Cabletron Systems CSX200 manual

Chapter 2: About the CSX200

In transparent bridging, the decision to forward the frame is based on this comparison. If the address indicates that the sending station and the destination station are on the same side of the bridge, the frame is not forwarded across the bridge. If the addresses do not indicate that, the bridge forwards the broadcast frame across the bridge to the other network(s).

Bridging allows frames to be sent to all destinations regardless of the network protocols used. It also allows protocols that cannot be routed (such as NETBIOS) to be forwarded, and optimizes internetwork capacity by localizing traffic on LAN segments. A bridge extends the physical reach of networks beyond the limits of each LAN segment. Filters can be used to increase network security in bridged networks, and restrict message forwarding by using user-built address tables (non-transparent bridging).

Routing — Routing provides a way to transfer user data from source to destination over different LAN and WAN links using one or more network protocol formats. Routing relies on routing address tables to determine the best path for each packet. Routing tables can be seeded (i.e., addresses for remote destinations are placed in the table along with network address masks and a metric for path latency). Routing tables are also built dynamically (i.e., the location of remote stations, hosts and networks are updated through inter-router protocols). Routing helps to increase network capacity by localizing traffic on LAN segments and broadcasts that would result from bridged traffic. It also provides security by isolating traffic on segmented LANs. Routing extends the world-wide reach of networks.

CSX200 Bridging and Routing — The CSX200 can operate as a bridge, a router, or both. The CSX200 operates as a router for network protocols that are supported when routing is enabled and operates as a bridge when bridging is enabled. When both bridging and routing are enabled, routing takes precedence over bridging; i.e., the CSX200 uses the protocol address information of the packet to route the packet to the correct destination. However, if the protocol is not supported, the CSX200 operates as a bridge and uses the MAC address information to send the packet.

Operation of the CSX200 is influenced by routing and bridging controls and filters set during CSX200 configuration. General IP routing, and routing or bridging from specific remote routers are controls set during the configuration process.

IEEE 802.1d Bridging — The CSX200 supports the IEEE 802.1d standard for LAN to LAN bridging. Bridging is provided over PPP and Frame Relay as well as adjacent LAN ports. Bridging uses the MAC (Machine Address and Control layer) address unique to each device. When configured as a bridge, the CSX200 bridges data packets to the destination, regardless of the network protocols used.

2-10 CSX200 Installation Guide