ICS 351: Today's plan

• Ethernet switches
• spanning tree protocols

Ethernet equipment

• much experience so far in lab with Ethernet
• different equipment used to connect Ethernet segments:
  • hubs: broadcast everything
  • switches: broadcast packets to 0xffffff, and packets for destinations that are not known, selectively transmit where possible
  • routers: forward packets among different IP networks
• hubs and switches work within a single IP network as a single broadcast medium (but switches don't always broadcast)
• traditional bridges had two interfaces, and forwarded everything from one interface to the other -- hubs and switches both implement this bridging function

Learning Switches

• if a switch gets a packet from A on interface I, it forwards the packet,
• and remembers that A can be reached on interface I
• the next time a packet for A is received on interface I', it is only forwarded on interface I (unless I == I', and then it is not forwarded)
• if there is no record of communication from A (within the last 60 seconds), the packet is broadcast on all interfaces except I'

Broadcast Storms

• given a network with redundant links
• if the network is connected by hubs, every packet will cause collisions with itself
• if the network is connected by switches, any broadcast packet will live forever
• packets may even be multiplied if there is more than one loop
• this is useless traffic that gets in the way of useful traffic -- a "broadcast storm"

Preventing broadcast storms

1. have no redundant links in the network, or
2. restrict "broadcast" forwarding by switches:
   • select a root switch, based on priority, using MAC addresses to break ties in case of equal priority
   • find a least-cost path to the root, reached via the root port for this switch
   • for each segment, determine a least-cost switch port to use to reach the root, the designated port for this segment
   • only forward broadcasts along root ports and designated ports

Rapid Spanning Tree Protocol, RSTP

• the regular spanning tree protocol can take tens of seconds to converge after a topology change
• instead, a switch can pre-select alternate ports that also lead to the root bridge
• broadcast data is only sent on alternate ports when it is determined that the root port is disconnected
• similarly for backup paths to individual segments
• switches also actively exchange their information, so one switch can quickly hand off forwarding to another switch