Introduction to IP Routing

Transcription

1 Introduction to IP Routing 1

2 Addressing Concepts Routing Protocols Statics & Defaults

3 ISO OSI Reference Model L7 L6 L5 L4 L3 L2 L1 Application Presentation Session Transport Network Layer Data Link Control Physical Layer The network layer provides connectivity and path selection between two hosts systems that could be located on geographically separated networks

4 Routing Protocols L7 L6 L5 L4 L3 L2 L1 Application Presentation Session Transport Network Layer Data Link Control Physical Layer Routing Information Protocol (RIP & RIPv2) Interior Gateway Routing Protocol (IGRP) Open Shortest Path First (OSPF) Protocol Intermediate System to Intermediate System (IS-IS) Enhanced IGRP (EIGRP) Border Gateway Protocol (BGP)

5 Router Functions Routing = building maps and giving directions Switching = moving packets between interfaces Routers are packet switches Path determination is overhead

6 Introduction to IP Addresses UNIX Host Internet TCP/IP UNIX Host Company A Company B Unique addressing allows communication between end stations Path choice is based on location Location is represented by an address

7 IP Addressing 32 Bits Network Host 8 Bits 8 Bits 8 Bits 8 Bits

8 IP Address Classes Class A: Class B: Class C: N H H H N N H H N N N H Class D: for multicast N = Network number assigned by RIPE/ARIN/LACNIC/APNIC/AFRINIC H = Host number assigned by network administrator

9 Host Addresses E0 E IP: IP: Routing Table Network Interface Network Host E0 E1

10 Subnet Addressing E0 E Network Subnet Host Routing Interface Table Network Interface e E0 E E1 E1

11 Subnet Mask IP Address Default Subnet Mask 8-bit Subnet Mask Network Host Network Host Network Subnet Host Use host bits, starting at the high order bit position

12 Discontiguous IP Subnet Where is ? A B C

13 Variable Length Subnet Mask A B Conserve IP addresses.14 C

14 Classless Prefix Notation /16 versus Summarizable blocks of subnets / / / / / / / / /21

15 Classless Versus Classful Routing Protocols Classless routing protocols advertise prefixes and match the longest prefix Subnets must be contiguous with classful routing protocols T1 Frame Relay k 56k 56k

16 Address Configuration Router (config-if) # ip address ip-address subnet-mask Assigns an address and subnet mask Starts IP processing on an interface

17 IP Address Configuration Router (config) # ip netmask-format {bitcount decimal hexadecimal} Sets format of network mask as seen in show commands bitcount /24 decimal hexadecimal xFFFFFF00

19 Convergence Time required for router to identify and use an alternate path Dependent on timer values and algorithm Difficult to predict precisely x

20 Convergence Fault detection time Serial lines: Detection immediate for carrier loss Two to three times 10 second keepalive otherwise Token Ring and FDDI: immediate Ethernet: two to three times keepalive EIGRP Hello can supersede keepalive Two to three times 5 seconds (default) OSPF Hello can supersede keepalive Dead Timer default: 40 seconds R1 Primary path X

21 Load Balancing T1 R2 T1 N1 R1 T1 R3 T1 R4 N2 Equal cost paths Rapid Failover

22 Load Balancing R2 T1 768K N1 R1 512K T1 R4 N2 R3 Unequal cost paths

23 Holddown I Will Ignore Routes to X While in Holddown x Sets minimum convergence time Prevents routing loops

24 Eliminate Holddown for Faster Convergence x 1. Link Fails 2. Networks Flagged as Unreachable Reduce or eliminate holddown, but Temporary routing loops cause traffic surge 3. Router Uses New Routes as Soon as They Arrive (As Long as Hop Count Is not Larger Than Original Hop Count)

25 Routing Loop: A Routing Disagreement Packets for network X Packets do not get to the destination Temporary traffic surge until convergence

26 Split Horizon Do not send routing data back in the direction from which it came

27 Split Horizon Frame Relay Network D 1 PVC A D S0 PVC PVC B C

28 Turn off Split Horizon A 2 B 2 C 2 D 1 Frame Relay Network PVC A D S0 PVC PVC B C

29 Metrics (Cost) Numeric value used to choose among paths RIP/RIPv2 is hop count OSPF/ISIS is interface cost (bandwidth) (E)IGRP is compound BGP is complicated Path determination depends on metric

31 Routing Table One Forwarding Table Per Protocol Network # Interface Next Hop Metric Age Source Ethernet [170/304793] 02:03:50 D Ethernet0 Ethernet0 Ethernet [110/9936] 02:03:50 O [120/3] 00:00:20 R C

32 Building the Routing Table Hardware state Dynamic Static Routes are learned from a protocol Routes are manually defined

33 Routing Protocols Routers are packet switches that forward traffic based on layer 3 logical addresses Routing protocol updates are exchanged by routers to learn about paths to other logical networks Each routing protocol offers features that can make it desirable as part of an internetwork design A B C I Know About: Network A Network B Network C Routing Update Exchanges Network Knowledge I Know About: Network X Network Y Network Z X Y Z

34 Routing Protocol Goals Optimal path selection Loop free routing Fast convergence Limited design administration Minimize update traffic Handle address limitations Support hierarchical topology Incorporate rapid convergence Easy to configure Adapts to changes easily and quickly Does not create a lot of traffic Scales to a large size Compatible with existing hosts and routers Supports variable length subnet masks and discontiguous subnets Supports policy routing

35 IP RIP Widely available Hop count metric Periodic update Easy to implement Usually free RFC 1058 Simple = limited Slow convergence No VLSM No discontiguous subnets Routing loops Count to infinity

36 RIP Distance Vector Net A E0 R1 R2 R3 Net B Net C S0 S0 S1 S0 E0 Net D Network Interface Network Interface Network Interface A E0 B S0 C S0 B S0 C S1 D E0 C S0 A S0 B S0 D S0 D S1 A S0 Send Routing Table to Neighbors

37 Broadcast Routing Updates RIP V1

38 RIP Metric Path A 1 Hop R2 Hops T1 T1 R1 56k R3 Path B 0 Hops

39 When to Use RIP Implementation in a few hours Good for stable links Good for small networks routed in host environment Multivendor environment Non redundant network

40 RIP V2 RFC 1723 Variable length subnet masks Route summarization Classless InterDomain Routing (CIDR) Multicast routing updates Authenticated updates using MD5

41 Multicast Routing Updates RIP V2

42 When to Use RIPv2 Implementation in a few hours Good for stable links Good for small networks routed in host environment Multivendor environment Non redundant network

43 OSPF Open Shortest Path First Link state or SPF technology Developed by OSPF working group of IETF (RFC 1253) Designed expressly for TCP/IP Internet environment Fast convergence Variable length subnet masks Discontiguous subnets No periodic updates Route authentication Delivered two years after IGRP

44 Link State Z s Link State Q s Link State Z Topology Information Is Kept in a Database Separate from the Routing Table Q Y A B C Q Z X X s Link State X OSPF IS IS NLSP

45 Link State Routing Neighbor discovery Constructing an LSA (Link State Advertisement) Distribute LSA Compute routes using SPF (Shortest Path First) On network failure New LSAs flooded All routers recompute routing tables

46 OSPF Areas and Rules Area Border Router Backbone area (0) must be present All other areas must have connection to backbone Backbone must be contiguous Do not partition area (0) Backbone Router Area 2 Area 3 Area 4 Area 0 Area 1 Internal Router Internet Autonomous System (AS) Border Router

47 When to Use OSPF Large hierarchical networks Complex networks, except Topology restrictive Additional network design VLSM Fast convergence Multivendor

48 Agenda Addressing Concepts Routing Protocols Statics & Defaults

49 Static Routes Routes configured manually Useful when few or just one route exist Can be administrative burden Frequently used for default route

50 Floating Static Routes A static route with a high distance Can be overridden by dynamic info T ISDN ip route

51 Default Routes Route used if no match is found in routing table Can be carried by routing protocols Two models Special network number: (IP) Flagged in routing protocol Protocols support multiple models

52 Creating a Default Route RIP, RIPv2: network IGRP, EIGRP: ip default-network OSPF: default originate

53 Comparison of Routing Protocols Link State Traditional Distance Vector Advanced Distance Vector Scalability Good Low Excellent Bandwidth Low High Low Memory High Low Moderate CPU High Low Low Convergence Fast Slow Fast Configuration Moderate Easy Easy

54 Internet Routing Protocols IP routing protocols are characterized as Name Type Proprietary Function Updates Metric VLSM Summ RIP DV No Interior 30 Sec Hops No Auto RIPv2 DV No Interior 30 Sec Hops Yes Auto IGRP DV Yes Interior 90 Sec Comp No Auto EIGRP Adv DV Yes Interior Trig Comp Yes Both OSPF LS No Interior Trig Cost Yes Man IS-IS LS No Int/Ext Trig Cost Yes Auto BGP DV No Exterior Trig N/ A N/A Man

55 Unicast Routing Protocols: RIP, OSPF

56 INTRA AND INTERDOMAIN ROUTING Routing inside an autonomous system is referred to as intradomain routing. Routing between autonomous systems is referred to as interdomain routing. 56

57 Autonomous systems 57

58 Popular routing protocols 58

59 DISTANCE VECTOR ROUTING In distance vector routing, the least cost route between any two nodes is the route with minimum distance. In this protocol each node maintains a vector (table) of minimum distances to every node The topics discussed in this section include: Initialization Sharing Updating When to Share Two-Node Loop Instability Three-Node Instability 59

60 Distance vector routing tables 60

61 Initialization of tables in distance vector routing 61

62 In distance vector routing, each node shares its routing table with its immediate neighbors periodically and when there is a change. 62

63 Updating in distance vector routing 63

64 Two-node instability 64

65 Three-node instability 65

66 RIP The Routing Information Protocol (RIP) is an intradomain routing protocol used inside an autonomous system. It is a very simple protocol based on distance vector routing. The topics discussed in this section include: RIP Message Format Requests and Responses Timers in RIP RIP Version 2 Encapsulation 66

67 Example of a domain using RIP 67

68 RIP message format 68

69 Request messages 69

70 RIP timers 70

71 RIP version 2 format 71

72 Authentication 72

73 Note: RIP uses the services of UDP on well-known port

74 14.4 LINK STATE ROUTING In link state routing, if each node in the domain has the entire topology of the domain, the node can use Dijkstra s algorithm to build a routing table. The topics discussed in this section include: Building Routing Tables 74

75 Figure state routing Concept of link 75

76 Figure knowledge Link state 76

77 Dijkstra algorithm 77

78 OSPF The Open Shortest Path First (OSPF) protocol is an intradomain routing protocol based on link state routing. Its domain is also an autonomous system. The topics discussed in this section include: Areas Metric Types of Links Graphical Representation OSPF Packets Link State Update Packet Other Packets Encapsulation 78