VLANs allow an administrator to segment networks based on factors such as function, project team, or application, without regard for the physical location of the user or device, as shown in the figure.
Devices within a VLAN act as if they are in their own independent network, even if they share a common infrastructure with other VLANs. Any switch port can belong to a VLAN.
Unicast, broadcast, and multicast packets are forwarded and flooded only to end devices within the VLAN where the packets are sourced. Each VLAN is considered a separate logical network.
Packets destined for devices that do not belong to the VLAN must be forwarded through a device that supports routing.
Multiple paths need to be managed so that Layer 2 loops are not created. The best paths are chosen, and an alternate path is immediately available should a primary path fail.
The Spanning Tree Protocol is used to maintain one loop-free path in the Layer 2 network, at any time.
Redundancy increases the availability of the network topology by protecting the network from a single point of failure, such as a failed network cable or switch.
When physical redundancy is introduced into a design, loops and duplicate frames occur. Loops and duplicate frames have severe consequences for a switched network. STP was developed to address these issues.
STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop.
A port is considered blocked when user data is prevented from entering or leaving that port. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops.
Blocking the redundant paths is critical to preventing loops on the network. The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring.
If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active.
Multilayer switches (also known as Layer 3 switches) not only perform Layer 2 switching but also forward frames based on Layer 3 and 4 information. All Cisco Catalyst multilayer switches support the following types of Layer 3 interfaces:
- Routed port – A pure Layer 3 interface similar to a physical interface on a Cisco IOS router.
- Switch virtual interface (SVI) – A virtual VLAN interface for inter-VLAN routing. In other words, SVIs are the virtual-routed VLAN interfaces.
A routed port is a physical port that acts similarly to an interface on a router, as shown in the figure. Unlike an access port, a routed port is not associated with a particular VLAN.
A routed port behaves like a regular router interface. Also, because Layer 2 functionality has been removed, Layer 2 protocols, such as STP, do not function on a routed interface.
However, some protocols, such as LACP and EtherChannel, do function at Layer 3. Unlike Cisco IOS routers, routed ports on a Cisco IOS switch do not support subinterfaces.
An SVI is a virtual interface that is configured within a multilayer switch, as shown in the figure. Unlike the basic Layer 2 switches discussed above, a multilayer switch can have multiple SVIs.
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