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This chapter describes how the CoreBuilder 2500 system operates in a subnetworked routing environment and how the system's bridging and routing model compares with traditional models. The chapter contains these topics:
Routing distributes packets over potentially dissimilar networks. A router is the device that accomplishes this task. Routers typically:
Figure 3-1 shows where routers are typically used in a network. CoreBuilder 2500 system routing connects subnetworks to the enterprise network, providing connectivity between devices within a workgroup, department, or building.
Figure 3-1 Traditional Routing Architecture
With the CoreBuilder 2500 system, you fit Ethernet switching capability into subnetworked environments. When you put the CoreBuilder 2500 system into such a network, the system streamlines your network architecture by routing traffic between subnetworks and switching within subnetworks. See Figure 3-2.
Figure 3-2 CoreBuilder 2500 Subnetwork Routing Architecture
The CoreBuilder 2500 system integrates bridging and routing. You can assign multiple switch ports to each subnetwork. See Figure 3-3.
Figure 3-3 Multiple Ethernet Ports per Subnetwork
Transparent bridging or Express switching (described in the CoreBuilder 2500 Operation Guide) switches traffic between ports that are assigned to the same subnetwork. Traffic traveling to different subnetworks is routed using one of the supported routing protocols.
In the following descriptions of bridging and routing on the CoreBuilder 2500 system, the term MAC address refers to a physical hardware address. The term network address refers to a logical address that applies to a specific protocol.
Because the CoreBuilder 2500 model of bridging and routing allows several segments to be connected to the same subnetwork, you can increase the level of segmentation in your network without creating new subnetworks or assigning new network addresses. Instead, you can use additional Ethernet ports to expand existing subnetworks.
This model differs from traditional bridging and routing, in which at most one port is connected to any subnetwork. Traditionally, to increase the level of segmentation in your network, you must create additional subnetworks and assign new network addresses to existing hosts.
The CoreBuilder 2500 system implements routing differently from the way bridging and routing usually coexist. Traditionally, network systems first try to route packets that belong to recognized protocols; all other packets are bridged. In the CoreBuilder 2500 model, the system first tries to bridge a packet. Then, if a packet's destination network address is not on the same subnetwork, the system routes the packet.
The next sections describe these approaches.
In traditional routing, the bridge or router determines what to do with a packet based on the packet's protocol. If the packet belongs to a recognized protocol, the packet is routed. Otherwise, the packet is bridged. Figure 3-4 illustrates traditional bridging:
1 . The packet enters the bridge or router.
2 . The bridge or router determines that the packet does not belong to a recognized routing protocol, so the packet is passed to the bridge.
3 . The bridge examines the destination MAC address and forwards the packet to the port where that address was learned.
Figure 3-4 Traditional Bridging
Figure 3-5 illustrates traditional routing:
1 . The packet enters the bridge or router.
2 . The bridge or router determines that the packet belongs to a recognized routing protocol, so the packet is passed to the router.
3 . The router examines the destination network address and forwards the packet to the interface (port) that is connected to the destination subnetwork.
Figure 3-5 Traditional Routing
The destination MAC address determines whether the CoreBuilder 2500 system bridges or routes a packet. Before a host system sends a packet to another host, the host system compares its own network address to the network address of the other host as follows:
Figure 3-6 illustrates CoreBuilder 2500 bridging:
1 . The packet enters the CoreBuilder 2500 system.
2 . The bridging layer examines the packet's destination MAC address. The destination MAC address does not correspond to the MAC address of one of the system ports configured for routing.
3 . The bridging layer selects a segment (port) based on the destination MAC address and forwards the packet to that segment.
Figure 3-6 CoreBuilder 2500 Bridging
Figure 3-7 illustrates CoreBuilder 2500 routing:
1 . The packet enters the CoreBuilder system.
2 . The bridging layer examines the packet's destination address. The destination address corresponds to the address of one of the system ports configured for routing (as opposed to a learned end station address).
3 . The packet is passed to the router interface that is associated with the port where the packet was received.
4 . The routing layer:
5 . The bridging layer then selects a segment (port) based on the destination MAC address and forwards the packet to that segment.
Figure 3-7 CoreBuilder 2500 Routing