switch Link is normally used as a backbone technology between network switches and/ or between the switch and a server that is equipped with a special token-ring-enabled ISL adapter card. Encapsulation and "de-capsulation" occurs within the switches at both ends of the backbone link, or within the server using special software. The simplicity of this approach means that no new hardware or software is required on workstations.

Before the advent of VLANs, client/ server broadcasts on a flat LAN literally brought slower PCs to a halt. Flat LANs were also plagued with access, security and troubleshooting issues. To address these problems, VLAN partitioning was implemented to break up the net into logically separate affinity groups. The ISL header carries the complete identification of the token-ring or Ethernet VLAN.

Implementing ISL requires no changes to existing token-ring or Ethernet end stations, applications, protocol stacks or source-route bridges to take advantage of the Fast Ethernet or Gigabit Ethernet backbone. Clients and servers continue to use token-ring and source-route bridged protocols without change, minimizing implementation cost and protecting installed base investment. No ring segment renumbering is required.

LAN switches with ISL uplinks provide high-speed switching from multiple token-ring and Ethernet segments and VLANs across the ISL backbone, without the need to introduce ATM switches and LAN Emulation. Servers using token-ring-enabled ISL network interface cards allow simultaneous access to common servers by token-ring and Ethernet clients via the same ISL link. Attaching routers via a single ISL connection allows communication between token-ring and Ethernet nets or between VLANs on the same net.

At a pace that makes business sense, client workstations can be migrated from token ring to Ethernet. As new PCs arrive with integrated Ethernet adapters, these users have instant access to the same resources as when they were attached to the net by token ring. ISL-attached servers can pump data at 100M bit/sec (or 400M bit/ sec using Fast EtherChannel) to token-ring and Ethernet client requestors.

Using ISL
More details from Cisco.