Is the idea that a switch is a multiport bridge accurate?

Question

I think there are at least 3 differences:

1. switches don't allow multiple mac addresses belonging to each port
2. Switches don't use the fast spanning tree and use different management mechanism
3. switches connect nics to form a lan, and bridge connect lans to form a bridged lan.

Answer 1

I think there are at least 3 differences:

• switches don't allow multiple mac addresses belonging to each port
• Switches don't use the fast spanning tree and use different management mechanism
• switches connect nics to form a lan, and bridge connect lans to form a bridged lan.

If you want to know what a bridge is, ask the people who standardized them... IEEE. I'm including a screenshot from IEEE 802.1D-2004, page 29.

IEEE 802.1D outlines what IEEE defines as a bridge. Basically a bridge:

• Relays and filters frames
• Maintains tables for filtering / relaying frames (the relay function is commonly accomplished via STP and mac-learning)
• Provides code for management of the functions above

Responding point-by-point...

• switches don't allow multiple mac addresses belonging to each port

Mac learning is required by IEEE's definition of a bridge, so if a switch doesn't learn multiple macs per port (and even some Linksys / Netgears will), then it wouldn't qualify as a bridge

• Switches don't use the fast spanning tree and use different management mechanism

Perhaps it's important to clarify which switch you're speaking of. Even some Linksys / Netgear switches run rapid spanning-tree.

• switches connect nics to form a lan, and bridge connect lans to form a bridged lan.

There is no IEEE-802.1D requirement for a bridge to connect LANs (using your words, not even sure I agree with the distinction you're making). They can connect different LAN technologies (such as Token Ring to Ethernet), but that's not part of IEEE's requirement for a bridge.

Answer 2

Yes, a switch is a multiport bridge. They both do IEEE 802.1D bridging.

Just like a switch is a multiport bridge, a hub is a multiport repeater.

A repeater/hub is a relatively dumb device that more or less just amplifies the signal and retransmits it out all the other ports. It works at the bottom of layer 2, and layer 1.

A bridge/switch is a relatively smart device that looks at the destination MAC address of each frame, compares it to a table it keeps of which MAC addresses are out which ports, and forwards the frame only to the port it knows that MAC address is on. It works at the upper levels of layer 2.

Addressing your points:

1. Switches do indeed support multiple MAC addresses per port. Usually thousands. 4096 is a commonly-seen number.

2. 802.1D defines both basic bridging (only forwarding frames to the right ports) as well as more advanced bridge features like Spanning Tree Protocol (STP) for detecting and avoiding network loops. Later on, 802.1w came along and defined Rapid STP (RSTP). Bridges and switches are not required to support STP or RSTP. Most small (say, 5 to 16 port) non-manageable switches like you commonly find on home networks and inside individual offices/cubicles don't support STP or RSTP. But the bigger manageable switches you find in wiring closets and server racks definitely do.

3. Back in the 10BASE-T networks of the mid-1990's when bridging was expensive, you'd use hubs to connect NICs to form a LAN, and a bridge in between hubs when you needed some traffic separation. Once bridging became cheaper in the later 1990's, vendors built multiport bridges and wanted to call attention to the fact that they direct traffic rather than just being dumb repeaters, so they called them switches to differentiate them from hubs.

Answer 3

A traditional "bridge" just converts from one type of datalink-layer technology to another. You might, for example, see a Token Ring to Ethernet Bridge, or wireless bridges that convert from wired Ethernet to Wi-Fi.

Repeaters do this, without coverting the type of technology.

Since switches really don't covert from one type of technology to another, it may be more accurate to call them "multi-port repeaters" rather than bridges. Bridge isn't necessarily an inaccurate term though - another property of bridges is that they do not modify the traffic passing through, and switches have that property.

switches don't allow multiple mac addresses belonging to each port

This isn't necessarily true.

Switches don't use the fast spanning tree and use different management mechanism

This is incorrect. RSTP is a protocol that Cisco switches use.

switches connect nics to form a lan, and bridge connect lans to form a bridged lan.

You should think of a "LAN" as a set of systems on the same subnet, regardless of connecting hardware. The relation between physical and logical connections is often blurred due to VLANs, VPNs, etc.