Network end-user devices are commonly connected using switched technology rather than using a shared media segment. Switched technology provides dedicated network bandwidth for each device on the network. Switched networks can support network infrastructure services, such as QoS, security, and management; a shared media segment cannot support these features.

In the past, LAN switches were Layer 2–only devices. Data link layer (Layer 2) switching supports multiple simultaneous frame flows. Multilayer switching performs packet switching and several functions at Layer 3 and at higher Open Systems Interconnection (OSI) layers and can effectively replace routers in the LAN switched environment. Deciding whether to deploy pure data link layer switches or multilayer switches in the enterprise network is not a trivial decision. It requires a full understanding of the network topology and user demands. When deciding on the type of switch to use and the features to be deployed in a network, consider the following factors:

■ Infrastructure service capabilities: The network services that the organization requires (IP multicast, QoS, and so on).

■ Size of the network segments: How the network is segmented and how many end devices will be connected, based on traffic characteristics.

■ Convergence time: The maximum amount of time the network will be unavailable in the event of network outages.

■ Cost: The budget for the network infrastructure. Note that multilayer switches are typically more expensive than their Layer 2 counterparts; however, multilayer functionality can be obtained by adding cards and software to a modular Layer 2 switch. The following sections examine the following infrastructure characteristics: convergence time, multilayer switching and Cisco Express Forwarding, IP multicast, QoS, and load sharing.

Convergence Time

Loop-prevention mechanisms in a Layer 2 topology cause the Spanning Tree Protocol (STP) to take between 30 and 50 seconds to converge. To eliminate STP convergence issues in the Campus Core, all the links connecting core switches should be routed links, not VLAN trunks. This also limits the broadcast and failure domains. In the case where multilayer switching is deployed everywhere, convergence is within seconds (depending on the routing protocol implemented) because all the devices detect their connected link failure immediately and act on it promptly (sending respective routing updates).

Multilayer Switching and Cisco Express Forwarding

multilayer switching denotes a switch’s generic capability to use information at different protocol layers as part of the switching process; the term Layer 3 switching is a synonym for multilayer switching in this context.

The three major components of MLS are as follows:

■ MLS Route Processor (MLS-RP): The MLS-enabled router that performs the traditional function of routing between subnets

■ MLS Switching Engine (MLS-SE): The MLS-enabled switch that can offload some of the packet-switching functionality from the MLS-RP

■ Multilayer Switching Protocol (MLSP): Used by the MLS-RP and the MLS-SE to communicate with each other

Protocol-Independent Multicast Routing Protocol

Protocol-Independent Multicast (PIM) is used by routers that forward multicast packets. The protocol-independent part of the name indicates that PIM is independent of the unicast routing protocol (for example, EIGRP or OSPF) running in the network. PIM uses the normal routing table, populated by the unicast routing protocol, in its multicast routing calculations.

PIM operates in one of the following two modes:

■ Sparse mode: This mode uses a “pull” model to send multicast traffic. Sparse mode uses a shared tree and therefore requires an RP to be defined. Sources register with the RP. Routers along the path from active receivers that have explicitly requested to join a specific multicast group register to join that group. These routers calculate, using the unicast routing table, whether they have a better metric to the RP or to the source itself; they forward the join message to the device with the better metric.

■ Dense mode: This mode uses a “push” model that floods multicast traffic to the entire network. Dense mode uses source trees. Routers that have no need for the data (because they are not connected to receivers that want the data or to other routers that want it) request that the tree be pruned so that they no longer receive the data.