Foundation Summary

MPLS provides a Layer 3 WAN alternative to traditional Layer 2 WAN technologies. It allows a secure, dynamic extension of an enterprise network across a service provider network. It also provides the network team in charge of the enterprise network some control over traffic classifications and prioritizations. This allows for preferential treatment of critical and timesensitive traffic over the WAN. Table 8-2 provides a brief review of traditional WAN topologies.

Table 8-2 Traditional WAN Topologies





Low-cost connectivity to all sites

Single point of failure at hub site can impact network service dramatically

Partial mesh

Moderate cost balanced with some redundancy in connectivity

Potential for significant service impact due to outages at key sites

Full mesh

Fully redundant; no site dependent on any other for connectivity

High cost

Redundant hub-and-spoke

More redundant than traditional hub-and-spoke with moderate incremental cost

Like a partial mesh, there is significant potential for service impact with the loss of key sites

Table 8-3 provides a brief review of the switching mechanisms in Cisco IOS Software.

Table 8-3 Cisco IOS Switching Mechanisms

Switching Mechanism



Process switching

Recursive routing lookup. Up-to-date information at all times.

Slow and inefficient repetition of lookups.

Fast switching (a.k.a cache-driven)

Interrupt code driven and significantly faster than process switching.

First packet is process switched. Difficulty with load balancing.

CEF switching (a.k.a topology-driven)

Full load balancing capable on per-packet basis or based on source address, destination address, or other characteristics.

High memory and CPU utilization. Should not be enabled on routers with insufficient horsepower.

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