Interfaces and Ports
Networking vendors use confusing terminology to describe the objects you use to connect networking devices to the outside world. They call them interfaces or ports, but they also call internal structures VLAN interfaces or subinterfaces.
This series of blog posts is trying to bring some order into that confusion.
- Router Interfaces and Switch Ports
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When I started implementing the netlab VLAN module, I encountered (at least) three different ways of configuring physical interfaces and bridging domains even though the underlying packet forwarding operations (and sometimes even the forwarding hardware) are the same. That confusopoly is guaranteed to make your head spin for years, and the only way to figure out what’s going on behind the scenes is to go back to the fundamentals.
- How Routers Became Bridges
- Network terminology was easy in the 1980s: bridges forwarded frames between Ethernet segments based on MAC addresses, and routers forwarded network layer packets between network segments. That nirvana couldn’t last long; eventually, a big enough customer told Cisco: “I don’t want to buy another box if I already have your too-expensive router. I want your router to be a bridge.”
- VLAN Interfaces and Subinterfaces
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Early bridges implemented a single bridging domain across all ports. Within a few years, we got multiple bridging domains within a single device (including bridging implementation in Cisco IOS). The capability to have multiple bridging domains stretched across several devices was still missing… until the modern-day Pandora opened the VLAN box and forever swamped us in the complexities of large-scale bridging.
- Routed Interfaces on Layer-3 Switches and Internal VLANs
- When configuring layer-3 switches, we pretend that some interfaces connect to the internal bridge (the switch ports) while other interfaces connect to the internal router (the routed interfaces). However, that’s not how the switch ASICs work; most switches have to do behind-the-scene magic to implement the ports+interfaces configuration model.