How VLANs Affect the Broadcast Domain

Theory

By convention, a collision domain is a contiguous wired and / or wireless half-duplex segment (typically using CSMA/CD), which is a subset of the subnet or vlan's broadcast domain.

• Collision domains are invariably a half-duplex segment, or a series of connected half-duplex segments.
  • Ethernet hubs / Wifi Access Points (within the same RF-channel width) start or continue a collision domain
  • Ethernet switches, routers, firewalls all form a boundary on a collision domain.
• Broadcast domains could have multiple half-duplex segments inside them.

How Ethernet switches control Collision Domains

Ethernet switches control collision domains because they can join half-duplex and full-duplex links within the same broadcast domain; Ethernet switches buffer frames to assist the transition between half-duplex and full-duplex links.

Example:

This shows a strange network which illustrates how collision domains are a contained within a broadcast domain, and how devices form boundaries for each one.

• Yellow areas are the span of the broadcast domain
• Grey area are the span of the collision domains
• Firewalls and routers are assumed in routed mode (i.e. not bridged mode)

But I do not understand how exactly routers resolves broadcast storm problem. Any explanation in detail?

When a router receives a packet, it gets inspected, then forwarded out the appropriate interface or it gets dropped. When a router receives a broadcast packet, it drops it (excluding directed-broadcasts, dhcp, etc).

When a switch receives a frame, it either forwards it on to a known interface or floods it out all of its ports if it doesn't know where to go. When a broadcast frame comes along, it get's flooded out all interfaces. Every machine in your segment sees it. Excessive amounts of these constitute a storm.

The most common way for a broadcast storm to happen is from a switching loop. If you somehow get a switching loop on your network, these broadcasts will perpetually send this data back and forth forever, or until you remove the loop. This will cause data to hit every machine on your segment. This can cause your network to stop.

When you have a router in between multiple layer 2 segments, each is inherently protected from the other. Remember, a router won't forward on broadcasts.

For instance:

+-----+    +------+    +-----+
|LAN 1|----|ROUTER|----|LAN 2|
+-----+    +------+    +-----+

LAN 1 can be all sorts of messed up, and LAN 2 will be none the wiser because ROUTER won't forward LAN 1 broadcast packets on to anyone.