COMS/CSEE 4140 Networking Laboratory Lecture 03 Salman Abdul Baset Spring 2008 Announcements No prelab due next week.
Download ReportTranscript COMS/CSEE 4140 Networking Laboratory Lecture 03 Salman Abdul Baset Spring 2008 Announcements No prelab due next week.
COMS/CSEE 4140 Networking Laboratory Lecture 03 Salman Abdul Baset Spring 2008 Announcements No prelab due next week. Instead a small assignment. Due next week for all students. Lab 3 will be covered over two weeks. Lab 3 (1-4) due next week before your lab slot. 2 Previous lecture… Data Link Protocols Topology: bus, ring, star Ethernet and 802.3 (bus and star topology) Switch and hub PPP protocol Address Resolution Protocol (ARP) CSMA/CD Ethernet/802.3, CSMA/CA 802.11 (WiFi) Proxy ARP, RARP, ARP cache, vulnerabilities IP addresses Structure, classful IP addresses, CIDR, subnetting, IPv6 3 Agenda More on CIDR Internet Protocol (IP) Internet Control Message Protocol (ICMP) IP forwarding Router architecture 4 Classless Inter-domain routing (CIDR) 1993 Full description RFC 1518 & 1519 Network prefix is of variable length Addresses are allocated hierarchically Routers aggregate multiple address prefixes into one routing entry to minimize routing table size 5 CIDR notation CIDR notation of an IP address: 128.143.137.144/24 /24 is the prefix length. It states that the first 24 bits are the network prefix of the address (and the remaining 8 bits are available for specific host addresses) CIDR notation can nicely express blocks of addresses An address block [128.195.0.0, 128.195.255.255] can be represented by an address prefix 128.195.0.0/16 How many addresses are there in a /x address block? 2 (32-x) 6 CIDR hierarchical address allocation ISP 128.1.0.0/16 128.2.0.0/16 128.0.0.0/8 128.59.0.0/16 University Foo.com Bar.com Library 128.59.44.0/24 128.59.16.150 CS 128.59.16.0/24 IP addresses are hierarchically allocated. An ISP obtains an address block from a Regional Internet Registry An ISP allocates a subdivision of the address block to an organization An organization recursively allocates subdivision of its address block to its networks 7 A host in a network obtains an address within the address block assigned to the network Regional Internet Registries (RIRs) Registration and management of IP address is done by Regional Internet Registries (RIRs) Where do RIRs get their addresses from: IANA maintains a high-level registry that distributes large blocks to RIRs RIR are administer allocation of: IPv4 address blocks IPv6 address blocks Autonomous system (AS) numbers There are currently five RIRs worldwide: APNIC (Asia/Pacific Region), ARIN (North America and Sub-Sahara Africa), LACNIC (Latin America and some Caribbean Islands) RIPE NCC (Europe, the Middle East, Central Asia, and African countries located north of the equator). AfriNIC (Africa) (100,663,296 IP addresses 5% of total IPv4 addresses!) 8 Hierarchical address allocation 128.59.16.[0 – 255] 128.59.16.150 128.59.0.0 – 128.59.255.255 128.0.0.0 - 128.255.255.255 ISP obtains an address block 128.0.0.0/8 [128.0.0.0, 128.255.255.255] ISP allocates 128.59.0.0/16 ([128.59.0.0, 128.59.255.255]) to the university. University allocates 128.59.16.0/24 ([128.59.16.0, 128.59.16.255]) to the CS department’s network A host on the CS department’s network gets one IP address 128.59.16.150 9 CIDR allows route aggregation You can reach 128.0.0.0/8 via ISP1 128.1.0.0/16 Foo.com ISP3 ISP1 128.2.0.0/16 I 128.0.0.0/8 128.0.0.0/8 ISP1 128.59.0.0/16 Bar.com University Library CS ISP1 announces one address prefix 128.0.0.0./8 to ISP2 ISP2 can use one routing entry to reach all networks connected to ISP1 10 What problems CIDR does not solve You can reach 128.0.0.0/8 And 204.1.0.0/16 via ISP1 ISP1 ISP2 128.0.0.0/8 204.0.0.0/8 ISP3 128.0.0.0/8 204.1.0.0/16 ISP1 204.1.0.0/16 ISP1 Mutil-home.com 204.1.0.0/16 An multi-homing site still adds one entry into global routing tables 11 Agenda More on CIDR Internet Protocol (IP) Internet Control Message Protocol (ICMP) IP forwarding Router architecture 12 IP: The waist of the hourglass IP is the waist of the hourglass of the Internet protocol architecture Multiple higher-layer protocols Multiple lower-layer protocols Only one protocol at the network layer. Applications HTTP FTP SMTP TCP UDP IP Data link layer protocols Physical layer protocols 13 Application protocol IP is the highest layer protocol which is implemented at both routers and hosts Application Application protocol Application TCP TCP protocol TCP IP Data Link Host IP IP protocol Data Link Data Link IP IP protocol Data Link Router Data Link Data Link IP protocol Data Link Router Data Link IP Network Access Host 14 IP Service Delivery service of IP is minimal IP provide provides an unreliable connectionless best effort service (also called: “datagram service”). Unreliable: IP does not make an attempt to recover lost packets Connectionless: Each packet (“datagram”) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence Best effort: IP does not make guarantees on the service (no throughput guarantee, no delay guarantee,…) Consequences: • Higher layer protocols have to deal with losses or with duplicate packets 15 • Packets may be delivered out-of-sequence IP Service IP supports the following services: one-to-one one-to-all one-to-several unicast broadcast (unicast) (broadcast) (multicast) multicast IP multicast also supports a many-to-many service. IP multicast requires support of other protocols (IGMP, multicast routing) 16 IP Datagram Format bit # 0 7 8 3 4 version header length 15 16 ECN DS Identification time-to-live (TTL) 23 24 31 total length (in bytes) 0 D M F F protocol Fragment offset header checksum source IP address destination IP address options (0 to 40 bytes) payload 4 bytes 20 bytes ≤ Header Size < 24 x 4 bytes = 60 bytes 20 bytes ≤ Total Length < 216 bytes = 65536 bytes 17 IP Datagram Format Question: In which order are the bytes of an IP datagram transmitted? Answer: Transmission is row by row For each row: 1. First transmit bits 0-7 2. Then transmit bits 8-15 3. Then transmit bits 16-23 4. Then transmit bits 24-31 This is called network byte order or big endian byte ordering. Note: Many computers (incl. Intel processors) store 32-bit words in little endian format. Others (incl. Motorola processors) use big endian. 18 Big endian vs. little endian • Conventions to store a multibyte data type • Example: a 4 byte Long Integer Byte3 Byte2 Byte1 Byte0 Little Endian Stores the low-order byte at the lowest address and the highest order byte in the highest address. Base Address+0 Byte0 Base Address+1 Byte1 Base Address+2 Byte2 Base Address+3 Byte3 Intel processors use this order Big Endian Stores the high-order byte at the lowest address, and the low-order byte at the highest address. Base Address+0 Byte3 Base Address+1 Byte2 Base Address+2 Byte1 Base Address+3 Byte0 Motorola processors use big endian. 19 Fields of the IP Header Version (4 bits): current version is 4, next version will be 6. Header length (4 bits): length of IP header, in multiples of 4 bytes DS/ECN field (1 byte) This field was previously called as Type-of-Service (TOS) field. The role of this field has been re-defined, but is “backwards compatible” to TOS interpretation Differentiated Service (DS) (6 bits): Used to specify service level (currently not supported in the Internet) Explicit Congestion Notification (ECN) (2 bits): New feedback mechanism used by TCP 20 Fields of the IP Header Identification (16 bits): Unique identification of a datagram from a host. Incremented whenever a datagram is transmitted Flags (3 bits): First bit always set to 0 DF bit (Do not fragment) MF bit (More fragments) Will be explained later Fragmentation 21 Fields of the IP Header Time To Live (TTL) (1 byte): Specifies longest paths before datagram is dropped Role of TTL field: Ensure that packet is eventually dropped when a routing loop occurs Used as follows: Sender sets the value (e.g., 64) Each router decrements the value by 1 When the value reaches 0, the datagram is dropped 22 Fields of the IP Header Protocol (1 byte): Specifies the higher-layer protocol. Used for demultiplexing to higher layers. 4 = IP-in-IP encapsulation 17 = UDP 6 = TCP 2 = IGMP 1 = ICMP IP Header checksum (2 bytes): A simple 16-bit long checksum which is computed for the header of the datagram. 23 Fields of the IP Header Options: Security restrictions Record Route: each router that processes the packet adds its IP address to the header. Typically never set. Why? Timestamp: each router that processes the packet adds its IP address and time to the header. (loose) Source Routing: specifies a list of routers that must be traversed. (strict) Source Routing: specifies a list of the only routers that can be traversed. Padding: Padding bytes are added to ensure that header ends on a 4-byte boundary 24 Maximum Transmission Unit Maximum size of IP datagram is 65535, but the data link layer protocol generally imposes a limit that is much smaller Example: Ethernet frames have a maximum payload of 1500 bytes IP datagrams encapsulated in Ethernet frame cannot be longer than 1500 bytes The limit on the maximum IP datagram size, imposed by the data link protocol is called maximum transmission unit (MTU) MTUs for various data link protocols: Ethernet: 802.3: 802.5: 1500 1492 4464 FDDI: 4352 ATM AAL5: 9180 PPP: negotiated 25 IP Fragmentation What if the size of an IP datagram exceeds the MTU? IP datagram is fragmented into smaller units. What if the route contains networks with different MTUs? FDDI Ring Host A MTUs: FDDI: 4352 Ethernet Router Host B Ethernet: 1500 • Fragmentation: • IP router splits the datagram into several datagram • Fragments are reassembled at receiver 26 Where is Fragmentation done? Fragmentation can be done at the sender or at intermediate routers The same datagram can be fragmented several times. Reassembly of original datagram is only done at destination hosts !! IP datagram H Fragm e nt 2 H2 Fragm e nt 1 H1 Router 27 What’s involved in Fragmentation? The following fields in the IP header are involved: version header length DS Identification time-to-live (TTL) Identification protocol total length (in bytes) ECN 0 DM F F Fragment offset header checksum When a datagram is fragmented, the identification is the same in all fragments Flags DF bit is set: Datagram cannot be fragmented and must be discarded if MTU is too small MF bit set: This datagram is part of a fragment and an 28 additional fragment follows this one What’s involved in Fragmentation? The following fields in the IP header are involved: version header length DS Identification time-to-live (TTL) Fragment offset Total length protocol total length (in bytes) ECN 0 DM F F Fragment offset header checksum Offset of the payload of the current fragment in the original datagram Total length of the current fragment 29 Example of Fragmentation A datagram with size 2400 bytes must be fragmented according to an MTU limit of 1000 bytes Header length: 20 Total length: 2400 Identification: 0xa428 DF flag: 0 MF flag: 0 Fragment offset: 0 Header length: 20 Total length: 448 Identification: 0xa428 DF flag: 0 MF flag: 0 Fragment offset: 244 IP datagram Header length: 20 Header length: 20 Total length: 996 Total length: 996 Identification: 0xa428 Identification: 0xa428 DF flag: 0 DF flag: 0 MF flag: 1 MF flag: 1 Fragment offset: 122 fragment offset: 0 Fragment 3 MTU: 4000 Fragment 2 Fragment 1 MTU: 1000 Router 30 Determining the length of fragments To determine the size of the fragments we recall that, since there are only 13 bits available for the fragment offset, the offset is given as a multiple of eight bytes. As a result, the first and second fragment have a size of 996 bytes (and not 1000 bytes). This number is chosen since 976 is the largest number smaller than 1000–20= 980 that is divisible by eight. The payload for the first and second fragments is 976 bytes long, with bytes 0 through 975 of the original IP payload in the first fragment, and bytes 976 through 1951 in the second fragment. The payload of the third fragment has the remaining 428 bytes, from byte 1952 through 2379. With these considerations, we can determine the values of the fragment offset, which are 0, 976 / 8 = 122, and 1952 / 8 = 244, respectively, for the first, second and third fragment. 31 Agenda More on CIDR Internet Protocol (IP) Internet Control Message Protocol (ICMP) IP forwarding Router architecture 32 Overview The IP (Internet Protocol) relies on several other protocols to perform necessary control and routing functions: Control functions (ICMP) Multicast signaling (IGMP) Setting up routing tables (RIP, OSPF, BGP, …) RIP ICMP OSPF IGMP BGP PIM Routing Control 33 Overview The Internet Control Message Protocol (ICMP) is a helper protocol that supports IP with facility for Error reporting Simple queries defined in RFC 792 ICMP messages are encapsulated as IP datagrams: IP header ICMP message IP payload 34 ICMP message format bit # 0 7 8 type 15 16 code 23 24 31 checksum additional information or 0x00000000 4 byte header: Type (1 byte): type of ICMP message Code (1 byte): subtype of ICMP message Checksum (2 bytes): similar to IP header checksum. Checksum is calculated over entire ICMP message If there is no additional data, there are 4 bytes set to zero. each ICMP messages is at least 8 bytes long 35 ICMP Query message ICMP query: Request sent by host to a router or host Reply sent back to querying host 36 Example of ICMP Queries Type/Code: Description The ping command uses Echo Request/ Echo Reply 8/0 0/0 Echo Request Echo Reply 13/0 14/0 Timestamp Request Timestamp Reply 10/0 9/0 Router Solicitation Router Advertisement 37 Example of a Query: Echo Request and Reply Ping’s are handled directly by the kernel Each Ping is translated into an ICMP Echo Request The Ping’ed host responds with an ICMP Echo Reply Host or Router Host or router 38 Example of a Query: ICMP Timestamp A system (host or router) asks another system for the current time. Time is measured in milliseconds after midnight UTC (Universal Coordinated Time) of the current day Sender sends a request, receiver responds with reply Type (= 17 or 18) Sender Receiver Timestamp Reply Code (=0) identifier Timestamp Request Checksum sequence num ber 32-bit sender tim estam p 32-bit receive tim estam p 32-bit transm it tim estam p 39 ICMP Error message ICMP error messages report error conditions Typically sent when a datagram is discarded Error message is often passed from ICMP to the application program 40 ICMP Error message ICMP Message from IP datagram that triggered the error IP header type ICMP header code IP header 8 bytes of payload checksum Unused (0x00000000) ICMP error messages include the complete IP header and the first 8 bytes of the payload (typically: UDP, TCP) 41 Frequent ICMP Error message Type Code 3 Description 0–15 Destination unreachable Notification that an IP datagram could not be forwarded and was dropped. The code field contains an explanation. 5 0–3 Redirect Informs about an alternative route for the datagram and should result in a routing table update. The code field explains the reason for the route change. 11 0, 1 Time exceeded Sent when the TTL field has reached zero (Code 0) or when there is a timeout for the reassembly of segments (Code 1) 12 0, 1 Parameter problem Sent when the IP header is invalid (Code 0) or when an IP header option is missing (Code 1) 42 Some subtypes of the “Destination Unreachable” Code Description Reason for Sending 0 Network Unreachable No routing table entry is available for the destination network. 1 Host Unreachable Destination host should be directly reachable, but does not respond to ARP Requests. 2 Protocol Unreachable The protocol in the protocol field of the IP header is not supported at the destination. 3 Port Unreachable The transport protocol at the destination host cannot pass the datagram to an application. 4 Fragmentation IP datagram must be fragmented, but the DF bit Needed in the IP header is set. and DF Bit Set 43 Example: ICMP Port Unreachable RFC 792: If, in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, the destination host may send a destination unreachable message to the source host. Scenario: No process is waiting at port 80 Client Server 44 Agenda More on CIDR Internet Protocol (IP) Internet Control Message Protocol (ICMP) IP forwarding Router architecture 45 Tenets of end-to-end delivery of datagrams The following conditions must hold so that an IP datagram can be successfully delivered 1. 2. 3. The network prefix of an IP destination address must correspond to a unique data link layer network (=LAN or point-to-point link or switched network). (The reverse need not be true!) Routers and hosts that have a common network prefix must be able to exchange IP datagrams using a data link protocol (e.g., Ethernet, PPP) Every data link layer network must be connected to at least one other data link layer network via a router. 46 Routing tables Each router and each host keeps a routing table which tells the router how to process an outgoing packet Main columns: 1. 2. 3. Destination address: where is the IP datagram going to? Next hop: how to send the IP datagram? Interface: what is the output port? Next hop and interface column can often be summarized as one column Routing tables are set so that datagrams gets closer to the its Destination Next interface destination Hop Routing table of a host or router IP datagrams can be directly delivered (“direct”) or is sent to a router (“R4”) 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 direct direct R4 direct R4 R4 eth0 eth0 serial0 eth1 eth0 47 eth0 Delivery with routing tables to: 20.2.1.2 48 Delivery of IP datagrams There are two distinct processes to delivering IP datagrams: 1. Forwarding: How to pass a packet from an input interface to the output interface? 2. Routing: How to find and setup the routing tables? Forwarding must be done as fast as possible: on routers, is often done with support of hardware on PCs, is done in kernel of the operating system Routing is less time-critical On a PC, routing is done as a background process 49 Processing of an IP datagram in IP Routing Protocol Static routing UDP TCP Demultiplex Yes routing table Lookup next hop Yes IP forwarding enabled? No Destination address local? No IP module Send datagram Discard Input queue Data Link Layer 50 IP router: IP forwarding enabled Host: IP forwarding disabled Processing of an IP datagram in IP Processing of IP datagrams is very similar on an IP router and a host Main difference: “IP forwarding” is enabled on router and disabled on host IP forwarding enabled if a datagram is received, but it is not for the local system, the datagram will be sent to a different system IP forwarding disabled if a datagram is received, but it is not for the local system, the datagram will be dropped 51 Processing of an IP datagram at a router Receive an IP datagram 1. 2. 3. 4. 5. 6. 7. 8. 9. IP header validation Process options in IP header Parsing the destination IP address Routing table lookup Decrement TTL Perform fragmentation (if necessary) Calculate checksum Transmit to next hop Send ICMP packet (if necessary) 52 Routing table lookup When a router or host need to transmit an IP datagram, it performs a routing table lookup Routing table lookup: Use the IP destination address as a key to search the routing table. Result of the lookup is the IP address of a next hop router, and/or the name of a network interface Destination address Next hop/ interface network prefix IP address of or next hop router host IP address or or loopback Name of a address network or interface default route 53 Type of routing table entries Network route Host route Destination address is an interface address (e.g., 10.0.1.2/32) Used to specify a separate route for certain hosts Default route Destination addresses is a network address (e.g., 10.0.2.0/24) Most entries are network routes Used when no network or host route matches The router that is listed as the next hop of the default route is the default gateway (for Cisco: “gateway of last resort) Loopback address Routing table for the loopback address (127.0.0.1) The next hop lists the loopback (lo0) interface as outgoing interface 54 Routing table lookup: Longest Prefix Match 128.143.71.21 Longest Prefix Match: Search for the routing table entry that has the longest match with the prefix of the destination IP address Search for a match on all 32 bits 2. Search for a match for 31 bits ….. 32. Search for a mach on 0 bits 1. Host route, loopback entry 32-bit prefix match Default route is represented as 0.0.0.0/0 0-bit prefix match Destination address Next hop 10.0.0.0/8 128.143.0.0/16 128.143.64.0/20 128.143.192.0/20 128.143.71.0/24 128.143.71.55/32 default R1 R2 R3 R3 R4 R3 R5 The longest prefix match for 128.143.71.21 is for 24 bits with entry 128.143.71.0/24 55 Datagram will be sent to R4 Route Aggregation Longest prefix match algorithm permits to aggregate prefixes with identical next hop address to a single entry This contributes significantly to reducing the size of routing tables of Internet routers Destination Next Hop Destination Next Hop 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.2.0.0/16 20.1.1.0/28 R3 direct direct R3 R2 R2 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.0.0.0/8 R3 direct direct R3 R2 56 How do routing tables get updated? Adding an interface: Adding a default gateway: Configuring an interface eth2 with 10.0.2.3/24 adds a routing table entry: Configuring 10.0.2.1 as the default gateway adds the entry: Destination Next Hop/ interface 10.0.2.0/24 eth2 Destination Next Hop/ interface 0.0.0.0/0 10.0.2.1 Static configuration of network routes or host routes Update of routing tables through routing protocols ICMP messages 57 Routing table manipulations with ICMP When a router detects that an IP datagram should have gone to a different router, the router (here R2) forwards the IP datagram to the correct router sends an ICMP redirect message to the host Host uses ICMP message to update its routing table (2) IP datagram (3) ICMP redirect (1) IP datagram R1 58 ICMP Router Solicitation ICMP Router Advertisement After bootstrapping a R1 host broadcasts an ICMP router solicitation. In response, routers send an ICMP router advertisement message Also, routers periodically broadcast ICMP router advertisement R2 ICMP router advertisement ICMP router advertisement ICMP router solicitation Ethernet H1 59 Agenda More on CIDR Internet Protocol (IP) Internet Control Message Protocol (ICMP) IP forwarding Router architecture 60 Router Components Hardware components of a router: Network interfaces Interconnection network Processor with a memory and CPU PC router: Processor Memory CPU Interconnection Network interconnection network is the (PCI) bus and interface cards are NICs All forwarding and routing is done Interface Card on central processor Interface Card Interface Card Commercial routers: Interconnection network and interface cards are sophisticated Processor is only responsible for control functions (route processor) Almost all forwarding is done on interface cards 61 Functional Components routing protocol Routing functions routing protocol routing table updates Control routing table Datapath: routing table lookup incoming IP datagrams IP Forwarding per-packet processing outgoing IP datagrams 62 Routing and Forwarding Routing functions include: route calculation maintenance of the routing table execution of routing protocols On commercial routers handled by a single general purpose processor, called route processor IP forwarding is per-packet processing On high-end commercial routers, IP forwarding is distributed Most work is done on the interface cards 63 Slotted Chassis R Pr o u t oc e (C esso PU r ) e cards Interfac Large routers are built as a slotted chassis Interface cards are inserted in the slots Route processor is also inserted as a slot This simplifies repairs and upgrades of components 64 Evolution of Router Architectures Early routers were essentially general purpose computers Today, high-performance routers resemble supercomputers Exploit parallelism Special hardware components Until 1980s (1st generation): standard computer Early 1990s (2nd generation): delegate to interfaces Late 1990s (3rd generation): Distributed architecture Today: Distributed over multiple racks 65 1st Generation Routers This architecture is still used in low end routers Arriving packets are copied to main memory via direct memory access (DMA) Interconnection network is a backplane (shared bus) All IP forwarding functions are performed in the central processor. Routing cache at processor can accelerate the routing table lookup. Route Processor CPU Cache Memory Shared Bus DMA DMA DMA Interface Card Interface Card Interface Card MAC MAC MAC Drawbacks: Forwarding Performance is limited by CPU Capacity of shared bus limits the number of interface cards that can be connected 66 2nd Generation Routers CPU Interface cards have local route cache and processing elements Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface Slow path: If routing table entry is not in cache, packet must be handled by central CPU Route Processor Keeps shared bus architecture, but offloads most IP forwarding to interface cards Cache Memory Shared Bus slow path fast path DMA DMA DMA Route Cache Route Cache Route Cache Memory Memory Memory MAC MAC MAC Interface Cards Drawbacks: Shared bus is still bottleneck 67 nd 2 Another Generation Architecture IP forwarding is done by separate components (Forwarding Engines) Forwarding operations: 1. Packet received on interface: 2. 3. Store the packet in local memory. Extracts IP header and sent to one forwarding engine Forwarding engine does lookup, updates IP header, and sends it back to incoming interface Packet is reconstructed and sent to outgoing interface. Control Bus Forwarding Bus (IP headers only) Data Bus Interface Cards Forwarding Engine Forwarding Engine Route Processor CPU CPU CPU Cache Cache Memory Memory Memory IP header IP datagram Memory Memory Memory MAC MAC MAC 68 3rd Generation Architecture Interconnection network is a switch fabric (e.g., a crossbar switch) Distributed architecture: Interface cards operate independent of each other No centralized processing for IP forwarding These routers can be scaled to many hundred interface cards and to aggregate capacity of > 1 Terabit per second Switch Fabric Switch Fabric Interface Switch Fabric Interface Route Processor Route Processing Route Processing CPU Memory Memory Memory MAC MAC 69 Basic Architectural Components Per-packet processing Routing Table Output Scheduling Switch Fabric Routing Decision Routing Table Forwarding Decision Routing Table Forwarding Decision 70 Cisco 2621XM Router 71 Router commands PDF file 72 Lab this week… Lab 3 (1-4) Common errors How to proceed? echo 1 > /proc/sys/net/ipv4/ip_forward Reboot machines. Wait for graphical system to start before you switch to other machine. Connect the cables. Configure IP addresses and perform the ping test. Interface does not work: change cable, then try other interface, then ask TA. Do not reboot routers in this lab. Three persons in a group One person attaches the cables The other person(s) configure IP addresses / routes on separate machines 73