3Com 3C63400-3AC-C - PathBuilder S700 Switch, Справочное руководство

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Application Module Overview 81
Ethernet Module
Overview
The Ethernet module (Dual Ethernet) is a port or application module that takes
legacy LAN traffic across a WAN ATM network using the PathBuilder S700. This
module is used to connect LAN segments and bridge or IP forward traffic across
the ATM network to a LAN/IP service.
The Ethernet module supports two 10BaseT Ethernet connections. It provides a
bridge function together with a SAR conversion to ATM cells. The Ethernet module
performs the following key functions:
n
Provides line interface for two 10BaseT Ethernet connections.
n
Performs bridge function and encapsulation according to RFC1483 with
multicast support.
n
Performs segmentation and assembly to ATM, with 14 programmable traffic
shapers on the transmit (PCR, SCR, MBS settings available).
n
Provides a wire speed forwarding rate of 14700 packets per port.
n
Supports Spanning Tree
Ethernet Module Standards Support
Table 26 lists the standards currently supported by the Ethernet module.
Ethernet Module Operation
The Ethernet module contains a high-performance CPU that performs all the
bridging and packet switching functions. The Ethernet CPU communicates to the
management CPU. Specific module information (serial number, type, MAC
address etc.) is stored and accessed through the management CPU. After bootup,
the local CPU requests actual run code from the management CPU. Figure 49
shows a functional block diagram for the Ethernet module.
The data flow in the Ethernet module is as follows:
n
Packets appearing on the 10BaseT ports from the LAN are checked for packet
integrity and stored in the shared memory area. Up to a Megabyte of memory
is reserved for data.
n
The CPU examines the packet's header in memory and a bridge operation is
performed to determine if the packet goes on the WAN. See “Bridging”, later
in this chapter, for details about the bridge operation.
n
If the packet is to go on the WAN, the bridge determines the VCC connection
for the packet destination.
n
The packet is then encapsulated according to RFC 1483 and the descriptor is
handed off to the SAR together with the associated ATM header descriptor.
n
The SAR append the AAL5 convergence sublayer to the packets and queues
the packets according the shaper you specify for that connection in the user
interface.
Table 26 Supported Ethernet RFC Standards
RFC Number Description
RFC 826 ARP
RFC 1042 Standard for the Transmission of IP Datagrams over IEEE.802 Networks
RFC 1483 Multiprotocol Encapsulation Over ATM Adaptation Layer
RFC 1577 Operation for ATM ARP and In ARP LLC/SNAP Encapsulation of IP, etc.