Link-layer switches. Jurassic Park* LANs with backbone hubs are good. LANs with backbone hubs are bad. Hubs, bridges, and switches

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1 Link-layer switches Jurassic Park* Hubs, bridges, and switches CS4 Cmputer Netwrks Department f Cmputer Science Wellesley Cllege *A multi-tier hub design. Switches 0- LANs with backbne hubs are gd. Prvide interdepartmental cmmunicatin.. Extend maximum distance between pairs f ndes. 3. Degrade gracefully. LANs with backbne hubs are bad. All LAN segments belng t the same cllisin dmain.. A hub may nt be able t intercnnect tw departments that use different Ethernet technlgies. 3. Limitatins f the varius Ethernet technlgies may cnstrain bth the ttal number f hsts as well as the gegraphical reach. Switches 0-3 Switches 0-4

2 Switches LANs with switches are gd Hubs eat bits; they are physical-level devices. Switches are link-level devices capable f full-fledge packet switching.. Switches permit interdepartmental cmmunicatin while preserving islated cllisin dmains.. Switches can intercnnect different LAN technlgies, including 0 Mbps, 00 Mbps, Cat 5, and fiber Ethernets. 3. There is n limit t hw large a LAN can grw when switches are used. The switch is transparent t the ndes; a nde addresses and sends a frame int the LAN unaware that a switch will be ding the dirty wrk. Switches 0-5 Switches 0-6 Switches filtering and frwarding Switch tables Filtering The ability t determine whether a frame shuld be frwarded t sme interface r simply drpped. Frwarding The ability t determine the interface t which a frame shuld be frwarded, and then t get it there. Switches 0- Switches 0-

3 Filtering & frwarding with switch tables When a switch receives a frame index switch table using MAC destinatin address if entry fund fr destinatin then { if destinatin is n segment frm which frame arrived then drp the frame else frward the frame n interface indicated } else fld* C sends a frame t D,.... Switch receives frame frm C Ntes in switch table that C is n interface Hwever, D is nt in table, s switch sends frame int interfaces and 3. Frame received by D. *Flding means frward the frame n all interfaces except the ne it arrived n. Switches 0-9 Switches and D replies But C wasn t riginally in switch table 3. D generates frame fr C, and sends it int the ether 4. The switch receives D s frame Ntes in switch table that D is n interface Switch knws C is n interface, s selectively frwards frame t interface Switches are selflearning; they build their tables are built autmatically. Fr each frame received, the switch stres in its table. The LAN address in the frame s surce field;. The interface frm which the frame arrived; 3. The current time. Switches 0- Switches 0-3

4 A switch r hub failure near the tp f a hierarchical structure is cause fr alarm. A switch r hub failure near the tp f a hierarchical structure is cause fr alarm. imprve fault tlerance, but have a serius side effect. imprve fault tlerance, but have a serius side effect. Hst in EE sends a frame t a hst n CS which first ges t the EE hub Switches 0-3 Switches 0-4 A switch r hub failure near the tp f a hierarchical structure is cause fr alarm. imprve fault tlerance, but have a serius side effect. The switches dn t knw where the hst lives, s... Spanning tree prtcl Switches cmmunicate with each ther in rder t determine a spanning tree, that is, a cnnected subset f the riginal tplgy that includes all ndes and has n lps. Switches virtually discnnect apprpriate interfaces t create the spanning tree. Switches 0-5 Switches 0-6 4

5 Fr example, If later a link fails, Electrical engineering may discnnect frm the tp ruter, while Systems engineering discnnects frm the bttm ruter. the switch can autmatically recnnect an interface, and run the spanning tree algrithm again and determine a new set f interfaces. Switches 0- Switches 0- S why a switch and nt a ruter? Weighing switches Prs. Plug and play.. Since they perate dwn a level, switches have relatively high packet filtering/frwarding rate. Cns. Spanning tree prtcl restricts effective tplgy, and. Cncentrates traffic n spanning tree links. 3. N prt in a bradcast strm. Switches 0-9 Switches -0 5

6 Weighing ruters Prs. Addresses hierarchical, s packets dn t usually cycle and we are nt restricted t trees.. Firewall prtectin against link-level bradcast strms. Cns. Nt plug-and-play; need IP addresses t be cnfigured.. One layer up; larger perpacket prcessing time than switches. Why use a backbne at all? The fllwing design is cheap and easy. It desn t use a backbne and hence has n central pint f failure. Switches 0- Switches 0- Dedicated access Institutinal netwrk Mdern Ethernet switches are high-perfrmance multi-interface bridges. Have large number f varius speed interfaces; hsts can have dedicated access t switch. Full-duplex switching. Switches 0-3 Switches 0-4 6

7 Stre-and-frward switching Cut-thrugh switching Remember the caravan f 0 cars traveling late at night (n cngestin) at 00 Km/hur. New-fangled tll bths can prcess each car and send it n its way (cut-thrugh). Old fashin tll bths prcess all 0 cars befre any car can leave (stre-and-frward). This cntributed t a transmissin delay f L/R. Of curse, this is nly pssible if there isn t a line. If there is, stre-and-frward and cut-thugh d pretty much the same thing. 00 km 00 km ten-car caravan tll bth tll bth ten-car caravan tll bth tll bth Switches 0-5 Switches 0-6 Summary cmparisn Prblems with the institutinal netwrk traffic islatin hubs switches ruters dedicated n yes yes yes plug & play yes yes n yes ptimal ruting cut thrugh n n yes n yes n n yes Grup traffic is lcalized t a single switch, but bradcast traffic is nt Limiting the scpe f such bradcast traffic wuld imprve LAN perfrmance*. Switches 0- *Nt t mentin security/privacy reasns. Switches 0-

8 Inefficient use f switches and managing users Mre, smaller grups wuld require mre (under-utilized) switches *And what d we d abut an emplyee that belngs t tw grups? Emplyees mving between grups wuld require physical cabling changes* Switches 0-9 Virtual Lcal Area Netwrks (VLANs) Switches that supprt VLANs allw multiple virtual lcal area netwrks ver a single physical LAN. Switch's prts are divided int VLAN grups, each frming its wn bradcast dmain.* VLANs slve all three f the previusly mentined prblems. But nw we have a new prblem: Hw can traffic frm EE be sent t CS? Electrical Engineering (VLAN prts -) Cmputer Science (VLAN prts 9-5) Electrical Engineering Cmputer Science (VLAN prts -) (VLAN prts 9-6) *That is, bradcast traffic frm ne prt can nly reach ther prts in the grup. Switches Hybrid ruter/switches One way t cnnect EE and CS is t cnnect a VLAN switch prt t an external ruter and cnfigure that prt t belng t bth EE and CS. T make Tim Cantin's life easier, vendrs prvide single devices that cntain bth a VLAN switch and a ruter. Electrical Engineering (VLAN prts -) ruter Cmputer Science (VLAN prts 9-5) Okay, here's anther prblem fr Tim Suppse nw sme EE and CS faculty are hused in separate buildings... and they still wuld like t be part f the department's VLAN. Switches 0-3 Switches 0-3

9 Well, we culd... Define a prt belnging t the CS VLAN n each switch and cnnect these prts t each ther. VLAN trunking Special prts n each switch are cnfigured as a trunk prts t intercnnect the tw VLAN switches And d the same fr prts belnging t the EE VLAN Electrical Engineering (VLAN prts -) Cmputer Science (VLAN prts 9-5) Prts,3,5 belng t EE VLAN Prts 4,6,, belng t CS VLAN *But this apprach desn t scale. *Fair enugh, but hw des a switch knw that a frame arriving n a trunk prt belngs t a particular VLAN? Switches 0-33 Switches 0-34 IEEE 0.Q* Yu say ptat... Type 0. frame Finally, there are tw ways t say ruter... 0.Q frame -byte Tag Prtcl Identifier (value: -00) Recmputed CRC Tag Cntrl Infrmatin ( bit VLAN ID field, 3 bit pririty field like IP TOS) *An extended frmat designed fr frames crssing a VLAN trunk. Switches 0-35 Switches