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  • 1. Hubs, Routers, and Switches B538 / Spring 2003

2. Hubs Operate at layer 2 of the network Data link layer Ethernet Main purposes Repeat traffic out all ports Media conversion inside a LAN Two-port version often called a repeater 3. A Hub in Action 4. A Hub in Action 5. A Hub in Action 6. Advantages of Hubs Easy to Understand Easy to Implement so theyre cheap 7. Disadvantages of Hubs Wasteful of bandwidth Why should host B have to share its link with a conversation between A and D? Poor security Why should host B get to share its link with a conversation between A and D? Packet sniffer on one port can monitor the traffic of all of the ports Cant have redundancy 8. Switches Also operate at layer 2 of the network Data link layer Ethernet Main purposes Repeat traffic out some ports Media conversion within a LAN Often called a bridge 9. Basic Switching Algorithm Maintain data structure called the switch forwarding table The forwarding table is indexed by MAC address and contains port numbers 10. Basic Switching Algorithm Packet arrives on port P with source S and destination D Set Fwd(S)=P If we have an entry Fwd(D) and Fwd(D)P, then send packet out Fwd(D) Otherwise, flood packet out all ports 11. A Switch in Action 12. A Switch in Action 13. A Switch in Action 14. A Switch in Action 15. A Switch in Action 16. A Switch in Action 17. A Switch in Action 18. A Switch in Action 19. A Switch in Action 20. Advantages of Switches Better use of bandwidth A lot of traffic is internal to a LAN We can reduce congestion by quite a bit Better for security Most traffic is delivered on a need to see basis This is not solid security Switches can be hijacked 21. Advantages of Switches Allow for redundant connections If all ports are active, redundant connections cause a loop and destroy life as we know it So switches run Spanning Tree Protocol (STP) to superimpose a tree of active switch connections onto the graph of actual connections Redundant connections are put in standby status 22. A Common Switch Problem The memory for switch forwarding tables is expensive Most of the entries are on the port that faces the rest of the network Why not save memory by making a network uplink port that doesnt learn any entries? This is a bad idea (but its been done) 23. A Common Switch Problem Host A is on the local LAN Host B is somewhere on the Internet Host A hasnt transmitted in a long time, so its not in the switch forwarding table Host B sends traffic to host A Because of the network uplink port, this packet is simply dropped Host A is invisible to the outside world! 24. Routers Operate at layer 3 of the network Network layer IP, IPX, Appletalk, etc. Main purposes To connect multiple networks together To enforce security policies 25. Internal Operation of Routers Basic job is to do longest-prefix matching against the routing table This is usually done in specialized hardware on the line cards themselves ASICs (Application-Specific Integrated Circuits) Only complex decisions (bad packets, TTL expired, etc.) are sent to the CPU 26. Longest-Prefix Match Most common method of doing longestprefix match is a radix tree Consider the network with a netmask of Today, we write this as Expressing this in binary, we get: 10011100 00111000 01100111 xxxxxxxx Use these bits to represent branches in a tree structure 27. Longest-Prefix Match 28. Longest-Prefix Match For speed, we usually use a higher radix (base) than 2 Other algorithms are a matter of current research Most of these trade space for efficiency This is one area where you could earn a lot of money very quickly 29. VLANs VLANs are virtual networks Traffic internal to one VLAN isnt visible to traffic on other VLANs even if its on the same switch Reduces the distribution of broadcast packets Enhances security within an enterprise 30. VLANs Defined by IEEE standard 802.1q Simply adds a new field to Ethernet frames containing a VLAN ID Range is 1 to 4,094 (12 bits) Range chosen to allow identified of tagged packets Older devices can have problems with tagged traffic 31. VLANs All VLANs give you is virtual hardware Theres nothing that a VLAN does that you cant do with more switches and routers This is really a way of more efficiently using resources, not a new technology

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