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I den här laborationen arbetar vi med ARP (Address Resolution Protocol) i Packet Tracer. Målet är att förstå hur enheter kommunicerar på lager 2 och lager 3 när en IP-adress ska översättas till en MAC-adress. Vi följer steg för steg hur ARP-förfrågningar, ARP-svar och ICMP-paket skickas genom nätverket, och hur switchar och routrar hanterar dessa.

Instruktionerna och frågorna är på engelska eftersom de följer den ursprungliga Cisco-laborationen i Packet Tracer. Det är viktigt att behålla dem på engelska för att träna på det språk och de kommandon som används i verkliga nätverkssammanhang och i CCNA-certifieringen. Svaren kan dock utvecklas och förklaras på engelska eller svenska beroende på sammanhang – här använder vi engelska svar på frågorna för att stämma överens med labbens format.

Objectives

Part 1: Examine an ARP Request
Part 2: Examine a Switch MAC Address Table
Part 3: Examine the ARP Process in Remote Communications

Background

This activity is optimized for viewing PDUs. The devices are already configured. You will gather PDU information in simulation mode and answer a series of questions about the data you collect.

Instructions

Part 1: Examine an ARP Request

Step 1: Generate ARP requests by pinging 172.16.31.3 from 172.16.31.2.

a. Click 172.16.31.2 and open the Command Prompt.

b. Enter the arp -d command to clear the ARP table. Close a command prompt

c. Enter Simulation mode and enter the command ping 172.16.31.3. Two PDUs will be generated. The ping command cannot complete the ICMP packet without knowing the MAC address of the destination. So the computer sends an ARP broadcast frame to find the MAC address of the destination.

d. Click Capture/Forward once. The ARP PDU moves to Switch1, while the ICMP PDU disappears, waiting for the ARP reply.

Open the PDU and record the destination MAC address.

Question: Is this address (Dest. MAC) listed in the table above?

No. The destination MAC is FF:FF:FF:FF:FF:FF, which is the Ethernet broadcast address, not a device MAC from the table.

e. Click Capture/Forward to move the PDU to the next device.

Question: How many copies of the PDU did Switch1 make?

Three. It flooded the ARP frame out Fa0/2, Fa0/3, and Gi0/1 (all ports except the incoming Fa0/1).

Question: What is the IP address of the device that accepted the PDU?

172.16.31.3. That’s the ARP target, so that host accepted the broadcast ARP request.

f.  Open the PDU and examine Layer 2.

Question: What happened to the source and destination MAC addresses?

When the ARP request is broadcast, the source MAC is the sender’s MAC (000C.85CC.1DA7 from 172.16.31.2) and the destination MAC is the broadcast address (FFFF.FFFF.FFFF).

When the ARP reply is generated, the fields are reversed:

Source MAC is now the responder’s MAC (0060.7036.2849 from 172.16.31.3).

Destination MAC is the original sender’s MAC (000C.85CC.1DA7).

In other words: the ARP request uses broadcast, but the ARP reply uses unicast back to the requester.

g. Click Capture/Forward until the PDU returns to 172.16.31.2.

Question: How many copies of the PDU did the switch make during the ARP reply?

During the ARP reply, the switch made only one copy of the PDU. Unlike the broadcast ARP request, the reply is unicast directly back to the original requester (172.16.31.2) on its specific port.

Step 2: Examine the ARP table.

a. Note that the ICMP packet reappears. Open the PDU and examine the MAC addresses.

Question: Do the MAC addresses of the source and destination align with their IP addresses?

Yes. The source IP 172.16.31.2 maps to MAC 000C.85CC.1DA7, and the destination IP 172.16.31.3 maps to MAC 0060.7036.2849—so the Layer-2 addresses correctly align with their corresponding Layer-3 endpoints for this ICMP packet.

b. Switch back to Realtime and the ping completes.

c. Click 172.16.31.2 and enter the arp –a command.

Question: To what IP address does the MAC address entry correspond?

It corresponds to 172.16.31.3, which has the MAC address 0060.7036.2849.

Question: In general, when does an end device issue an ARP request?

An end device issues an ARP request whenever it needs to send a frame to an IP address on the same local network but does not yet know the corresponding MAC address.

Part 2: Examine a Switch MAC Address Table

Step 1: Generate additional traffic to populate the switch MAC address table.

a. From 172.16.31.2, enter the ping 172.16.31.4 command.

b. Click 10.10.10.2 and open the Command Prompt.

c. Enter the ping 10.10.10.3 command.

Question: How many replies were sent and received?

Each ping generated four ICMP Echo Replies, and all four replies were received successfully.

Close a command prompt

Step 2: Examine the MAC address table on the switches.

a. Click Switch1 and then the CLI tab. Enter the show mac-address-table command.

Question: Do the entries correspond to those in the table above?

Yes. The MAC addresses listed (0002.1640.8D75, 000C.85CC.1DA7, 00E0.F7B1.8901) match the devices and interfaces shown in the addressing table.

b. Click Switch0, then the CLI tab. Enter the show mac-address-table command.

Question: Do the entries correspond to those in the table above (Switch0)?

Yes. Switch0 also learns the correct MAC address from the connected devices according to the addressing table:

• • • 0001.6458.2501 → Router0 G0/0 (Gig0/1)
    • 0060.2F84.4AB6 → host 10.10.10.2 (Fa0/2)
    • 0060.4706.572B → host 10.10.10.3 (Fa0/2)

Question: Why are two MAC addresses associated with one port?

De två adresserna (0060.2F84.4AB6 för 10.10.10.2 och 0060.4706.572B för 10.10.10.3) dyker upp på samma port (Fa0/2) därför att de båda är kopplade via en Access Point (AP).

Part 3: Examine the ARP Process in Remote Communications

Step 1: Generate traffic to produce ARP traffic.

a. Click 172.16.31.2 and open the Command Prompt.

b. Enter the ping 10.10.10.1 command.

c. Type arp –a.

Question: What is the IP address of the new ARP table entry?

172.16.31.1
The PC must resolve the MAC of its default gateway (Router0’s G0/0) to reach the remote 10.10.10.0/24 network.

d. Enter arp -d to clear the ARP table and switch to Simulation mode.

e. Repeat the ping to 10.10.10.1.

Question: How many PDUs appear?

Two.
One ARP request to find the MAC of the gateway, and one ICMP Echo Request (ping).

Close a command prompt

f. Click Capture/Forward. Click the PDU that is now at Switch1.

Question: What is the target destination IP destination address of the ARP request?

172.16.31.1
The ARP request is for the local gateway’s IP, not the remote host.

g. The destination IP address is not 10.10.10.1.

Question: Why?

Because ARP only works within the local broadcast domain. To reach a remote network (10.10.10.0/24), the host must send the packet to its default gateway (172.16.31.1). The PC therefore issues an ARP request for the gateway’s IP, not for the remote target.

Step 2: Examine the ARP table on Router1.

a. Switch to Realtime mode. Click Router1 and then the CLI tab.

b. Enter privileged EXEC mode and then the show mac-address-table command.

Question: How many MAC addresses are in the table?

The MAC address table on Router1 is empty — there are 0 entries.

Question: Why?
Because a router is not a switch. Routers do not learn or maintain a switch-style MAC address table that maps ports to multiple MACs. Instead, a router only knows its own interface MAC addresses and keeps track of neighbors in the ARP table, not in a MAC address forwarding table.

c. Enter the show arp command.

Questions: Is there an entry for 172.16.31.2?

Yes. Since Router1 is directly connected to the 172.16.31.0/24 network, it learns the MAC address of host 172.16.31.2 when that host generates traffic.

Questions: What happens to the first ping in a situation where the router responds to the ARP request?

The first ping fails or times out because the router must first resolve the MAC address via ARP. Once the ARP reply is received and cached, subsequent pings succeed.

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