# Chapter 3 Ethernet Bridges & Switches, ATM Switching Professor Rick Han University of Colorado at Boulder

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Chapter 3 Ethernet Bridges & Switches, ATM Switching Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

Prof. Rick Han, University of Colorado at Boulder Announcements Previous lecture online Reminder: Programming assignment #1 is due Feb. 19 Homework #2 will be available on the Web site on Thurs. Feb. 7 Shifted Office Hours Today: 4:30-5:30 pm Reading in Chapter 3 3.2: Ethernet bridges and switches 3.1, 3.3: ATM packet switching Skip 3.4 Next, Ethernet bridges, switches, and ATM

Prof. Rick Han, University of Colorado at Boulder Recap of Previous Lecture Interconnecting Ethernet LANs Ethernet Repeaters & Hubs – Physical Layer Amplify analog signal Problems: Limited range Amplify noise Same collision domain Can’t connect LANs with different bit rates Ethernet Bridges – Layer 2 Forward Ethernet frames Construct a table for frame forwarding When a frame arrives, put in table

Prof. Rick Han, University of Colorado at Boulder Recap of Previous Lecture (2) Ethernet Bridges – Layer 2 Frame forwarding rules: If dest. and src on same LAN, don’t forward frame If dest. and src on diff. LAN, route frame to dest. LAN If dest. unknown, forward to all outgoing interfaces Advantages Can connect LANs with different bit rates Separate collision domains Indefinite range No noise amplification Problems: Loops can develop, causing endless packet forwarding Packet multiplication effect Solution: spanning trees

Prof. Rick Han, University of Colorado at Boulder Problems With Bridges Bridges can interconnect LANs and have multiple paths between every node Inadvertent Layer 2 Bridge Purposely for robustness, in case highest tier fails Problem: Frames can cycle forever in a loop and multiply to crash LAN!

Prof. Rick Han, University of Colorado at Boulder Problems With Bridges: Packet Multiplication Effect Suppose all bridges have just booted Suppose A wants to send to Z Bridge 4 Bridge 1 Bridge 2 Bridge 3 Bridge 1 sends A’s frame to LAN 5 & 4 These two frames propagate to Bridge 3, where they multiply into 4 copies Exponentially multiplying copies! A Z LAN1 LAN3 LAN2 LAN4 LAN5

Prof. Rick Han, University of Colorado at Boulder Problems With Bridges: Endless Looping Suppose all bridges have just booted Suppose A wants to send to Z Bridge 4 Bridge 1 Bridge 2 Bridge 3 Bridge 2 sends frame to LAN 2 Bridge 3 sends frame to LAN 3 Bridge 4 -> LAN 4 Back to LAN 1 Frames can cycle forever! A Z LAN1 LAN3 LAN2 LAN4

Prof. Rick Han, University of Colorado at Boulder Solution: Spanning Tree Algorithm Invented by Radia Perlman, modified into 802.1d spanning tree standard Bridges communicate with each other to set up a spanning tree that has no loops Bridge 4 Bridge 1 Bridge 2 Bridge 3 Disconnect some interfaces, though physical link exists Some frames may take long route though shorter direct route exists A Z LAN1 LAN3 LAN2 LAN4 Some bridges may become orphans

Prof. Rick Han, University of Colorado at Boulder Rules to Build Spanning Tree 1.Elect a root bridge with the smallest global id 2.Each bridge computes its shortest distance to root 3.Each LAN selects a forwarding/designated bridge closest to root LAN CLAN D LAN E LAN A Bridge1Bridge 2 Bridge 3 Bridge 4 Spanning tree = root + forwarding bridges Root forwards frames on all outgoing ports If dest. not on LAN, send via forwarding bridge Eliminates loops! Root

Prof. Rick Han, University of Colorado at Boulder Control Messages to Build Spanning Tree Each bridge creates a configuration message: Each bridge floods its initial configuration message on each of its ports/LANs: Each bridge stores “best” config msg for each port/LAN A config msg C1 is better than stored config msg C2 if: 1.Root id of C1 < root id of C2 2.Root id’s equal and distance of C1 < distance of C2 3.If root id’s and distances equal, C1 is better than C2 if transmitting bridge on C1 is lower than C2

Prof. Rick Han, University of Colorado at Boulder First, Elect the Root If advertised root of new config msg C1 has smaller id, then Stop sending out its own bridge id config msg’s Forward new smaller id on all outgoing ports LAN CLAN D LAN E LAN A Bridge1 Bridge 2 Bridge 3 Bridge 4 Higher id config messages are discarded. Eventually, lowest ID bridge suppresses all other bridges’ config msg’s Root bridge knows it is the root because the lowest ID is its own

Prof. Rick Han, University of Colorado at Boulder First, Elect the Root (2) Example: Regardless of the config msgs exchanged by Bridges 2,3, and 4, as soon as Bridge 1 floods its config msg to Bridge 2 and 4, they both: LAN CLAN D LAN E LAN A Bridge1 Bridge 2 Bridge 3 Bridge 4 stop sending out their own bridge id config msg’s and Begin forwarding Bridge 1’s config msg on all outgoing ports Eventually, Bridge 3 also stops sending its config msg’s

Prof. Rick Han, University of Colorado at Boulder Next, Build Shortest-Path Forwarding Tree to Root Conceptually, build shortest-path forwarding tree after electing the root But, as the root’s config msg floods the network, notice that the shortest-path tree can simultaneously be calculated Thus, piggyback on Bridge 1’s config msg flooding to set up the shortest path tree to root: Each bridge increments by one the distance, as it receives Bridge 1’s config msg, and forwards config msg with Bridge 1 as root to all outgoing ports

Prof. Rick Han, University of Colorado at Boulder Next, Build Shortest-Path Forwarding Tree to Root (2) When a bridge receives a config message from another bridge on same LAN with Bridge 1 as root, it stops sending config messages on that port/LAN if: Other bridge is closer to root Other bridge is same distance from root, but has a lower ID Thus, a bridge de-selects itself as the designated forwarding bridge for that port/LAN

Prof. Rick Han, University of Colorado at Boulder Next, Build Shortest-Path Forwarding Tree to Root (3) Bridge 1 floods its config message Bridge D is part of a loop, and will receive multiple config msg’s from Bridge 1 Bridge D deselects itself from both LANs because Bridges 2 & 3 are closer to root Bridge 1 Bridge 1 Bridge D Bridge 2Bridge 3

Prof. Rick Han, University of Colorado at Boulder Next, Build Shortest-Path Forwarding Tree to Root (4) Bridge 4 is designated forwarding bridge for LAN A, since it closer to root than Bridge 3 on LAN A Bridge 3 removes itself For LAN B, Bridge 2 is designated forwarding bridge Bridge 3 removes itself LAN CLAN D LAN E LAN A Bridge1 Bridge 2 Bridge 3 Bridge 4

Prof. Rick Han, University of Colorado at Boulder Topology Change Root bridge periodically sends keep-alive messages If this is not heard locally, then local bridges start the spanning-tree algorithm all over again Handles the case when root bridge failed Handles the case when intermediate bridge failed, and the network becomes a… Partitioned network Non-partitioned network

Prof. Rick Han, University of Colorado at Boulder Ethernet Switches Essentially, the same as bridges, with support for many more interfaces: Still forward frames based on destination address Still construct forwarding table based on source address Special routing fabric to speed frame routing from input interface to output interface

Prof. Rick Han, University of Colorado at Boulder 80/20 Rule Position a bridge so that 80% of traffic on a segment is local 20% is forwarded Higher throughput, because each LAN has its own conversation Example: place users of Server 1 on same LAN. Server 1 could be a file/Web server Server 2Server 1

Prof. Rick Han, University of Colorado at Boulder Why Not Bridge Ethernet Indefinitely? Couldn’t really bridge cross-country Delay accumulates in each bridge Many bridges, due to small segment sizes Many different types of LAN’s, e.g. Token Ring and FDDI, with completely different addressing schemes Ethernet …?

Prof. Rick Han, University of Colorado at Boulder ATM Switching Point-to-Point Links Interconnect Switches Closer to Internet topology Don’t connect shared-media segments Host A Switch C Switch D Switch B Switch E Host F

Prof. Rick Han, University of Colorado at Boulder ATM Switching (2) Big difference with Internet routing: ATM uses virtual circuits to route packets Packet switching, but with fixed-length cells 48 bytes + 5 bytes header Why fixed-length cells? Optimized hardware in switch can get higher throughput Why 48 bytes? Europe and US couldn’t agree, one wanted 64 bytes and another 32 bytes, so they split the difference

Prof. Rick Han, University of Colorado at Boulder ATM Adaptation Layer 3/4 Due to small packet sizes, need a layer above ATM to fragment and reassemble long packets ATM Adaptation Layer (AAL) 3/4 IP packets can be encapsulated in ATM packets: IP over ATM ATM operates as a part of Internet backbone Since ATM is network layer protocol, then still need link layer – SONET, e.g. encapsulate IP over ATM over SONET too much overhead!

Prof. Rick Han, University of Colorado at Boulder Virtual Circuit Routing Create a virtual circuit path across an interconnected mesh of switches Each packet is labeled with a virtual circuit ID in its header Host A Switch C Switch D Switch B Switch E Host F

Prof. Rick Han, University of Colorado at Boulder Virtual Circuit Routing (2) Each node chooses an unused VC number on a leg of circuit Each switch maintains a routing table mapping VC on input interface to VC on output interface Host A Switch C Switch D Switch B Switch E Host F 7 88 10

Prof. Rick Han, University of Colorado at Boulder Virtual Circuit Routing (3) Host A Switch C Switch D Switch B Switch E Host F 7 88 10 Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI From A7To E88 Switch B Routing Table Any cell with VCI=7 from A Is (1) relabeled with VCI=88 (2) Then routed onto E interface

Prof. Rick Han, University of Colorado at Boulder Virtual Circuit Routing (4) Host A Switch C Switch D Switch B Switch E Host F 7 88 10 Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI From B88To F10 Switch E Routing Table Any cell with VCI=88 from B Is (1) relabeled with VCI=10 (2) Then routed onto F interface VC’s have local scope

Prof. Rick Han, University of Colorado at Boulder Setting up VC Routing Tables Permanent Virtual Circuits (PVC) are set up by a network administrator Switched Virtual Circuits (SVC) are set up by sending control signals into the network Send setup message with dest. address Assume for now that switches can determine the best outgoing interface to forward a setup packet on when the setup packet arrives As setup message courses through network, each switch picks its incoming VCI (an unused #) When setup msg reaches destination, send acknowledgment back along same path, so each upstream switch knows VCI chosen by downstream switch

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