Network Redundancy: STP, EtherChannel, and Inter-VLAN Routing

Posted on Mar 18, 2025 in Computer Engineering

Network Redundancy Technologies

MAC Database Instability

  • If there is more than one path for a frame to be forwarded, an endless loop can result.

Broadcast Storms

  • Switches forward broadcast frames to all ports except the port the broadcast frame first entered.
  • If a Layer 2 loop is formed, broadcast frames are forwarded endlessly, creating a broadcast storm.

Issues with Layer 1 Redundancy: Duplicate Unicast Frames

  • Most upper-layer protocols are not designed to recognize or cope with duplicate transmissions.

Spanning Tree Protocol (STP)

Introduction to Spanning-Tree Algorithm

  • STP ensures only one logical path exists between all network destinations by intentionally blocking redundant paths that could cause a loop.

Spanning Tree Protocol List

  • STP or IEEE 802.1D-1998
  • PVST+
  • IEEE 802.1D-2004
  • Rapid Spanning Tree Protocol (RSTP)

PVST+ Network Characteristics

  • A network can run an independent STP instance for each VLAN.

STP Port States

  • Blocking
  • Listening
  • Learning
  • Forwarding
  • Disabled

In a PVST+ environment, the extended switch ID ensures each switch has a unique BID for each VLAN.

  • RSTP is the preferred protocol for preventing Layer 2 loops in a switched network environment.
  • UplinkFast and BackboneFast are not compatible with RSTP.

Repairing Spanning-Tree Problems

  • One way to correct spanning-tree failure is to manually remove redundant links.

First-Hop Redundancy Protocols

  • Hot Standby Router Protocol (HSRP)
  • Virtual Router Redundancy Protocol version 2 (VRRPv2)
  • Gateway Load Balancing Protocol (GLBP)

Link Aggregation (EtherChannel)

Introduction to Link Aggregation

  • Link aggregation allows the creation of logical links comprised of several physical links.

Advantages of EtherChannel

  • Most configurations are performed on the EtherChannel interface.
  • Relies on existing switch ports—no need for upgrades.
  • Load balances between links on the same EtherChannel.

Implementation Restrictions

  • EtherChannel can consist of up to 16 compatibly configured Ethernet ports.

Verifying EtherChannel

  • The show interface port-channel command displays the general status of the EtherChannel.
  • The show etherchannel summary command provides one line of information per port channel.
  • The show etherchannel port-channel command provides information about a specific port channel.

Inter-VLAN Routing

What is Inter-VLAN Routing?

  • Inter-VLAN routing is a process for forwarding network traffic from one VLAN to another using a router.

Router-on-a-Stick Inter-VLAN Routing

  • The router-on-a-stick approach uses a single physical interface to route between VLANs.
  • Only one of the router’s physical interfaces is used.

Multilayer Switch Inter-VLAN Routing

  • Multilayer switches can perform Layer 2 and Layer 3 functions, replacing the need for dedicated routers.
  • Multilayer switches support dynamic routing and inter-VLAN routing.
  • By default, an SVI is created for the default VLAN (VLAN 1). This allows for remote switch administration.
  • SVIs advantages include:

DHCPv4 Address Allocation Methods

DHCPv4 uses three different address allocation methods:

  • Manual Allocation – The administrator assigns a pre-allocated IPv4 address to the client, and DHCPv4 communicates only the IPv4 address to the device.
  • Automatic Allocation – DHCPv4 automatically assigns a static IPv4 address permanently to a device, selecting it from a pool of available addresses.
  • Dynamic Allocation – DHCPv4 dynamically assigns, or leases, an IPv4 address from a pool of addresses for a limited period of time chosen by the server, or until the client no longer needs the address.

Stateless Address Autoconfiguration (SLAAC)

Stateless Address Autoconfiguration (SLAAC) is a method in which a device can obtain an IPv6 global unicast address without the services of a DHCPv6 server.