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L2 Switching Concepts and MAC Table Operation

28 questions

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Question 21Question →

Host-X sends an Ethernet frame destined for Host-Y through a Cisco Catalyst switch. The switch receives the frame on interface FastEthernet0/2. At the time of arrival, Host-X's source MAC address is not present in the switch MAC address table, while Host-Y's destination MAC address is already mapped to FastEthernet0/8. Place the internal processing steps performed by the Layer 2 switch in the correct chronological sequence from first to last.

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Answer

The correct chronological sequence begins with receiving the frame on FastEthernet0/2, followed by learning the source MAC address (MAC-X), performing a table lookup for the destination MAC address (MAC-Y), matching MAC-Y to FastEthernet0/8, and finally forwarding the frame solely out interface FastEthernet0/8.
The correct order follows standard Cisco Layer 2 switching logic: 1) Ingress frame reception on interface FastEthernet0/2; 2) Source MAC address inspection and MAC table entry creation for FastEthernet0/2; 3) Destination MAC address table lookup; 4) Identification of egress port FastEthernet0/8; 5) Unicast transmission out FastEthernet0/8.

Step-by-Step Solution

1
Receive Ingress Frame
The switch physical port FastEthernet0/2 accepts the frame header containing source MAC-X and destination MAC-Y.
Layer 2 processing requires the physical frame to enter an ingress interface first.
2
Source MAC Learning
Source MAC-X is mapped to FastEthernet0/2 in the switch MAC address table.
Switches update or populate their CAM/MAC table based on ingress source MAC addresses.
3
Destination MAC Lookup
The switch queries the MAC address table for destination MAC-Y.
The switch needs to decide whether to flood or unicast-forward the ingress frame based on destination MAC state.
4
Match Egress Port
The lookup resolves MAC-Y to interface FastEthernet0/8.
A matching entry prevents unknown unicast flooding.
5
Unicast Frame Forwarding
The frame is transmitted directly out FastEthernet0/8.
Known unicast destination entries result in single-port forwarding.

Key Concept

Layer 2 Ethernet Switching Process and MAC Address Table Learning and Forwarding Logic
Question 22Question →

Switch SW2 receives an ingress Ethernet frame on interface GigabitEthernet0/5 configured as an access port in VLAN 20. The frame contains a source MAC address of 0010.ab12.3456 and a destination MAC address of 0010.ab99.8877. A review of SW2's MAC address table reveals that neither MAC address currently exists in the table for VLAN 20. Which two actions does the switch take upon processing this frame? (Select two.)

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Answer: Records MAC address 0010.ab12.3456 associated with interface GigabitEthernet0/5 in the VLAN 20 MAC address table.; Floods the frame out all active interfaces assigned to VLAN 20, excluding interface GigabitEthernet0/5.

Answer

The switch records the source MAC address 0010.ab12.3456 dynamically under GigabitEthernet0/5 in VLAN 20, and floods the unknown unicast frame out all active ports in VLAN 20 except the ingress port GigabitEthernet0/5.
When a switch receives a frame, it performs two distinct steps: learning and forwarding. First, it reads the incoming frame's source MAC address and associates it with the receiving port and VLAN in its MAC address table. Second, it checks the destination MAC address. Because the destination is not in the table, it treats the frame as unknown unicast and floods it out all active ports in the same VLAN except the receiving port.

Step-by-Step Solution

1
Inspect the source MAC address of the ingress frame.
The source MAC address 0010.ab12.3456 is learned on port GigabitEthernet0/5 for VLAN 20 and added/updated in the MAC address table.
Switches build and refresh their Layer 2 forwarding table using incoming source MAC addresses.
2
Inspect the destination MAC address of the ingress frame.
The destination MAC address 0010.ab99.8877 is not found in VLAN 20's MAC address table, classifying it as unknown unicast.
A lookup in the MAC table determines whether known unicast forwarding or unknown unicast flooding is required.
3
Forward the unknown unicast frame according to Layer 2 rules.
The frame is flooded to all active ports belonging to VLAN 20, except GigabitEthernet0/5.
Flooding ensures delivery to the unknown destination while avoiding sending traffic back out the receiving interface.

Key Concept

Layer 2 Switch Learning and Unknown Unicast Flooding
Estimated Time:1m 30s
Question 23Question →

A network workstation with MAC address 0010.7b33.44a1 is relocated from switch port GigabitEthernet0/1 to port GigabitEthernet0/2 on the same VLAN. Before the switch's MAC address table aging timer expires, the workstation sends an Ethernet frame out GigabitEthernet0/2 destined for a server with MAC address 0010.7b99.88c2. The server's MAC address is already dynamically mapped to port GigabitEthernet0/4 in the MAC address table. How does the switch process this incoming frame?

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Answer: The switch updates the MAC address table entry for 0010.7b33.44a1 to point to GigabitEthernet0/2, then forwards the frame directly out GigabitEthernet0/4.

Answer

The switch updates the MAC address table entry for the moving host to reflect the new ingress port (GigabitEthernet0/2) and forwards the frame directly out the destination port (GigabitEthernet0/4).
When a switch receives a frame, it reads the source MAC address to update its MAC address table. If the source MAC address is already listed but associated with a different interface, the switch instantly updates the entry to point to the new ingress interface (GigabitEthernet0/2). Next, the switch looks up the destination MAC address. Because the destination MAC address (0010.7b99.88c2) is already known and mapped to GigabitEthernet0/4, the switch performs known unicast forwarding directly to GigabitEthernet0/4.

Step-by-Step Solution

1
Examine the source MAC address and ingress interface of the incoming frame.
Source MAC 0010.7b33.44a1 arrives on GigabitEthernet0/2.
Switches inspect the source MAC address of every ingress frame to build and maintain the MAC address table.
2
Update the MAC address table entry if the source MAC is missing or associated with a different port.
The existing entry for 0010.7b33.44a1 is updated from GigabitEthernet0/1 to GigabitEthernet0/2.
Dynamic MAC learning automatically updates port associations when a host moves to a new switch port within the same VLAN.
3
Lookup the destination MAC address in the MAC address table to determine forwarding behavior.
Destination MAC 0010.7b99.88c2 is found mapped to GigabitEthernet0/4.
Known unicast frames are forwarded exclusively out the specific port mapped in the MAC address table.

Key Concept

Layer 2 MAC Address Table Learning and Forwarding Operations
Estimated Time:1m 0s
Question 24Question →

An engineer connects a new host to interface FastEthernet0/12 on a Cisco Catalyst switch. The host immediately sends a unicast Ethernet frame intended for a server whose MAC address is already dynamically learned on interface FastEthernet0/18. Arrange the operational steps in the exact chronological order that the switch executes upon receiving this frame.

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Answer

The correct operational sequence begins with frame reception on FastEthernet0/12, followed by source MAC address learning, destination MAC lookup in the table, identifying the egress port match on FastEthernet0/18, and finally unicast forwarding out FastEthernet0/18.
Layer 2 switch operation dictates a strict sequence of events upon frame arrival: frame ingress occurs first, followed immediately by source MAC address learning (adding or updating the source MAC and interface association in the MAC address table). Next, the switch performs a table lookup for the destination MAC address, identifies the matching egress interface (FastEthernet0/18), and lastly forwards the unicast frame directly out that specific interface.

Step-by-Step Solution

1
Identify the ingress port event.
The switch receives the raw frame on FastEthernet0/12.
Frame processing cannot occur until the physical interface receives and buffers the frame.
2
Perform ingress source MAC learning.
The switch associates the frame's source MAC address with interface FastEthernet0/12 in its Layer 2 MAC address table.
Cisco switches always process source MAC learning prior to destination forwarding logic.
3
Perform destination MAC table lookup.
The switch checks its MAC address table for the frame's destination address.
The destination MAC determines whether the frame will be unicast forwarded, flooded, or filtered.
4
Evaluate the table lookup result.
An existing mapping points to interface FastEthernet0/18.
Since the server's MAC was previously learned, the destination lookup succeeds.
5
Execute frame forwarding.
The switch transmits the frame solely out interface FastEthernet0/18.
Known unicast frames are forwarded strictly to the matching egress port, preventing unnecessary broadcast domain traffic.

Key Concept

L2 Switch Ingress Processing, MAC Address Learning, and Known Unicast Forwarding
Question 25Question →

A Cisco Catalyst switch with an empty MAC address table receives an ingress Ethernet frame on interface GigabitEthernet1/0/24 assigned to VLAN 50. The frame contains a source MAC address of 001B.D455.A1B2 and a destination MAC address of 001B.D488.F9E0. Which processing action does the switch perform upon receiving this frame?

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Answer: It records source MAC address 001B.D455.A1B2 associated with interface GigabitEthernet1/0/24 in VLAN 50 in its MAC address table and floods the frame out all active interfaces in VLAN 50 except GigabitEthernet1/0/24.

Answer

The switch records the source MAC address 001B.D455.A1B2 associated with interface GigabitEthernet1/0/24 in VLAN 50 in its MAC address table and floods the frame out all active interfaces in VLAN 50 except GigabitEthernet1/0/24.
When a switch receives a frame, it always performs MAC learning by binding the source MAC address to the ingress port and VLAN. If the destination MAC address is not found in the MAC address table for that VLAN, the switch performs an unknown unicast flood, transmitting the frame out all operational ports within the VLAN except the port on which the frame arrived.

Step-by-Step Solution

1
Inspect the ingress frame's source MAC address
The switch reads source MAC address 001B.D455.A1B2 on port GigabitEthernet1/0/24 in VLAN 50.
Layer 2 switches continuously learn source MAC addresses and bind them to their ingress interface and VLAN ID.
2
Update the switch MAC address table
An entry mapping 001B.D455.A1B2 to GigabitEthernet1/0/24 in VLAN 50 is dynamically created with an aging timer.
Populating the table allows future unicast traffic destined for 001B.D455.A1B2 to be forwarded directly out GigabitEthernet1/0/24.
3
Lookup the destination MAC address 001B.D488.F9E0 in the MAC address table
The MAC address table lookup results in a miss (unknown unicast).
The initial state of the MAC address table was completely empty.
4
Execute unknown unicast forwarding logic
The frame is replicated and sent out all active ports assigned to VLAN 50, excluding GigabitEthernet1/0/24.
Flooding ensures the unknown host receives the frame without causing split-horizon interface loops.

Key Concept

Layer 2 MAC Address Table Learning and Unknown Unicast Flooding Logic
Question 26Question →

A Cisco switch receives an Ethernet frame on interface GigabitEthernet0/1 assigned to VLAN 10. The frame features a source MAC address of 0050.56a1.1111 and a destination MAC address of 0050.56b2.2222. The switch MAC address table currently contains an entry for 0050.56b2.2222 associated with interface GigabitEthernet0/3 on VLAN 10, but has no existing entry for 0050.56a1.1111. Which two actions does the switch take upon processing this ingress frame? (Select two.)

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Answer: It records an entry in the MAC address table mapping source MAC address 0050.56a1.1111 to interface GigabitEthernet0/1 on VLAN 10.; It forwards the frame directly out interface GigabitEthernet0/3 without transmitting it on any other port.

Answer

The switch learns the source MAC address by associating 0050.56a1.1111 with ingress port GigabitEthernet0/1 in VLAN 10, and forwards the frame directly out egress port GigabitEthernet0/3 based on the known destination MAC table entry.
Layer 2 switches process Ethernet frames in two distinct phases. First, the switch reads the frame's source MAC address (0050.56a1.1111) and updates its MAC address table by binding that address to ingress port GigabitEthernet0/1 under VLAN 10. Second, the switch checks its MAC table for the destination MAC address (0050.56b2.2222). Since an entry already exists pointing to GigabitEthernet0/3 in VLAN 10, the switch forwards the frame directly out interface GigabitEthernet0/3.

Step-by-Step Solution

1
Examine incoming frame headers for source MAC address processing.
The switch reads source MAC 0050.56a1.1111 on ingress interface GigabitEthernet0/1 (VLAN 10).
Layer 2 switches inspect the source MAC address of every ingress frame to dynamically build and maintain the MAC address table.
2
Update the MAC address table.
A new dynamic MAC address table entry is created: MAC 0050.56a1.1111 → GigabitEthernet0/1 (VLAN 10).
The source MAC was not previously in the table, so a new mapping is recorded to enable future return traffic forwarding.
3
Examine incoming frame headers for destination MAC lookup.
The switch searches the MAC address table for destination MAC 0050.56b2.2222 in VLAN 10.
Destination lookup determines whether the frame requires point-to-point forwarding or unknown unicast flooding.
4
Forward the frame to the destination interface.
The frame is forwarded directly out GigabitEthernet0/3.
Because destination MAC 0050.56b2.2222 is known in the MAC table on port GigabitEthernet0/3, unicast forwarding is performed without flooding.

Key Concept

Layer 2 Ethernet switching operates on a two-step process per frame: (1) Learn source MAC address to ingress port mapping, and (2) Forward based on destination MAC address lookup (known unicast forwarding vs. unknown unicast flooding).
Question 27Question →

A Cisco Catalyst switch receives an Ethernet frame on interface FastEthernet0/10. The frame has a source MAC address of 0011.2233.4455 and a destination MAC address of 00aa.bbcc.dd88. The switch MAC address table already contains an active dynamic entry for 0011.2233.4455 mapped to interface FastEthernet0/10, but contains no entry for 00aa.bbcc.dd88. Interfaces FastEthernet0/10 through FastEthernet0/15 are active access ports assigned to VLAN 10, while interface FastEthernet0/16 is an access port assigned to VLAN 20. Which action does the switch perform upon receiving this frame?

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Answer: Refreshes the aging timer for MAC 0011.2233.4455 on FastEthernet0/10 and floods the frame out interfaces FastEthernet0/11 through FastEthernet0/15.

Answer

The switch refreshes the aging timer for the existing source MAC entry on interface FastEthernet0/10 and floods the frame out all other active interfaces in VLAN 10 (FastEthernet0/11 through FastEthernet0/15).
When a switch receives a frame, it inspects the source MAC address to update or refresh its MAC address table entry. Next, it looks up the destination MAC address. Because the destination MAC is not in the table, the switch treats the frame as an unknown unicast frame and floods it out all operational interfaces in the same VLAN (VLAN 10), except for the receiving interface (FastEthernet0/10).

Step-by-Step Solution

1
Examine the source MAC address and update the MAC address table.
The source MAC address 0011.2233.4455 is already mapped to interface FastEthernet0/10. The switch resets/refreshes the aging timer for this entry.
MAC learning occurs on every incoming frame. If the entry exists on the correct port, its aging timer is reset.
2
Examine the destination MAC address to determine the forwarding decision.
The destination MAC address 00aa.bbcc.dd88 is not present in the MAC address table (unknown unicast).
Switches use MAC table lookup for unicast forwarding. Absence of an entry triggers unknown unicast flooding.
3
Identify the target egress interfaces for flooding.
The frame is flooded out FastEthernet0/11 through FastEthernet0/15. It is excluded from FastEthernet0/10 (ingress port) and FastEthernet0/16 (different VLAN).
Flooding is constrained within the ingress VLAN boundary and strictly avoids reflecting traffic back out the receiving interface.

Key Concept

Layer 2 Unknown Unicast Flooding and MAC Table Aging
Question 28Question →

A network host sends a unicast Ethernet frame to a Layer 2 switch. The source MAC address of the frame is not currently present in the switch's MAC address table, but the destination MAC address is already mapped to an active interface. In what sequence does the switch perform operations to process and forward this frame? Arrange the switch operations in the correct chronological order from first to last.

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Answer

The correct sequence of switch operations is: receiving the frame on the ingress interface, examining and recording the source MAC address in the MAC address table, looking up the destination MAC address in the table, identifying the corresponding egress interface, and forwarding the unicast frame directly out that egress interface.
When an Ethernet frame arrives on a switch port, the switch first learns or updates the mapping of the frame's source MAC address to the ingress interface in its MAC address table. Next, it performs a lookup for the destination MAC address. Because the destination MAC address is already known, the switch identifies the single mapped egress interface and forwards the frame directly out that port without flooding.

Step-by-Step Solution

1
Frame Ingress
The Ethernet frame enters the switch port buffer.
Layer 2 processing begins as soon as the physical frame is received on an interface.
2
Source MAC Learning
The unknown source MAC address is mapped to the ingress port in the MAC address table.
Switches continuously build their MAC address table by examining the source MAC address of every incoming frame.
3
Destination MAC Lookup
The switch searches its MAC address table for the destination MAC address.
Forwarding decisions are determined by comparing the destination MAC address against existing table entries.
4
Egress Port Selection
The switch identifies the target egress port linked to the destination MAC address.
A valid table match maps the destination MAC address directly to a specific physical or logical interface.
5
Unicast Forwarding
The frame is transmitted out the target egress port.
Known unicast frames are forwarded strictly out the designated interface rather than being flooded out all ports.

Key Concept

Layer 2 MAC Address Table Learning and Known Unicast Forwarding Logic
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