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