HP 200 Series Services And Applications page 33

Architecture and Technology
Routing Software Technology
Routing Software Technology
A router is a layer three—network layer—device. Packets are routed using
the network-layer addresses in the network protocol header of the packets.
Each "routable" protocol suite, such as TCP/IP or Novell IPX (here called a
routing service), manages its own forwarding based on its own address
tables, routing protocols, and other routing configuration parameters as a
separate software module. Each service's software module is referred to as a
redirector.
The HP routers also perform as bridges, which operate at layer two—the
data-link layer—independent of and invisible to the network-layer and
higher-layer protocols. Packets are forwarded by a bridge using the station
addresses (also called physical or MAC or Ethernet addresses) in the
data-link-layer header header of the packets. The bridging service manages
its forwarding based on address tables and other bridging configuration
parameters as a software module of its own. Features for the transparent or
source routing bridge, including the spanning tree protocol, are also part of
the bridging software module. The redirector for bridging is here called the
bridge redirector.
Protocols for the higher layers, such as transmission (TCP) and application
(TFTP, FTP) layers, are not used in routing decisions on the router, but
support of these protocols depends on the selection of these data-link-layer
and network-layer protocols (TFTP requires IP at the network layer), and
some of these services are applications provided on the router.
All software modules—redirectors—operate concurrently.
Each physical LAN or WAN link (here called a line)—the cable attached to a
port on the router—is also a logical entity called a circuit. A circuit group is
another logical entity; it consists of one or more circuits. Each circuit group
is the network interface to which each routing or bridging service can
forward or route packets. Circuit groups are useful for hiding load sharing
and automatic backup from the network layer.
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