CISSP考试指南笔记：4.8 网络组件

Repeaters

A repeater provides the simplest type of connectivity because it only repeats electrical signals between cable segments, which enables it to extend a network. Repeaters work at the physical layer and are add-on devices for extending a network connection over a greater distance.

A hub is a multiport repeater.

Bridges

A bridge is a LAN device used to connect LAN segments. It works at the data link layer and therefore works with MAC addresses.

When using bridges, you have to watch carefully for broadcast storms. Because bridges can forward all traffic, they forward all broadcast packets as well.

A local bridge connects two or more LAN segments within a local area.

A remote bridge can connect two or more LAN segments over a MAN by using telecommunications links.

A translation bridge is needed if the two LANs being connected are different types and use different standards and protocols.

The following list outlines the functions of a bridge:

• Segments a large network into smaller, more controllable pieces.
• Uses filtering based on MAC addresses.
• Joins different types of network links while retaining the same broadcast domain.
• Isolates collision domains within the same broadcast domain.
• Bridging functionality can take place locally within a LAN or remotely to connect two distant LANs.
• Can translate between protocol types.
• Forwarding Tables

bridges use transparent bridging.

If transparent bridging is used, a bridge starts to learn about the network’s environment as soon as it is powered on and as the network changes. It does this by examining frames and making entries in its forwarding tables.

Many bridges use the Spanning Tree Algorithm (STA), which adds more intelligence to the bridges.

If source routing is allowed, the packets contain the necessary information within them to tell the bridge or router where they should go.

External devices and border routers should not accept packets with source routing information within their headers, because that information will override what is laid out in the forwarding and routing tables configured on the intermediate devices.

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