HON’s Wiki # Network Architecture

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Contents

• Topologies
  • Flat
  • Three-layer Hierarchical Model
  • Collapsed Core
  • Collapsed Distribution
  • Spine-Leaf
• Terms
• Miscellanea

Topologies

Flat

• A single L2 domain/layer.
• Switching, routing and firewalling is all done by the same device or device stack, with clients directly connected.

Three-layer Hierarchical Model

• Appropriate for large networks spanning multiple regions (e.g. multiple buildings).
• Scales well.
• Focuses on north-south traffic.
• Consists of three layers.
• Access layer:
  • L2 switches.
  • Connected to clients.
  • Typically one access-layer VLAN spans one or a few access switches.
  • Should implement first-hop security (MAC, IPv4, IPv6).
  • Connected upstream to distribution switches.
• Distribution layer:
  • Aka “distro” layer.
  • L3 switches.
  • Terminates access-layer VLANs.
  • Implements features like filtering and QoS.
  • May manage individual WAN connections.
  • Connected upstream to core routers and ptionally interconnected with other distribution switches.
• Core layer:
  • Routers.
  • Provides a backbone between distribution regions and toward external networks.
  • Focuses entirely on high bandwidth, low latency, high reliability and high resilience.
  • Avoids anything that may slow down traffic, like access lists, policy enforcement, etc.
  • It’s possible to connect multiple core routers and distribution switches by using a switch.

Collapsed Core

• Similar to the three-layer hierarchical model, but with the core and distribution layers collapsed into the same devices.
• This generally means that there is only one routed layer.
• Distro layer devices may be interconnected directly or through one or more core switches (not routers) which are not themselves interconnected.
• Appropriate for medium/small sites without multiples regions, where a separate core network is not needed.

Collapsed Distribution

• Similar to the three-layer hierarchical model, but with the distribution and access layers collapsed.
• Generally not very useful.

Spine-Leaf

• A type of Clos network (non-hierarchical).
• Two or three layers: Leaf layer, spine layer and an optional super-spine layer (for larger networks).
• Leaf switches connect to every spine switch and not to any other leaf switches.
• Spine routers (or switches) are not connected to any other spine routers.
• Hosts connect only to leaf switches.
• All spine-leaf links are L3 (routed).
• Every pair of leaf switches are always two hops away from each other.
• Routers to external areas, firewalls and load balancers are added connected leaf switches called border leaves.
• Large spine-leaf networks may be broken into multiple networks where the spine rouers are connected to routers in the super-spine layer.
• Focuses on east-west traffic.
• Requires ECMP for optimal utilization.
• Well suited for data centers.
• Well suited for VXLAN for allowing hosts to move easily between leaf switches.

Terms

• Equal-cost multi-path routing (ECMP): Routing strategy for forwarding over multiple best paths to the same destination.
• Oversubscription: Less uplink capacity than downlink capacity. Appropriate when downstream devices rarely use the uplink simultaneously, allowing them to share the uplink capacity without sacrifice.

Miscellanea

• Q-in-Q (simple) or VXLAN (sophisticated) may be used to span VLANs over different areas.

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