7. Analyzing Ethernet LAN Switching

What are the default settings of the swith that allow it be put to work out of the box?

- All interfaces are enabled by default - All interfaces are assigned to VLAN1 - 10/100 and 10/100/1000 interfaces use autonegotiation by default - The MAC learning, forwarding , flooding logic all works by default - STP is enabled by default

What switches decision does an LAN switch have to make on each and every frame it receives?

- Forward the frame out some other port - Ignore it

How does a LAN Switch make it switching decision?

1. Deciding when to forward a frame or when to filter ( not forward) a frame, based on the destination MAC Address 2. Preparing to forward frames by learning MAC addresses by examining the source MAC address of each frame received by the switch 3. Preparing to forward only one copy of the frame to the destination by creating a (Layer 2) loop-free environment with other switches by using Spanning Tree Protocol (STP)

What is the value of inactivity timer of a MAC table entry before it is timed out (removed)?

300 seconds on many switches

What is a LAN broadcast frame?

A frame destined to the Ethernet broadcast address of FFFF:FFFF:FFFF, this process helps deliver a copy of the frame to all devices in the LAN.

What are the two states STP assigns a port?

Blocking Forwarding

What is the Ultimate role of a LAN Switch?

Forward Frames

What are advantages of STP?

Frames do not loop infinitely, which makes the LAN usable.

Draw Switch Forwarding and Filtering Decision

Fred sends a frame with destination address 0200.2222.2222 (Barney's MAC address). The switch compares the destination MAC address (0200.2222.2222) to the MAC address table, matching the bold table entry. That matched table entry tells the switch to forward the frame out port F0/ 2, and only port F0/ 2.

What time of memory does the MAC address table use?

MAC Table uses content-addressable memory (CAM), a physical memory that has great lookup capabilities. The size of the MAC table depends on the size of the CAM. Each switch model has a different CAM, which equals different amount of entries in the MAC table/

What does the forwarding state in STP do to a port?

Permits the interface to send and receive data frames.

What information is displayed as a result of the show interfaces status command?

Port Name: ex: Fa0/1 Status: ex: connected/not connected VLAN: ex:assigned vlan Duplex ex:: a-full, auto Speed: ex: a-100, auto

Which command will display all the mac address entries for one VLAN?

Show mac address-table dynamic interface VLAN1

Which command will display all the mac address learned from a particular port?

Show mac address-table dynamic interface fa0/1

How do switches make decisions to forward and filter frames, learn MAC addresses, and use STP to avoid loops? List the steps.

Step 1. Switches forward frames based on the destination MAC address: A. If the destination MAC address is a broadcast, multicast, or unknown destination unicast (a unicast not listed in the MAC table), the switch floods the frame. B. If the destination MAC address is a known unicast address (a unicast address found in the MAC table): i. If the outgoing interface listed in the MAC address table is different from the interface in which the frame was received, the switch forwards the frame out the outgoing interface. ii. If the outgoing interface is the same as the interface in which the frame was received, the switch filters the frame, meaning that the switch simply ignores the frame and does not forward it. Step 2. Switches use the following logic to learn MAC address table entries: A. For each received frame, examine the source MAC address and note the interface from which the frame was received. B. If it is not already in the table, add the MAC address and interface it was learned on. Step 3. Switches use STP to prevent loops by causing some interfaces to block, meaning that they do not send or receive frames.

How do Switches build address tables?

Switches build the address table by listening to incoming frames and examining the Source MAC address in the frame. If a frame enters the switch and the source MAC address is not in the MAC address table, the switch creates an entry in the table.

What are some disadvantages of STP?

Takes a some work to balance traffic across the redundant alternate links

What does a switch do when it does not the destination MAC address of a frame?

The switch will forward the frame out all interfaces (except the incoming interface) using a process called flooding.

What are the one primary jobs a LAN Switch is responsible for?

To Forward frames to the correct destination (MAC) address. And to achieve that goal, switches use logic---logic based on the source and destination MAC address in each frames Ethernet header.

What is a frame whose destination address is unknown called?

Unknown unicast frame

How does the MAC address inactivity timer work?

Value starts at 0 and moves up in seconds, onces it reaches the value specified the switch will delete the entry form the MAC table. If a frame is received for an existing entry the entries inactivity timer is reset to 0

Which command will remove the dynamic entries from MAC table?

clear mac address-table dynamic

What command would you use to delete the VLAN configuration?

delete vlan.dat

what commands earses the startup-config

erase startup-config

what commands erase the startup-config?

erase startup-config

which commands would you use to reset a switch to factory?

erase startup-config delete vlan.dat reload

Which command would you use to restart the switch

ewloS

What does a switch do when it receives a frame and already has the source address in MAC table?

resets inactivity timer back to 0

which command would you use to display information of a particular interface?

show interface f0/1 status (returns single line) or show interface f0/1 (returns detailed output)

Which option of the show interfaces fa0/1 command will display list interface statistics incoming/outgoing frames, list number of unicast, multicast, and broadcast frames in and out directions, and total byte count for those frames.

show interfaces fa0/1 counter The counter parameter of the show itnerfaces command list all statistics for incoming and outgoing frames.

Which command displays interfaces status

show interfaces status

What command displays the switch mac address table?

show mac address-table

Which command will display MAC address aging time?

show mac address-table aging-time Shows Global Value for max age

Which command will display MAC address count?

show mac address-table count Shows how $ of Dynamic MAC # of Static MAC Total MAC

What command displays the switches dynamically learned MAC address only?

show mac address-table dynamic

Which command would you use to find a particular mac address in the mac address table?

show mac address-table dynamic address 1111:2222:3333 searches mac address table for the mac address at the end of the command and list its VLAN, Port, Type(Dynamic, Static)

What does the blocking state in STP do to a port?

the interface cannot forward or receive data frames. If the correct subset of the interfaces is blocked, only a single currently active logical path exists between each pair of LANs.