Understanding Spanning Tree Protocol: Preventing Loops in LAN Networks, Slides of Network security

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Spanning Tree Protocol for

Bridges/Switches

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Bridge = Switch

• Bridge/switches are layer-2 switching device (look at the Ethernet destination address to decide how to forward a frame)
• Used to separate collision domains (segment)
  • Reduce collision probability
  • Increase aggregate bandwidth
  • Extend LAN’s length
• Can form a large and complicated network topology.

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A Topology Example

Bridge/switch

Router

Same subnet

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Bridge’s Features

• Listen promiscuously
• Store and forward packets (not cut-through)
• Learn where a packet should be forwarded to
• Use spanning tree algorithm to avoid loops

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Desired Improvements

• If the source and destination hosts are on

the same segment (port), the bridge need

not forward a received packet to all of other

segments (ports).

• If the source and destination hosts are on

different segments (ports) and the

destination host is on port i, the received

packet should be forwarded to port i only,

rather than all of other ports.

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Simple Method 1

• Humane type in addresses for each port and this database is keep at the bridge. (^) 8- 8

Simple Method 3

• Let layer 2 addresses embed layer-

addresses (LAN).

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Management headache

• All these simple methods cause management headache. - In method 1 and 2, the manager needs to key in many layer-2 address or range. - In method 2, the hosts need to be placed in a particular way. - In method 3, the hosts need to be able to configure its own layer-2 address. This may generate the same layer- 2 addresses by error.
• The best method is that a bridge can learn which host is on which port itself.

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Example 1

The bridge concludes that Q and A are on the same segment, therefore the Q to A packet can be discarded. 8- 16

Example 2

Bridge B1 cannot distinguish between hosts that are on LAN 2 and LAN 3 because B2 connect LAN 2 and LAN 3 transparently.

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Example 2

From B1’s point of view, the network topology looks like this. 8- 19

Example 2

From B2’s point of view, the network topology looks like this.

The learning bridge concept works for any tree topology! 8- 20