Network Interconnection: Bridging CS 57 Fll 6 6 Kenneth L. Clvert All rights reserved
The Prolem We know how to uild (rodcst) LANs Wnt to connect severl LANs together to overcome scling limits Recll: speed of light limits on efficiency of MAC protocols Gol: mke set of interconnected LANs "look like" single, lrge LAN Ech sttion uses the sme protocol it normlly would to trnsmit to ny sttion on the extended LAN But the LANs my e different technologies (e.g. Ethernet, WiFi) Use stndrd IEEE 8 ddresses to identify destintions
Solution Approch We get to design specil oxes (ISs) to interconnect LANs Cll these ridges Ech ridge cts like sttion on ech LAN to which it is connected IS
Solution Approch We get to design specil oxes (ISs) to interconnect LANs Cll these ridges Ech ridge cts like sttion on ech LAN to which it is connected B IS A B A
Lerning Bridge Bridge keeps tle mpping ddresses to interfces Bridge looks t source ddresses of received pckets to populte tle Pcket with source B received on interfce indictes tht B is in tht direction Addr i/f B B IS A A B Prolem: Where is A?
Lerning Bridge If the destintion ddress is not in the tle Forwrd out ll interfces other thn the incoming one Reply (if ny) revels loction of destintion Addr i/f B B IS A B A
Lerning Bridge If the destintion ddress is not in the tle Forwrd out ll interfces other thn the incoming one Reply (if ny) revels loction of destintion B IS Addr i/f B A A B A
Bridge Processing For ech received frme:. Rememer the interfce it rrived on. Look up the source ddress in the tle If not present, dd (source ddr, incoming i/f) to tle 3. Look up the destintion ddress in the tle If found: trnsmit frme on the indicted interfce, if different from the incoming i/f If not found: flood, i.e. trnsmit on ll interfces except incoming Steps nd 3 cn e done in prllel Works when pths include multiple ridges Content-ddressle memory (CAM) is idel for implementing this processing
Advntges of Bridging Communiction on different LANs cn occur in prllel Any sttion cn rech ny other sttion s if they were connected to the sme LAN Heterogenous LANs cn e ccomodted Miniml dministrtive overhed Bridges populte tles utomticlly No dditionl ddress ssignment required
Issues The lerning ridge lgorithm works provided there re no cycles in the network The interconnected network topology is constrined to e tree Only one pth etween ny pir of nodes If this constrint is violted, rodcst frmes circulte forever! Drwcks of the tree topology constrint: Any ridge filure prtitions the network Cpcity limited y minimum-rte LAN Hevy trffic on strtup (when no ddresses re known)
Brodcst Loops B 3 A B 5
Roustness Solution: Spnning Tree Protocol Allow ritrry topologies Including cycles of ridges nd LANs Bridges collorte to construct spnning tree (overly) on the network Before ny pckets cn e flooded Ech ridge determines which of its interfces re prt of the tree Interfces not in the tree re not used in the forwrding lgorithm All interfces prticipte in the spnning tree lgorithm When prtition occurs, protocol djusts the spnning tree
Roustness Solution: Spnning Tree Protocol 3 5
Roustness Solution: Spnning Tree Protocol 3 No trffic forwrded over these interfces 5
Spnning Tree Construction. Elect root ridge Bridge with lowest ID. Ech ridge determines its shortest pth to root Interfce closest to the root is lwys prt of the tree 3. Elect "designted ridge" for ech LAN Bridge with shortest pth to root Brek ties using lowest ID. Any interfce connecting ridge to LAN for which it is designted ridge is in the tree; ny others re not
Spnning Tree Protocol Ech ridge mintins two glol stte vriles Root ID Distnce to Root nd one per-interfce vrile: designted ridge on this LAN Ech ridge rodcsts messges contining triples: (Who I think is root, distnce to tht root, my ID) Triples re lexicogrphiclly ordered (,7,) < (3,,5) (,,5) < (,,5) (,, ) < (,, )
Spnning Tree Protocol Ech ridge initilly proclims itself s root Ech ridge updtes its stte vriles sed on the smllest triples it sees Root ID := Root ID from smllest triple yet seen Distnce to Root := Distnce from smllest triple + (unless smllest triple is tht ridge's) Designted Bridge on LAN := sender of smllest triple seen on tht LAN Bridges turn off interfces not in the tree
Spnning Tree Protocol Exmple Root: Distnce: Des. : Des. : Distnce: Des. : Des. : 3 Root: 3 Distnce: Des. : 3 Des. : 3 Root: 5 Distnce: Des. : 5 Des. : 5 5 c Root: Distnce: Des. : Des. : Des. c: Root: Distnce: Des. : Des. :
Spnning Tree Protocol Exmple (,,) Root: 5 Distnce: Des. : 5 Des. : 5 (,,) (,,) (,,) 5 (,,) (5,,5) Distnce: Des. : Des. : (,,) c Root: Distnce: Des. : Des. : Des. c: Root: Distnce: Des. : Des. : (3,,3) 3 (,,) (,,) (,,) (,,) Root: 3 Distnce: Des. : 3 Des. : 3 Root: Distnce: Des. : Des. :
Spnning Tree Protocol Exmple (,,) Distnce: Des. : Des. : (,,) (,,) (,,) 5 (,,) (5,,5) Distnce: Des. : Des. : (,,) c Root: Distnce: Des. : Des. : Des. c: Distnce: Des. : Des. : (3,,3) 3 (,,) (,,) (,,) (,,) Root: Distnce: Des. : Des. : Root: Distnce: Des. : Des. :
Spnning Tree Protocol Exmple (,,) Distnce: Des. : Des. : (,,) (,,) (,,) 5 (,,5) (,,5) Distnce: Des. : Des. : (,,) c Root: Distnce: Des. : Des. : Des. c: Distnce: Des. : Des. : (,,3) 3 (,,3) (,,) (,,) (,,) Root: Distnce: Des. : Des. : Root: Distnce: Des. : Des. :
Spnning Tree Protocol Exmple (,,) Distnce: Des. : Des. : 5 (,,) (,,) (,,) 5 (,,5) (,,5) Distnce: Des. : Des. : (,,) c Distnce: Des. : 3 Des. : 5 Des. c: Distnce: Des. : Des. : (,,3) 3 (,,3) (,,) (,,) (,,) Distnce: Des. : Des. : 3 Root: Distnce: Des. : 3 Des. :
Spnning Tree Protocol Exmple (,,) Distnce: Des. : Des. : 5 (,,) (,,) (,,) 5 (,,5) (,,5) Distnce: Des. : Des. : (,,) c Distnce: Des. : Des. : 5 Des. c: Distnce: Des. : Des. : (,,3) 3 (,,) (,3,) (,,) (,,) Distnce: Des. : Des. : 3 Root: Distnce: Des. : 3 Des. :
Spnning Tree Protocol Exmple (,,) Distnce: Des. : Des. : 5 (,,) (,,) (,,) 5 (,,5) (,,5) Distnce: Des. : Des. : (,,) c Distnce: Des. : Des. : 5 Des. c: Distnce: Des. : Des. : (,,3) 3 (,,) (,3,) (,,) (,,) Distnce: Des. : Des. : Distnce: 3 Des. : Des. :
Spnning Tree Protocol Exmple Distnce: Des. : Des. : (,,) Distnce: Des. : Des. : (,,3) 3 Distnce: Des. : Des. : Distnce: Des. : Des. : 5 5 (,,5) c Distnce: Des. : Des. : 5 Des. c: (,,) (,3,) Distnce: 3 Des. : Des. :
Tree Mintennce Pcket forwrding egins only fter tree construction hs converged Bridges periodiclly rodcst their triples fter convergence Detect ridge filure
Spnning Tree Protocol: Repir Distnce: Des. : Des. : (,,) Distnce: Des. : Des. : (,,3) 3 Distnce: Des. : Des. : Distnce: Des. : Des. : 5 5 (,,5) c Distnce: Des. : Des. : 5 Des. c: (,,) (,3,) Bridge fils Distnce: 3 Des. : Des. :
Spnning Tree Protocol: Repir Distnce: Des. : Des. : (,,) Distnce: Des. : Des. : (,,3) 3 Distnce: Des. : Des. : Distnce: Des. : Des. : 5 5 (,,5) c (,,3) (,3,) Distnce: 3 Des. : Des. : (,3,)
Spnning Tree Protocol: Repir Distnce: Des. : Des. : (,,) Distnce: Des. : Des. : (,,3) 3 Distnce: Des. : Des. : 3 Distnce: Des. : Des. : 5 5 (,,5) c (,,3) (,3,) Distnce: 3 Des. : Des. : (,3,)
Spnning Tree Protocol: Repir Root: Distnce: Des. : Des. : Distnce: Des. : Des. : 3 Root: 3 Distnce: Des. : 3 Des. : 3 Root: 5 Distnce: Des. : 5 Des. : 5 5 c Root: Distnce: Des. : Des. : Des. c: Root: Distnce: Des. : Des. :
Trnsprent Bridging: Summry End Systems: no chnge Extended LAN looks exctly like regulr LAN IS's: uild forwrding tle vi locl computtion Topology constrined to e (logiclly) tree IS's communicte to construct spnning tree overly Fctors limiting sclility Net-wide rodcst when ddress is not known Tree topology limits cpcity etween LANs IS tles eventully contin every destintion ddress
Bridging with Source Routes Used with IBM's Token Ring Protocol (lso in 8.5) No constrint on forwrding topology Assumptions: Ech LAN hs unique -it ID (mx 96 LANS) Ech Bridge on LAN hs -it ID (unique etween two LANs) Not trnsprent: Sources (ESs) must plce route description in ech pcket = sequence of lternting LAN nd ridge IDs Bridges (ISs) forwrd pckets from LAN to LAN sed on route description in pcket
Source Routing Frme Formt Source Addr RII it Dest Addr RIF Pylod FCS Routing Control LAN ID Bridge ID LAN ID Bridge ID... LAN ID Source Route Type Length Dir it "Directed" or "All-pths Explorer" MTU Bridge Algorithm for Directed Frmes:. Find ID of LAN received on in Source Route. If next Bridge ID = my ID: forwrd onto "next" LAN Direction it determines if "next" = left or right Note: Assumptions imply tht LAN-Bridge-LAN ID sequences re unique!
How Sources Find Pths Source rodcsts "All Pths Explorer" frme Ech ridge rodcsts explorer frme to every LAN except the one it ws received on As frme is forwrded, ridge ppends: Its own ID ID of LAN forwrded on to the source route Destintion returns explorer frmes to Source s directed frmes (reversing pth vi Direction it) Source chooses the "est" pth Looping prevention: no ridge forwrds n explorer frme more thn once
Pth Discovery vi Explorer Frmes A B 3 C E 5 D - F
Pth Discovery vi Explorer Frmes A B 3 C D//E E D//E D/5/C 5 D//F D F
Pth Discovery vi Explorer Frmes A D//E/3/B B D/5/C//A D//E/3/B 3 C D//F//E E 5 D//E//F D//F//E D F
Pth Discovery vi Explorer Frmes D//E/3/B A D/5/C//A//B D//E/3/B//A D//F//E/3/B D/5/C//A//B D//F//E/3/B 3 B C E 5 D F
Pth Discovery vi Explorer Frmes D//E/3/B A D//F//E/3/B//A D/5/C//A//B D//F//E/3/B B D//E/3/B//A//C D/5/C//A//B/3/E 3 C D/5/C//A//B/3/E E 5 D F
Pth Discovery vi Explorer Frmes D//E/3/B A D//F//E/3/B//A//C 3 D/5/C//A//B D//F//E/3/B B C E 5 D/5/C//A//B/3/E//F D/5/C//A//B/3/E//F D F
Pth Discovery vi Explorer Frmes D//E/3/B A D/5/C//A//B D//F//E/3/B B 3 C E 5 D F
Pth Discovery vi Explorer Frmes B/3/E//D A B//A//C/5/D B/3/E//F//D B 3 C E 5 D F
Route Selection & Cching Source is free to choose ny returned route Typiclly choose the one rriving first! Other possiilities: Ech explorer hs n MTU field tht collects the minimum MTU long the pth; choose the pth with lrgest MTU Avoid some LANs known to e hevily loded Any other policy Sources should cche routes for efficiency Re-explore periodiclly to ctch chnges
Source Routing Bridging Summry End Systems er the urden: Route discovery Route selection, cching But unused routes consume no overhed! Intermedite systems (Bridges): life is simple Completely stteless No "ckground" protocol etween ridges Works with ny topology Fctors limiting sclility Explorer pckets crete hevy lod, wste cpcity