Network Troubleshooting and Configuration Guide

VLAN Configuration and Troubleshooting

Common VLAN Issues and Solutions

VLAN Mismatch

Problem: RTA uses the same sub-interface for VLAN 20 and VLAN 30.

Solution: Ensure each VLAN is assigned a unique sub-interface.

Adding VLANs

Problem: How to add VLAN 10?

Solution: Switch1(config)# vlan 10

Incorrect VLAN Tagging

Problem: The dot1Q encapsulation command contains the wrong VLAN for PC1 and PC2.

Solution: Verify and correct the VLAN tag in the dot1Q command.

Trunk Mode Verification

Problem: Network admin verification: Gi1/1 is configured as trunk mode, but Gi1/1 is in the default VLAN.

Solution: Ensure Gi1/1 is correctly configured for trunk mode and assigned to the appropriate VLANs.

Inter-VLAN Communication

Problem: Communication between VLANs is not working.

Solution: Verify that the switch port Gi1/1 is in trunk link mode.

IP Address Configuration

Problem: Incorrect IP address configuration on GigabitEthernet0/0.30 for VLANs 15, 30, and 45.

Solution: Verify and correct the IP address configuration for the affected VLANs.

VLAN Trunk Configuration

Problem: Configuring VLANs on Gi0/0.

Solution: switchport mode trunk

Missing VLANs on Trunk

Problem: No VLAN number is added to the trunk in this design for VLAN 5.

Solution: Add the required VLANs to the trunk configuration.

Sub-interface IP Address Mismatch

Problem: The IP address on the sub-interface is incorrectly paired with the VLAN for RT1, Host1, Host2, and Host3.

Solution: Verify and correct the IP address and VLAN mapping on the sub-interfaces.

Routing Configuration and Troubleshooting

Default and Static Routes

Default Route Configuration

Problem: How to configure connectivity between PC A and PC B?

Solution: Configure one default route from R1 to ISP and one static route on ISP to R1.

ISP Route Configuration

Problem: ISP network engineer needs to configure a default route.

Solution: ip route 0.0.0.0 0.0.0.0 s0/0/0 and ip route 0.0.0.0 0.0.0.0 s0/1/0

Static Route Configuration

Problem: How to configure a static route for 10.0.0.0/8?

Solution: A(config)# ip route 10.0.0.0 255.0.0.0 192.168.1.2 and remove the incorrect route: A(config)# no ip route 10.0.0.0 255.0.0.0 172.16.40.2

Border Router Configuration

Problem: Network admin needs to configure a default route on the border router.

Solution: Border(config)# ip route 0.0.0.0 0.0.0.0 s0/0/1

Inter-LAN Routing

Problem: How to route from LAN A to LAN C via Router A?

Solution: A(config)# ip route 192.168.4.0 255.255.255.0 192.168.3.2

Static Route to R2 LAN

Problem: R1 cannot reach R2's LAN.

Solution: R1 needs a static route to R2's LAN.

Dynamic Routing Protocols

OSPF Configuration and Troubleshooting

OSPF Neighbors

Problem: Identifying OSPF neighbors in Net B.

Solution: R1, R3, R5, and R7 are the OSPF neighbors.

RIP Configuration

Problem: Analyzing show ip route output for RIP.

Solution: RIP version 2 is running on this router and its RIP neighbor. The no auto-summary command has been used on the neighboring RIP router.

Interface Identification

Problem: Which interface is 172.16.0.66 on?

Solution: Serial 0/0/1

Dynamic Routing Protocol Comparison

Link-State Protocols

• OSPF
• IS-IS

Distance-Vector Protocols

• EIGRP
• RIPv2

Dynamic Routing

• Automatically adds new networks to the routing table.
• Best option for large networks.

Static Routing

• Can overload routing.
• Generally used in internet routing networks.

OSPF Commands and Concepts

OSPF Commands

• show ip ospf neighbor: Displays neighbor ID of 3.3.3.3.
• show ip ospf: Displays routing process "ospf 10" with ID 1.1.1.1.
• show ip protocols: Displays routing protocol as "ospf 10".
• show running-config: Displays bandwidth (BW) 1544 Kbit/sec.

OSPF Router ID Selection

1. Configured router ID 1.1.1.1
2. Loopback interface IP address 172.16.1.1
3. Loopback interface address 10.1.1.1
4. Serial interface IP address 192.168.10.1

OSPF Terminology

• DUAL (Diffusing Update Algorithm): The algorithm used by OSPF (Shortest Path First).
• Adjacency Database: Contains details of neighboring routers.
• Link-State Database: Contains the topology table.
• Single-Area OSPF: All routers are in the backbone area.

Routing Protocol Metrics

• RIP: Hop count
• EIGRP: Bandwidth and delay
• OSPF: Bandwidth (cost)

Additional Notes

RIP Version

The version 2 command is used.

DR/BDR Election

DR and BDR election occurs in multi-access networks.

OSPF Metric

The OSPF metric is cost.