Foundation Summary

Study Material For Cisco Ccna Ccnp And Ccie Students

Study Material For Cisco Students

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The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your CCNA exam, a well-prepared CCNA candidate should know, at a minimum, all the details in each "Foundation Summary" section before going to take the exam.

Transparent bridges forward frames when necessary and do not forward when there is no need to do so, thus reducing overhead. To accomplish this, transparent bridges perform three actions:

1. Learning MAC addresses by examining the source MAC address of each frame received by the bridge

2. Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address

3. Creating a loop-free environment with other bridges by using the Spanning Tree Protocol The following list provides a quick review of the basic logic used by a switch or bridge:

1. A frame is received.

2. If the destination is a broadcast or multicast, forward on all ports except the port in which the frame was received.

3. If the destination is a unicast and the address is not in the address table, forward on all ports except the port in which the frame was received.

4. If the destination is a unicast and the address is in the address table, and if the associated interface is not the interface in which the frame arrived, forward the frame out the one correct port.

5. Otherwise, filter (do not forward) the frame.

The internal processing algorithms used by switches vary among models and vendors; regardless, the internal processing can be categorized as one of the methods listed in Table 9-6.

Table 9-6 Switch Internal Processing

Switching Method

Description

Store-and-forward

The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame. (The FCS is in the Ethernet trailer.)

Cut-through

The switch performs the address table lookup as soon as the destination address field in the header is received. The first bits in the frame can be sent out the outbound port before the final bits in the incoming frame are received. This does not allow the switch to discard frames that fail the FCS check. (The FCS is in the Ethernet trailer.)

Fragment-free

This performs like cut-through switching, but the switch waits for 64 bytes to be received before forwarding the first bytes of the outgoing frame. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frames in error because of a collision will not be forwarded.

General definitions for a collision domain and a broadcast domain are as follows:

■ A collision domain is a set of network interface cards (NICs) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.

■ A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC will be received by all other NICs in the same broadcast domain.

Figure 9-12 shows a typical example of the definition of collision domains, while Figure 9-13

shows broadcast domains in the same network.

Figure 9-12 Collision Domains

Figure 9-12 shows a typical example of the definition of collision domains, while Figure 9-13

shows broadcast domains in the same network.

Figure 9-13 Broadcast Domains
Table 9-7 summarizes the reasons STP places a port in forwarding or blocking state. Table 9-7 STP: Reasons for Forwarding State

Characterization of Port

Explanation

All root bridge's ports

The root bridge is always the designated bridge on all connected segments.

Each nonroot bridge's root port

The root port is the port that receives the lowest-cost BPDU from the root.

Each LAN's designated port

The bridge that forwards the lowest-cost BPDU onto the segment is the designated bridge for that segment.

All other ports

All ports that do not meet the other criteria are placed into a blocking state.

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