DATA COMMUNICATION LAB (EC-605) - Unit 2 Short Notes

UNIT 2: DATA LINK LAYER & LOCAL NETWORKING LAB

2.1 Ethernet Fundamentals & LAN Device Configuration

2.1.1 Ethernet Frame Structure & Field Analysis

• MAC Addressing (48-bit): Format XX:XX:XX:XX:XX:XX. First byte's LSB determines type:

  • Unicast: LSB = 0 (e.g., 00:1A:2B:3C:4D:5E)

  • Multicast: LSB = 1 (e.g., 01:00:5E:00:00:01 for IPv4 multicast)

  • Broadcast: FF:FF:FF:FF:FF:FF

• Standard Ethernet II Frame Fields:

• Ethernet II vs. IEEE 802.3:

  • Ethernet II: Uses Type field (e.g., 0x0800). Dominant in TCP/IP networks.

  • IEEE 802.3: Uses Length field (indicates data size) + 802.2 LLC/SNAP header for protocol identification. Rarely used in modern IP networks.

[!TIP] Exam Focus: Know the 6 MAC address fields in order. Distinguish Type (Ethernet II) from Length (802.3). FCS is computed by sender and checked by receiver.

2.1.2 Switch Operation & MAC Address Table Management

• Switching Methods:

  • Store-and-Forward: Receives entire frame, checks FCS, then forwards. Error-checking, higher latency.

  • Cut-Through: Forwards after reading Destination MAC only. Lower latency, no error-checking.

• MAC Address Table (CAM Table): Built dynamically.

  • Learning: Switch records Source MAC and incoming port.

  • Aging: Entries deleted after ~5 minutes of inactivity (default).

• Frame Forwarding Decision:

  1. If Destination MAC in table: Forward out associated port (unicast).

  2. If Destination MAC unknown/broadcast/multicast: Flood out all ports in same VLAN except incoming port.

  3. If Destination MAC on same port as incoming: Filter (do not forward).

[!TIP] Common Pitfall: A switch floods unknown unicast frames, not just broadcasts. This is a key behavior.

2.1.3 Basic Switch CLI Configuration (Cisco)

Switch> enable

Switch# configure terminal

Switch(config)# hostname SW1

SW1(config)# line console 0

SW1(config-line)# password cisco

SW1(config-line)# login

SW1(config-line)# exit

SW1(config)# line vty 0 15

SW1(config-line)# password cisco

SW1(config-line)# login

SW1(config-line)# exit

SW1(config)# banner motd #Unauthorized Access Prohibited#

SW1(config)# interface vlan 1

SW1(config-if)# ip address 192.168.1.10 255.255.255.0

SW1(config-if)# no shutdown

SW1(config-if)# exit

SW1(config)# ip default-gateway 192.168.1.1  ! For Layer 2 switch management

SW1(config)# end

SW1# copy running-config startup-config

[!TIP] Critical: no shutdown enables an interface. ip default-gateway is needed on Layer 2 switches for remote management; Layer 3 switches use SVIs with routing.

2.2 VLANs (Virtual LANs) & Trunking

2.2.1 VLAN Concept & Benefits

• Definition: A VLAN is a logical broadcast domain. Ports in same VLAN act as if connected to same switch.

• Benefits:

  • Security: Segment sensitive departments (e.g., Finance VLAN).

  • Broadcast Control: Reduce broadcast traffic size.

  • Flexibility: Group users by function, not physical location.

• VLAN ID Range:

  • Normal Range (1-1005): Stored in vlan.dat file. VLAN 1 is default (cannot be deleted).

  • Extended Range (1006-4094): Supported in VTP transparent mode only.

2.2.2 Access Port Configuration

SW1(config)# interface fastEthernet 0/1

SW1(config-if)# switchport mode access      ! Force access mode

SW1(config-if)# switchport access vlan 10   ! Assign to VLAN 10

[!TIP] Native VLAN: On a trunk, frames for the Native VLAN are sent untagged. Default is VLAN 1. Change for security.

2.2.3 Trunking Protocols: IEEE 802.1Q

• Tagging Mechanism: Inserts 4-byte header between Source MAC and Type fields.

  • TPID (Tag Protocol Identifier): 0x8100 (identifies 802.1Q frame).

  • TCI (Tag Control Information):

    • PCP (Priority Code Point): 3 bits for QoS.

    • DEI (Drop Eligible Indicator): 1 bit (formerly CFI).

    • VLAN ID: 12 bits (1-4094).

• Trunk Configuration:

SW1(config)# interface fastEthernet 0/24

SW1(config-if)# switchport mode trunk

SW1(config-if)# switchport trunk allowed vlan 10,20,30  ! Restrict allowed VLANs

• DTP (Dynamic Trunking Protocol) Modes:

  | Mode | Behavior | | :--- | :--- | | access | Never trunk (force access) | | trunk | Always trunk (force trunk) | | dynamic desirable | Actively attempt to form trunk | | dynamic auto | Passively wait for other side to request trunk | | nonegotiate | Disable DTP (use with non-Cisco devices) |

[!TIP] Security Best Practice: Set trunk ports to trunk or nonegotiate and explicitly list allowed VLANs (switchport trunk allowed vlan ...). Avoid dynamic auto as it can form unintended trunks.

2.2.4 VLAN Trunking Lab Verification

SW1# show vlan brief          ! Lists VLANs and their member ports

SW1# show interfaces trunk    ! Shows trunk status, native VLAN, allowed VLANs

SW1# show interfaces fastEthernet 0/24 switchport  ! Detailed port mode info

2.3 IPv4 Subnetting & IP Addressing on Network Devices

2.3.1 Subnet Mask Interpretation & Custom Subnet Masks

• Notations: 255.255.255.0 = /24 = 11111111.11111111.11111111.00000000

• Key Formulas:

  • Number of Subnets: $$\displaystyle 2^n $$ (where n = bits borrowed from host portion). Note: Modern practice counts all-zeros and all-ones subnets.

  • Hosts per Subnet: $$\displaystyle 2^h - 2 $$ (where h = remaining host bits). Subtract 2 for network & broadcast addresses.

  • Subnet Increment: $$\displaystyle 2^{(number\ of\ host\ bits)} $$ in the octet where subnetting occurs.

• Example: Subnet 192.168.1.0/24 into subnets with min. 20 hosts.

  1. Need 20 hosts → $$\displaystyle 2^h - 2 \geq 20 $$ → $$\displaystyle h=5 $$ (since $$\displaystyle 2^5-2=30 $$).

  2. Original host bits = 8. Borrow $$\displaystyle 8-5=3 $$ bits for subnetting.

  3. New mask: /27 = 255.255.255.224.

  4. Subnet increment = $$\displaystyle 2^5 = 32 $$ in 4th octet.

  5. Subnets: 192.168.1.0/27, 192.168.1.32/27, 192.168.1.64/27, etc.

[!TIP] Step-by-Step Subnetting: 1) Determine required hosts → find h. 2) Calculate new prefix (/). 3) Find increment. 4) List subnets. Always draw the subnet block!

2.3.2 Configuring IP Addresses

• On Host (Windows CLI): ipconfig /all to view. Static via GUI or netsh.

• On Router Interface:

R1(config)# interface gigabitEthernet 0/0

R1(config-if)# ip address 192.168.10.1 255.255.255.0

R1(config-if)# no shutdown

• On Switch (SVI - Switched Virtual Interface): For management or Layer 3 switch routing.

SW1(config)# interface vlan 10

SW1(config-if)# ip address 192.168.10.10 255.255.255.0

SW1(config-if)# no shutdown

2.3.3 Practical Subnetting Lab Documentation Template

2.4 ARP, ICMP, and Basic Network Troubleshooting Tools

2.4.1 Address Resolution Protocol (ARP)

• Purpose: Resolves known IPv4 address to unknown MAC address on local network.

• Process:

  1. Host A (IP_A, MAC_A) wants to send to IP_B on same subnet.

  2. ARP Request: Broadcast (FF:FF:FF:FF:FF:FF), "Who has IP_B? Tell IP_A."

  3. ARP Reply: Host B unicasts to MAC_A: "IP_B is at MAC_B."

  4. Host A caches mapping (IP_B, MAC_B) in ARP table (typically 2-10 min).

• Commands:

  • Windows: arp -a (view), arp -d * (delete).

  • Cisco: show arp or show ip arp.

[!TIP] ARP Spoofing: Malicious host sends fake ARP replies to poison cache, enabling man-in-the-middle attacks. Use arp -a to check for duplicate IP-MAC pairs.

2.4.2 Internet Control Message Protocol (ICMP)

• Key Messages for Troubleshooting:

  • Echo Request (Type 8) / Echo Reply (Type 0): Used by ping.

  • Destination Unreachable (Type 3): Sent by router when no route or port unreachable.

  • Time Exceeded (Type 11): Sent by router when TTL reaches 0 (used by traceroute).

• Tools:

  • ping <IP>: Tests L3 reachability. Measures RTT (Round-Trip Time).

  • tracert <IP> (Windows) / traceroute <IP> (Linux/Cisco): Shows path and RTT to each hop. Identifies routing loops or failures.

2.4.3 Command-Line Troubleshooting Utilities

[!TIP] Systematic Troubleshooting: Start local (ipconfig), then same subnet (ping gateway), then remote (ping remote IP). Use tracert if remote ping fails. Check ARP cache if same-subnet ping fails.

2.5 Introduction to Static Routing & Router Configuration

2.5.1 Router Fundamentals

• Role: Connects different broadcast domains (subnets/VLANs). Operates at Layer 3.

• Default Gateway: The router interface IP address that hosts use to send traffic outside their subnet.

2.5.2 Configuring Static Routes

• Syntax:

  
  R1(config)# ip route <destination-network> <subnet-mask> <next-hop-IP>
  
  R1(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2
  
  

  • Or using exit-interface (only for point-to-point links):

  
  R1(config)# ip route 192.168.2.0 255.255.255.0 gigabitEthernet 0/1
  
  

• Administrative Distance (AD): Static routes have AD = 1 (very trustworthy). Directly connected interfaces have AD = 0.

[!TIP] When to use next-hop vs. exit-interface: Use next-hop IP for multi-access networks (like Ethernet). Use exit-interface for point-to-point serial links. Using exit-interface on Ethernet can cause routing issues if next-hop is down but interface is up.

2.5.3 Basic Router CLI Configuration

R1> enable

R1# configure terminal

R1(config)# hostname BranchRouter

BranchRouter(config)# banner login #Authorized Personnel Only#

BranchRouter(config)# enable secret cisco123   ! Encrypted

BranchRouter(config)# line console 0

BranchRouter(config-line)# password cisco

BranchRouter(config-line)# login

BranchRouter(config-line)# exit

BranchRouter(config)# line vty 0 4

BranchRouter(config-line)# password cisco

BranchRouter(config-line)# login

BranchRouter(config-line)# exit

BranchRouter(config)# interface gigabitEthernet 0/0

BranchRouter(config-if)# ip address 10.0.0.1 255.255.255.252

BranchRouter(config-if)# no shutdown

BranchRouter(config-if)# exit

BranchRouter(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

BranchRouter(config)# end

BranchRouter# show ip route          ! Verify static route (marked 'S')

BranchRouter# show ip interface brief ! Verify interface IP and status

BranchRouter# copy running-config startup-config

2.6 Network Analysis with Wireshark / Packet Tracer Simulation

2.6.1 Capturing and Filtering Traffic

• Start Capture: Select interface → click shark fin.

• Stop Capture: Click red square.

• Essential Display Filters:

  • eth.addr == 00:11:22:33:44:55 (specific MAC)

  • ip.addr == 192.168.1.1 (specific IP)

  • arp (all ARP packets)

  • icmp (all ICMP packets)

  • vlan (all 802.1Q tagged frames)

  • eth.type == 0x0800 (IPv4 frames)

  • eth.type == 0x0806 (ARP frames)

2.6.2 Analyzing Key Protocols in Unit 2

[!TIP] Packet Tracer Simulation Mode: Use "Edit Filters" to simulate Wireshark filters. Click a packet to see "Outbound" and "Inbound" PDU details layer-by-layer.

2.7 Integrated Lab Scenarios (Synthesis of Topics)

2.7.1 Multi-VLAN, Multi-Subnet Lab with Inter-VLAN Routing

Scenario: Two switches (SW1, SW2), one router (R1). VLANs 10 (Sales) and 20 (Engineering). Hosts in each VLAN on different switches. Steps:

1. Design Subnets: e.g., 192.168.10.0/24 for VLAN10, 192.168.20.0/24 for VLAN20.

2. Configure VLANs on both switches:

   
   SW1(config)# vlan 10
   
   SW1(config-vlan)# name Sales
   
   SW1(config)# vlan 20
   
   SW1(config-vlan)# name Engineering
   
   

3. Assign Access Ports: Connect host ports to correct VLANs (switchport access vlan 10).

4. Configure Trunk between SW1 & SW2:

   
   SW1(config-if)# switchport mode trunk
   
   SW1(config-if)# switchport trunk allowed vlan 10,20
   
   

5. Router-on-a-Stick (R1): Create sub-interfaces for each VLAN.

   
   R1(config)# interface gigabitEthernet 0/0.10
   
   R1(config-subif)# encapsulation dot1Q 10
   
   R1(config-subif)# ip address 192.168.10.1 255.255.255.0
   
   R1(config)# interface gigabitEthernet 0/0.20
   
   R1(config-subif)# encapsulation dot1Q 20
   
   R1(config-subif)# ip address 192.168.20.1 255.255.255.0
   
   R1(config)# interface gigabitEthernet 0/0
   
   R1(config-if)# no shutdown
   
   

6. Configure Hosts: Set IPs in correct subnet, default gateway = router sub-interface IP (e.g., 192.168.10.1 for VLAN10).

7. Verification:

   • show vlan brief on switches (check trunk port shows VLANs).

   • show interfaces trunk (check allowed VLANs).

   • show ip route on router (should see directly connected routes for each sub-interface).

   • ping from host in VLAN10 to host in VLAN20 → should succeed.

   • show arp on router → should see MACs for both VLANs.

2.7.2 Basic Network Troubleshooting Scenario

Symptom: Host A (VLAN10, 192.168.10.10/24) cannot ping Host B (VLAN20, 192.168.20.20/24). Systematic Diagnosis:

1. Host A Local Config: ipconfig → Correct IP/mask/gateway (192.168.10.1)? ✅

2. Same-Subnet Connectivity: Ping Host A's default gateway (192.168.10.1). Fails?

   • Check switch port mode (show interfaces fa0/1 switchport). Should be access vlan 10.

   • Check VLAN membership (show vlan brief). Is port in VLAN 10?

   • Check trunk between switches (show interfaces trunk). Is VLAN 10 allowed?

   • Check router sub-interface config (show running-config interface g0/0.10). Correct encapsulation dot1Q 10 and IP?

3. If gateway ping succeeds: Issue is with VLAN20 or router config.

   • Repeat steps for Host B's side.

   • On router: show ip route → Are both 192.168.10.0/24 and 192.168.20.0/24 listed as connected (C)? If not, sub-interface is down (no shutdown missing?).

4. Use Wireshark/Packet Tracer Simulation:

   • Capture on Host A. Does it send ARP for gateway? If no ARP request, gateway IP is wrong or same subnet.

   • Does ARP reply come? If not, check physical connectivity and switch VLAN.

   • Does ping packet leave Host A with VLAN 10 tag? (In simulation, check outbound PDU).

   • At router: Does sub-interface g0/0.10 receive the tagged frame? Does it strip tag and route?

   • Does router send packet out g0/0.20 with VLAN 20 tag?

   • Does Host B receive it? Check Host B's ARP for router's VLAN20 IP.

2.8 (Optional/Advanced) Spanning Tree Protocol (STP) Basics

2.8.1 Purpose of STP

• Prevents Layer 2 Loops in redundant switched topologies.

• Mechanism: Elects a Root Bridge. All other switches determine a single Root Port (best path to root) and Designated Port (forwarding port for each segment). All other ports become Blocking Ports (no forwarding, but listen for BPDUs).

2.8.2 Observing STP in Packet Tracer

• BPDU (Bridge Protocol Data Unit) Types:

  • Configuration BPDU: Contains Bridge ID, Root ID, path cost. Sent every 2 seconds by Root.

  • Topology Change (TCN) BPDU: Notifies of topology change.

• Port States: Blocking → Listening → Learning → Forwarding (default 30-50 sec convergence).

• Commands:

  
  SW1# show spanning-tree
  
  SW1# show spanning-tree summary
  
  SW1# show spanning-tree interface fastEthernet 0/1 detail  ! See port role/state
  
  

• In Simulation Mode: Filter for stp. Observe BPDUs flowing from Root Bridge. Non-root bridges forward superior BPDUs toward root.

[!TIP] Root Bridge Election: Lowest Bridge ID wins. Bridge ID = Priority (default 32768) + MAC address. To force a switch as root, set lower priority: spanning-tree vlan 1 priority 4096.