1CS 640CS 640 Introduction to Computer NetworksLecture 19CS 640Today’s lecture• Ethernet bridging– The learning bridge– Spanning tree algorithm for bridges– VLANs (Virtual LANs)• Auxiliary protocols between layers 2 and 3–ARP– DHCPCS 640Why use Ethernet bridges?• Larger networks• More concurrent communication• Extend Ethernet without changing computersA B C DCollision!!!2CS 640Basic transparent bridgeA B C DFrom A To CFrom A To CFrom A To CBridgeCS 640Basic transparent bridgeA B C DFrom A To BFrom A To BFrom A To BBridgeCS 640Learning bridgeA B C DBridgeFrom A To CFrom A To CFrom A To CI known A is to the left3CS 640Learning bridgeA B C DBridgeFrom B To AFrom B To AI known A is to the leftI known B is to the leftCS 640Learning bridges• Initially forward frames onto all ports• Learn where hosts are based on source address• For known addresses only forward to right port• Multicast/broadcast go to all ports• Terminology– For twisted pair based Ethernet• Repeaters are called hubs• Bridges are called switches– A segment or collision domain has wires and hubs• LAN vs. extended LANCS 640What happens with cyclesA BC DBridge 1Bridge 2From A To XFrom A To X4CS 640Solution: eliminate cycles• On power up Ethernet bridges run a distributed spanning tree algorithm– Node with lowest ID is root– Spanning tree is the tree of shortest paths to root– Break ties based on bridge IDs• Ports that are not part of the spanning tree are turned off to data traffic– All cycles eliminatedCS 640Spanning tree algorithm details• Message format: (root, distance, bridgeID)• Each bridge stores best message for each port• Each bridge picks port closest to root• Best message is flooded with distance incremented• Each segment “elects” a designated bridge• The root repeats message• Messages are timed outACEDBKFHJGIB5B2B3B7B4B1B6CS 640Ethernet today• Limits to growth of bridged LANs– Traffic due to broadcast/multicast frames still goes to all segments– Throughput at root switch becomes bottleneck• Can have faster switches towards the middle• Bonus for switched Ethernet: better security– Hosts cannot snoop on others’ traffic– Today switches have advanced features (VLANs)5CS 640Today’s lecture• Ethernet bridging– The learning bridge– Spanning tree algorithm for bridges– VLANs (Virtual LANs)• Auxiliary protocols between layers 2 and 3–ARP– DHCPCS 640Why restrict reachability?• Security – multiple defenses– Sometimes you don’t want some computers to communicate with the outside world• Performance– Protect the performance of virtual networks from the effects of the rest of the traffic– VLANs cut down on broadcast traffic• And sharing infrastructure reduces costs!!!CS 640VLANs• Bridges never forward frames from one VLAN to the other• Each port is assigned to one VLAN• Some ports assigned to many VLANs– Must add VLAN ID to framesWXB1 B2YZVLAN 100VLAN 100VLAN 200VLAN 2006CS 640Today’s lecture• Ethernet bridging– The learning bridge– Spanning tree algorithm for bridges– VLANs (Virtual LANs)• Auxiliary protocols between layers 2 and 3–ARP– DHCPCS 640Address Translation• To build Ethernet frame carrying IP packet, sender needs to find out Ethernet address of destination• Cannot encode it in 32 bit IP address• ARP (Address Resolution Protocol)– Table of IP to MAC address bindings– Broadcast request if IP address not in table– Target machine responds with its MAC address– Table entries are discarded if not refreshedCS 640ARP Details • Request Format– HardwareType: type of physical network (e.g., Ethernet)– ProtocolType: type of higher layer protocol (e.g., IP)– HLEN & PLEN: length of physical and protocol addresses– Operation: request or response – Source/Target-Physical/Protocol addresses• Other details– Table entries timeout in about 10 minutes– Add entry to table with source of packets you receive • If entry exists, update/reset timer– Do not refresh table entries upon reference7CS 640ARP Packet FormatTargetHardwareAddr (bytes 2 – 5)TargetProtocolAddr (bytes 0 –3)SourceProtocolAddr (bytes 2 – 3)Hardware type = 1 ProtocolType = 0x0800SourceHardwareAddr (bytes 4 – 5)TargetHardwareAddr (bytes 0 – 1)SourceProtocolAddr (bytes 0 – 1)HLen = 48 PLen = 32 OperationSourceHardwareAddr (bytes 0 – 3)0 8 16 31CS 640Today’s lecture• Ethernet bridging– The learning bridge– Spanning tree algorithm for bridges– VLANs (Virtual LANs)• Auxiliary protocols between layers 2 and 3–ARP– DHCPCS 640Dynamic Configuration• Computers running IP needs some network configuration information (own address, router’s addr.)• Can store information in per host configuration file – Hard to manage– Does not help with laptops moving between WLANs• Dynamic configuration is the primary method for IP address allocation used today– IP address management centralized in DHCP server– No persistent configuration information stored on computers8CS 640Dynamic Host Configuration Protocol• Server keeps pool of available IPs for use on demand– Computers request address when booting– IP addresses leased (may renew to keep same IP address)• Administrator may assign permanent IP addresses to given hosts (as identified by Ethernet address)• DHCP also manages other IP-related configuration• The protocol relies on broadcast to find DHCP
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