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Lecture 8

CSE 123 Lecture 8: L8 10/15/18

Computer Science and Engineering
Course Code
CSE 123
Alex Snoeren

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Spanning Tree Algorithm
- each bridge sends “periodic” configuration messages (speaks the same protocol)
- ALL nodes think they’re the root “initially”
- every bridge has a # when they’re manufactured (globally unique!)
- each bridge updates route/Root upon receipt
- smaller root address is better
- select port with lowest cost (bridge #) to root as “root port”
- to break TIES, bridge w/ smaller address is better
- join the tree in the direction that puts you CLOSER to the root!
- recruit other bridges by “advertising” your ports where you are BEST!
- rebroadcast new config to ports for which we’re “best”
- you are closer to the root than ANY of the messages you’ve been receiving on other
- don’t bother sending config to LANs w/ “better” options
- adds 1 to distance, send new configs on ports that haven’t told us about a shorter path
to the root
- you don’t forward any frames on a port on which you are not “best” (logically TURNED
Important Details
- what if root bridge FAILS?
- AGE configuration info
- if NOT refreshed for MaxAge seconds then “delete” root and recalculate spanning tree
- if config msg is received with a more “recent” age, then recalculate spanning tree
- applies to ALL bridges! (not just root)
- temporary loops
- don’t start forwarding packets “immediately” → wait some time for convergence
Switched Ethernet
- Hosts directly connected to a bridge (exclusive connection!)
- learning + spanning tree protocol
- wire is NEVER busy unless you’re using it!
- bridge where EVERY port connects DIRECTLY to a host!
- switch supports parallel forwarding
- A to B and A’ to B’ simultaneously
- generally full duplex as well (can go one way AND the other)
- switch backplane capacity “varies”
- ideally, non-blocking (can run at FULL line rate on ALL ports)
- need to be sending to each unique destination!
Ex: Ethernet switch connecting to 6 hosts
- Allows for 6 packets to be sent simultaneously (Why?)
- Each host can be SENDING AND RECEIVING
- BUT, each packet that comes out had to come in somewhere!
Layer-2 Forwarding
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