IP Forwarding ProcedureExample Network:R AR BR CR DB172.16.96.165172.16.96.129172.16.96.128/26172.16.96.65172.16.96.64/27172.16.96.66172.16.96.96/27172.16.96.214172.16.96.192/27172.16.96.193172.16.96.224/28172.16.96.225172.16.96.67172.16.96.226172.16.96.240/28AIP Addresses Identify an Interface not a machineeth0eth1(Reference : “ IP Fundamentals” by Maufer Part II)(Read Forouzan Chapters 6 and 7))In Sending An IP Packet From A to BA 172.16.96.214B 172.16.96.165STEP 1:Since B 172.16.96.165 [ 10101100.00010000.01100000.101]00101and A’s network 172.16.96.192/27 [10101100.00010000.01100000.110]00000do not match in masked off significant part (Prefix):A must send IP packet to a default gateway which A must know (by set up) to be RBA must send Address Resolution Protocol (ARP) request to find out RB MAC.A then sendsFCSMAC DA MAC SA0X0800RB ( MAC)MACAEtherTypeIPA IPBIP packet dataIP SA IP Destination AddressIP PacketMAC FrameFrame ControlSequence ( CRC )……STEP 2:At RB IP frame is extracted and IP Destination Address is examined and compared for longest matchin a forwarding table.For Example:Known Prefixes Next – Hop Gateway Interface Metric172.16.96.64/27 172.16.96.67172.16.96.96/27 172.16.96.66172.16.96.128/26 172.16.96.65 eth0172.16.96.192/27 172.16.96.193172.16.96.224/28 172.16.96.225172.16.96.240/28 172.16.96.226 LaterThe destination prefix that has the most leading bits in common with the IP DA 172.16.96.165 is172.16.96.128/26172.16.96.165 = 10101100.00010000.01100000.10100101172.16.96.128/26 = [10101100.00010000.01100000.10]000000PrefixThis means we need to forward the packet to whoever has 172.16.96.65 as an IP address.CAUTION: we want to forward at the MAC layer so need to use ARP to find MAC addressfor 172.16.96.65. DO NOT CHANGE IP DA!!!!!FCSRC MAC RB MAC0x0800IPAIPBIP Packet DataIPMACMAC DA MAC SAether TypeIP SA IP DANew time to live => new IP header check sumDifferent value from step 1Frame ControlSequence (CRC)STEP 2 :STEP 3 :At RC IP frame is extracted and IP destination address is examined and compared for longest match in a forwarding table. For Example:Known Prefixes Next – Hop Gateway Interface Metric172.16.96.64/27 172.16.96.65172.16.96.96/27 172.16.96.66172.16.96.128/26 172.16.96.129 eth1172.16.96.192/27 172.16.96.67172.16.96.224/27 172.16.96.67 LaterDifference from before - since RC is directly connected to destinations subnetwork, the subnetworklayer destination MAC address MACB and IP destination address ( IPB ) are for the same machine.Longest prefix match of same old IP DA is with 172.16.96.128/26FCSMAC DA MAC SAMAC B (MAC) RC0X0800IP SA IP DAIPAIPBIP PacketDataEtherTypeAside: Routing protocols ( RIP or OSPF ) build forwarding tables (or can do by hand )What is the difference between routing and switching?LAN interconnection devices operate a “ OSI ” (Open Systems Interconnection Reference Model)Layers:Layer 1 : Physical LayerLayer 2: Data Link LayerLayer 3: Network LayerApplicationTransportInternetSubnetworkApplicationPresentationSessionTransportNetwork Data Link PhysicalInternet ProtocolStackOpen SystemsInterconnectionReference ModelLayer 1: Repeaters ( Not too common now) HUBS ( Very common ) HUB To Backbone Four HUB rule - only 4 hub hops allowed in a “ Collision Domain ”Hubs are physical layer devices that just “Repeat” what they see, errored frames and all. 10 Base-T pinout and Crossover cable HUB attachment Functionality Endstation1 -- TD+ 2 -- TD- 3 -- RD+ 456 -- RD- 78Hub Side1 -- TD+2 -- TD-3 -- RD+456 -- RD-78Endstation1 -- TD+ 2 -- TD- 3 -- RD+ 456 -- RD- 78Endstation1 TD+2 TD-3 RD+456 RD-78Layer 2:Bridges ( Not common now )Switches ( What we call bridges now )Layer - 2 switches ( Way cool marketing term)Forwarding decisions based only on data link layer header, that is the MAC DA. Use a table made fromobserving which addresses are seen on each port.No more than 7 bridges in diameter.Today’s ethernet environments use lots of ethernet switches, reducing the collision domain.Collision domain is the set of ethernet segments that can directly hear each others frames.Worst case is all inputs want to go out on the same output. Must have some buffering and some sort of fairness algorithm insideLayer 3: routers( Historically Called a Gateway When Different Protocols Were Involved )Routers forward packets based on the destination address at network layer ( Layer 3 ) May see “Layer - 3 switching” term this just means Routing !LAN 1 LAN 2321Early RoutingInitially called gateways ( Gateway From One Network To Another ) and later called routers. Used to connect different physical networks into larger unified network.Packets contain a destination address, router attempts to match with one of many possible entriesin a table of destination networks, sends out appropriate interface.First generation router architecture:ProcessorInterface InterfaceShared BusInterfaceProcessor runs routing protocols and maintains forwarding table of next HOP routersPackets flow from interface up to processor then back down to
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