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4.2. Hybrid L2/L3 Designs

4.2. Hybrid L2/L3 Designs​

Operators have sought to limit the impact of data-plane faults and build large-scale topologies through implementing routing protocols in either the Tier 1 or Tier 2 parts of the network and dividing the Layer 2 domain into numerous, smaller domains. This design has allowed data centers to scale up, but at the cost of complexity in managing multiple network protocols. For the following reasons, operators have retained Layer 2 in either the access (Tier 3) or both access and aggregation (Tier 3 and Tier 2) parts of the network:

  • Supporting legacy applications that may require direct Layer 2 adjacency or use non-IP protocols.

  • Seamless mobility for virtual machines that require the preservation of IP addresses when a virtual machine moves to a different Tier 3 switch.

  • Simplified IP addressing = less IP subnets are required for the data center.

  • Application load balancing may require direct Layer 2 reachability to perform certain functions such as Layer 2 Direct Server Return (DSR). See [L3DSR].

  • Continued CAPEX differences between L2- and L3-capable switches.

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