Describing the Campus Infrastructure Module
Routed network issues

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.