Analyzing Ethernet LAN Switching

Question 1

Overview Of Switching Logic

Answer 1

1. Deciding when to forward a frame or when to filter 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 loop free environment with other switches by using Spanning Tree Protocol (STP)

Question 2

Forwarding Known Unicast Frames

Answer 2

To decide where a frame should be forward, a switch builds a DYNAMIC TABLE that lists MAC addresses and outgoing interfaces

Compares the frame’s destination MAC address to the table, and decides whether to forward or filter

Switch’s MAC address table is also known as switching table, Content Addressable Memory (CAM) table

Question 3

Forwarding choice of switch

Answer 3

Uses a forward versus filter decision schema, because the switch can also choose not to forward frames and not send frame out some ports

Question 4

Unicast frame

Answer 4

Frames are called unicast frames when the destination address is known and is sent to only one host through one port.

Port is listed in the MAC table entry for that MAC address

Question 5

Secondary Function Of Switch

Answer 5

Build an address table through learning the MAC address of data packets and the interfaces the data packet flows through

Switches build the address table by listening to incoming frames and examining the source MAC address in 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, MAC address to Interface

Question 6

Unknown Unicast

Answer 6

For a switch, when there is no matching entry in the table, switches forward the frame out all interfaces (except the incoming interface) using a process called flooding ( These frames are called unknown unicast frames )

Flooding means that the switch forwards copies of the frame out all ports, except the port on which the frame was received

Question 7

LAN broadcast frames

Answer 7

Ethernet broadcast address of FFFF.FFFF.FFFF ( Process helps deliver a copy of the frame to all devices in the LAN)

Question 8

Avoiding the Loops (3rd Function)

Answer 8

The third primary feature of LAN switches is loop prevention, as implemented by Spanning Tree Protocol (STP)

W/o STP, any flooded frames would loop for an indefinite period of time, redundancy causes looping, flooding of frame would rotate around switches and each switch would flood the same frame

Process, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments

Question 9

Con of STP

Answer 9

Takes some work to balance traffic across the redundant alternate links

Question 10

STP

Answer 10

STP causes each interface on a switch to settle into either a blocking state or a forwarding state.

Blocking means that the interface cannot forward or receive data frames

Forwarding means that the interface can send and receive data frames

If a correct subset of the interfaces is blocked, only a single currently active logical path exists between each pair of LANs

If a frame repeatedly floods the network the network would be congested as each switch stage would propagate n numbers of frames out of its ports

Question 11

Switching Summary

Answer 11

1. If the destination MAC address is a broadcast, multicast, or unknown destination unicast, the switch floods the frame
2. If the destination MAC is a known unicast address;
   a. If the outgoing interface listed in the MAC address table is different from which the frame was received, the switch forwards the frame out the outgoing interface
   b. 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
3. 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
4. Switches use STP to prevent loops by causing some interfaces to block, meaning that they do not send or receive frames

Question 12

clearing Switch’s configuration

Answer 12

erase startup-config
delete vlan.dat
reload
hostname SW1

Question 13

mac addresses command

Answer 13

show mac address-table dynamic
show mac address-table dynamic address 127.0.0.1
show mac address-table dynamic fa 0/1
show mac address-table dynamic valn1

Question 14

show status

Answer 14

show interfaces status
show interface fa 0/1 status
show vlan status

Question 15

Frame flooding in VLAN setup

Answer 15

If a frame enters via a port in VLAN 1, then the switch will forward or flood that frame out other ports in VLAN1 only, and not out any ports that happen to be assigned to another VLAN

Question 16

show packet counters

Answer 16

show interface fa0/1 counters

Question 17

Switch management of MAC address table

Answer 17

MAC address do not remain in the table indefinitely

Switch will remove the entries due to:

• Aging
• Table filling

Aging - Switch removes entries that have not been used for a defined number of seconds

• Listen to the ports, if a new MAC address is received, the switch add the correct entry to the table.
• If the entry already exist, resets inactivity timer back to 0

Question 18

show amount of MAC entries and MAC aging time

Answer 18

show mac address-table aging-time

show mac address-table count

Question 19

clearing mac address table

Answer 19

VLAN, clear mac address-table dynamic vlan 1

Interface, clear mac address-table dynamic interface fa0/1

MAC, clear mac address-table dynamic address