A major limitation of a Layer 2 switch is that they cannot switch traffic
between Layer 3 network segments (IP subnets for example). Traditionally, this
was done using a router. Unlike switches, a router acts as a broadcast boundary
and does not forward broadcasts between its interfaces. Additionally, a router
provides for an optimal path determination process. The router examines each
incoming packet to determine which route the packet should take through the
network. Also, the router can act as a security device, manage quality of
service and apply network policy. Although routers used in conjunction with
Layer 2 switches resolve many issues, some concerns still remain:
- When security or traffic management components, such as access lists, are
configured on router interfaces, the network may experience delays as the
router processes each packet in software.
- When routers are introduced into a switched network, end-to-end VLANs are
no longer supported because routers terminate the VLAN.
- Routers are more expensive per interface than Layer 2 switches, so their
placement in the network should be well planned. Non-hierarchical networks by
their very nature require more interconnections and hence more routed
interfaces.
- In a non-hierarchical network, the number of router interconnections may
result in peering problems between neighboring routers.
- Because traffic flows are hard to determine, it becomes difficult to
predict where hardware upgrades are needed to mitigate traffic
bottlenecks.