Chapter 13 Routing Protocols (RIP, OSPF, and BGP)

1. Chapter 13Routing Protocols (RIP, OSPF, and BGP)

2. Outline • INTERIOR AND EXTERIOR ROUTING • RIP • OSPF • BGP

3. Introduction • An internet is a combination of networks connected by routers • How to pass a packet from source to destination ? • Which of the available pathways is the optimum pathway ? • Depends on the metric: • Metric: a cost assigned for passing through a network • A router should choose the route with the smallest metric

4. Introduction (Cont.) • The metric assigned to each network depends on the type of protocol • RIP (Routing Information Protocol) • Treat each network as equals • The cost of passing through each network is the same: one hop count • Open Shortest Path First (OSPF) • Allow administrator to assign a cost for passing through a network based on the type of serviced required • For example, maximum throughput or minimum delay • Border Gateway Protocol (BGP) • The criterion is the policy, which can be set by the administrator

5. Introduction (Cont.) • Routing table can be static or dynamic • An internet needs dynamic routing tables • Dynamic routing table is achieved by the routing prococols

6. 13.1 INTERIOR AND EXTERIOR ROUTING The McGraw-Hill Companies, Inc., 2000

7. Interior and Exterior Routing • An internet can be so large that one routing protocol cannot handle the task of updating routing table of all routers • Thus, an internet is divided into autonomous systems (AS) • AS is a group of networks and routers under the authority of a single administration

8. Interior and Exterior Routing • Interior routing • Routing inside an autonomous system • Each AS can chose its own interior routing protocol • Examples: RIP and OSPF • Exterior routing • Routing between autonomous systems • Only one exterior routing protocol is usually used for exterior routing • Examples: BGP

9. Figure 13-1 Popular Routing Protocols The McGraw-Hill Companies, Inc., 2000

10. Example • R1, R2, R3 and R4 use an interior and an exterior routing protocol • Solid thin lines • Interior routing protocol • Broken thick lines • Exterior routing protocol

11. Figure 13-2 Autonomous Systems The McGraw-Hill Companies, Inc., 2000

12. 13.2 RIP: Routing Information Protocol The McGraw-Hill Companies, Inc., 2000

13. RIP • RIP: Routing Information Protocol • Based on distance vector routing • Use the Bellman-Ford algorithm for calculating the routing tables

14. Distance Vector Routing • Each router periodically shares its knowledge about the entire internet with its neighbors • Sharing knowledge about the entire AS • At the start, a router’s knowledge may be sparse • But, how much it knows is unimportant, it sends whatever it has • Sharing only with neighbors • Sends its knowledge only to neighbors • Sharing at regular intervals

15. RIP Updating Information • Routing table is updated on receipt of a RIP response message • Receipt: a response RIP message • Add one hop to the hop count for each advertised destination • Repeat the following steps for each advertised destination • If (destination not in the routing table) • Add the advertised information to the table • Else • If (next-hop field is the same) • Replace retry in the table with the advertised one • Else • If (advertised hop count smaller than one in the table) • Replace entry in the routing table • Return

16. Example of Updating a Routing Table

17. Initializing the Routing Table • When a router is added to a network • It initializes a routing table for itself using its configuration file • The table contains only the directly attached networks and the hop count (= 1)

18. Updating the Routing Table • Each routing table is updated upon receipt of RIP message • Using the RIP updating message algorithm shown above

19. Initial Routing Tables in a Small Autonomous System

20. Final Routing Tables for the Previous Figure

21. RIP Message Format • Command: 8-bit • The type of message: request (1) or response (2) • Version: 8-bit • Define the RIP version • Family: 16-bit • Define the family of the protocol used • TCP/IP: value is 2 • Address: 14 bytes • Defines the address of the destination network • 14 bytes for this field to be applicable to any protocol • However, IP currently uses only 4 bytes, the rest are all 0s • Distance: 32-bit • The hop count from the advertising router to the destination network

22. Figure 13-6 RIP Message Format The McGraw-Hill Companies, Inc., 2000

23. Requests and Response • RIP uses two type of messages • Request and response • Request • Sent by a router that has just come up or has some time-out entries • Can ask specific entries or all entries

24. Figure 13-7 Request Messages The McGraw-Hill Companies, Inc., 2000

25. Requests and Response (Cont.) • Response: solicited or unsolicited • A solicited response: sent only in answer to a request • Contain information about the destination specified in the corresponding request • An unsolicited response: sent periodically • Every 30s • Contains information about the entire routing table • Also called update packet

26. Example 1 • What is the periodic response sent by router R1 in the next slide? • Assume R1 knows about the whole autonomous system.

27. Figure 13-8 Example 1 The McGraw-Hill Companies, Inc., 2000

28. Solution • R1 can advertise three networks 144.2.7.0, 144.2.9.0, and 144.2.12.0. • The periodic response (update packet) is shown in the next slide.

29. Figure 13-9 Solution to Example 1 The McGraw-Hill Companies, Inc., 2000

30. Timers in RIP • RIP uses three timers • Periodic timer • Expiration timer • Garbage collection timer

31. Figure 13-10 RIP Timers The McGraw-Hill Companies, Inc., 2000

32. Periodic Timer • Periodic timer • Control the advertising of regular update message • Although protocol specifies 30 s, the working model uses a random number between 25 and 35 s • Prevent routers update simultaneously

33. Expiration Timer • Govern the validity of a route • Set to 180 s for a route when a router receives update information for a route • If a new update for the route is received, the timer is reset • In normal operation, this occurs every 30 s • If timer goes off, the route is considered expired • The hop count of the route is set to 16, which means destination is unreachable

34. Garbage Collection Timer • When a route becomes invalid, the router does not immediately purge that route from its table • It continues advertise the route with a metric value of 16 • A garbage collection timer is set to 120 s for that route • When the count reaches zero, the route is purged from the table • Allow neighbors to become aware of the invalidity of a route prior to purging

35. Example 2 • A routing table has 20 entries. • It does not receive information about five routes for 200 seconds. • How many timers are running at this time?

36. Solution • The timers are listed below: • Periodic timer: 1 • Expiration timer: 20 - 5 = 15 • Garbage collection timer: 5

37. Problems in RIP • Slow Convergence • Instability

38. Slow Convergence • A change somewhere in the internet propagates very slowly through the rest of the internet • For example • A change is Net1, R1 updates itself immediately • R1->R2: an average of 15 s • R2->R3: an average of 15 s • … • Thus, R1-Rn: an average of 15 x n s

39. Figure 13-11 Slow Convergence The McGraw-Hill Companies, Inc., 2000

40. Solution • Limit the hop count to 15 • Prevent data packet from wandering around forever • Thus, an autonomous system using RIP is limited to a diameter of 15 • The number 16 is considered infinity and designates an unreachable network

41. Figure 13-12 Hop count The McGraw-Hill Companies, Inc., 2000

42. Instability • An internet running RIP can become unstable • A packet could go from one router to another in a loop

43. Instability

44. Solutions • Triggered Update • Split Horizons • Poison Reverses

45. Triggered Update • If there is no change, updates are sent at the 30-s intervals • If there is a change, the routers sends out its new table immediately

46. Split Horizons • A router must distinguish between different interface • If a router received route updating message from an interface • This same updated information must not be sent back through this interface • Example • B receives information about Net1 and Net2 through its left interface • This information is updated and passed on through the right interface but not to the left

47. Figure 13-14 Split Horizon The McGraw-Hill Companies, Inc., 2000

48. Poison Reverse • A variation of split horizons • Information received is used to update routing table and then passed out to all interface • However, a table entry that has come through one interface is set to a metric of 16as it goes out through the same interface • For example • Router B has received information about Net1 and Net2 through its left interface • Thus, it sends information out about Net1 and Net2 with a metric of 16

49. Figure 13-15 Poison Reverse The McGraw-Hill Companies, Inc., 2000

50. RIP Version 2 • Does not augment the length of the message of each entry • Only replace those fields in version 1 that were filled with 0s with some new fields