Can Layer 3 switching increase bandwidth?

Traditionally, routing improved network performance only in places where bandwidth was limited. For instance, routers would prioritize traffic on congested WAN interfaces to guarantee performance of more mission-critical applications. Routers have also been used to isolate traffic to certain network segments, thereby preventing Layer 2 broadcasts from flooding the network. With this in mind, is there any benefit to routing or Layer 3 switching in networks with high bandwidth - for example, Fast Ethernet and Gigabit Ethernet? Absolutely.

Recall that Layer 2 switched networks are limited to a single active path between two endstations as specified in IEEE 802.1 Spanning Tree. Of course, Spanning Tree prevents loops in the network, but it also prevents network traffic from load-sharing across multiple paths through the network. Layer 3 switching, by contrast, allows multiple traffic streams to follow multiple paths through the network.

Consider a network with three 100M bit/sec paths between two LANs. In a traditional Spanning Tree environment, one path would be active and two would be in standby (or "blocking") mode. Using Layer 3 switching in this environment could provide up to 300M bit/sec of aggregate throughput. In this case, Layer 3 switching delivers three times the previous bandwidth without changing any link topologies or speeds. (Of course, any given stream would be limited to 100M bit/sec since routers generally forward a traffic stream across only one path at a time.)

Compare this approach with link aggregation, which allows multiple links between neighboring switches or servers. While link aggregation allows multiple parallel links, or loops, in a Spanning Tree environment, those links can exist only between directly attached devices. Therefore, they cannot follow different paths through the network to converge several hops later. By contrast, Layer 3 switching allows just such multiple paths through the entire network, and traffic between two LANs may follow multiple routes across the net.

One of the main drawbacks to such Layer 3 switching - other than the cost compared to that of Layer 2 switching - is the obvious overhead in configuring and managing such paths. However, if such management enables additional bandwidth in the network - especially if that bandwidth is currently unused - then the benefits may significantly outweigh the drawbacks. Moreover, if one of the links fails and a path is no longer available, Layer 3 switching can redirect traffic around the failure just as Layer 2 switching would do through Spanning Tree. The difference is that, with Layer 3 switching, the backup paths are always used; whereas, with Layer 2 switching, they are idle until a network failure occurs.

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