Understanding VLANs and Spanning Tree Protocol in Networking

Posted on Sep 4, 2024 in Computers

Benefits of VLANs with Switches

Administrative Advantages

**Virtual LANs based on Diverse Criteria:** Create VLANs based on workgroup, protocol, or service, not just physical location.
**Simplified Device Moves:** Easily move devices to new VLANs by modifying port assignments.
**Controlled Broadcast Traffic and Collision Domains:** Manage broadcast traffic and create collision domains based on logical criteria.
**Enhanced Security:** Isolate traffic within a VLANs for improved security.
**Network Traffic Load Balancing:** Distribute traffic logically rather than physically for optimized performance.

Advantages Over Routers

**Easier Administration:** Switches are simpler to manage compared to routers.
**Cost-Effectiveness:** Switches are generally less expensive than routers.
**Higher Performance:** Switches introduce less latency, resulting in better performance.

When Routers Are Still Necessary

**WAN Traffic Filtering:** Routers are essential for filtering traffic on Wide Area Networks.
**Inter-Network Routing:** Routers are required for routing traffic between separate networks.
**Inter-VLAN Routing:** Routers are needed to route packets between different VLANs.

VLAN Configuration Commands

Task	Command(s)
Define a VLAN (Optional name; must be unique)	switch(config)#vlan <1-4094> switch(config-vlan)#name WORD1-4094
Delete a VLAN (Ports become inactive and need reassignment)	switch(config)#no vlan <1-4094>
Assign Ports to a VLAN (VLAN created automatically if it doesn’t exist)	switch(config-if)#switchport access vlan <1-4094>
Show VLAN List	switch#show vlan switch#show vlan brief
Show Specific VLAN Information	switch#show vlan id <1-4064>

VLAN Configuration Example

Creating VLAN 12 named IS_VLAN and assigning port 0/12:

switch#config t
switch(config)#vlan 12
switch(config-vlan)#name IS_VLAN
switch(config-vlan)#interface fast 0/12
switch(config-if)#switchport access vlan 12

Spanning Tree Protocol: An Overview

Benefits of Spanning Tree Algorithm

**Elimination of Bridging Loops:** Prevents network loops for stable connectivity.
**Redundant Paths:** Provides backup paths for increased network resilience.
**Dynamic Role Configuration:** Automatically assigns roles to switches for efficient operation.
**Automatic Recovery:** Recovers from topology changes or device failures without manual intervention.
**Optimal Path Identification:** Determines the best path between network devices.

Spanning Tree Roles

Role	Characteristics
Root Bridge	The master bridge; single in the network; determined by lowest Bridge ID (BID); broadcasts configuration messages; all ports forward messages.
Designated Bridge	Forwards packets; selected automatically; only one per segment to prevent loops.
Backup Bridge	Listens and builds bridge database but doesn’t forward; takes over if root or designated bridge fails.

Spanning Tree Port States

Port State	Description
Disabled	Powered on but inactive; doesn’t participate in network communication.
Blocking	Initial state; receives packets but doesn’t process them; backup bridge ports are always in this state.
Listening	Transition state; allows network to stabilize after changes.
Learning	Receives packets and builds bridge database (MAC address to port mapping).
Forwarding	Root and designated bridges; can receive and forward packets.

Spanning Tree Port Types

Port Type	Description
Root Port	Designated switch port with lowest cost to root bridge; single per designated switch; always forwarding.
Designated Port	Single port per segment; allows sending and receiving frames; selected based on lowest cost to root bridge; always forwarding.
Blocking Port	Any port that’s not a root or designated port; prevents loops; always blocking.

Spanning Tree Port Selection Criteria

**Lowest Cost to Root Bridge:** Determines root/designated port based on path cost (e.g., 10 Mbps = 1000, 100 Mbps = 19, 1 Gbps = 4, 10 Gbps = 2).

If costs are equal, bridge ID (priority and MAC address) is compared. Lowest bridge ID wins. If bridge IDs are equal, lowest MAC address wins. If two ports on a switch have the same cost, the port with the lowest port ID (priority and port number) becomes the designated port.

Spanning Tree Enhancements

**Port Fast:** Allows ports without connected switches to immediately transition to forwarding state, preventing unnecessary delays.
**Uplink Fast:** Enables quick recovery by using an alternate path if the root port or link fails.