Bài giảng môn Mạng máy tính - Chapter 4: Distance vector routing protocols

Chapter 4 – Distance Vector Routing Protocols CCNA Exploration version 4.0 Học viện mạng Bách Khoa - Website: www.bkacad.com 2 Objectives • Identify the characteristics of distance vector routing protocols. • Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP). • Describe the processes to maintain accurate routing tables used by distance vector routing protocols. • Identify the conditions leading to a routing loop and explain the implications for router performance. • Recognize that distance vector routing protocols are in use today Học viện mạng Bách Khoa - Website: www.bkacad.com 3 Introduction to Distance Vector Routing Protocols Học viện mạng Bách Khoa - Website: www.bkacad.com 4 Distance Vector Routing Protocols 28 routers shown in the figure Học viện mạng Bách Khoa - Website: www.bkacad.com 5 Distance Vector Routing Protocols • Dynamic routing protocols help the network administrator overcome the time-consuming and exacting process of configuring and maintaining static routes. • What happens when a link goes down? • How do you ensure that redundant paths are available? Học viện mạng Bách Khoa - Website: www.bkacad.com 6 Distance Vector Routing Protocols Routing Information Protocol (RIP): RFC 1508 • Hop count is used as the metric • Maximum hop count is 15 • Routing updates are broadcast (ver1) or multicast (ver2) every 30 seconds Học viện mạng Bách Khoa - Website: www.bkacad.com 7 Distance Vector Routing Protocols Interior Gateway Routing Protocol (IGRP) • Proprietary protocol developed by Cisco. • Bandwidth, delay, load and reliability are used to create a composite metric. • Routing updates are broadcast every 90 seconds, • IGRP is the predecessor of EIGRP and is now obsolete Học viện mạng Bách Khoa - Website: www.bkacad.com 8 Distance Vector Routing Protocols Enhanced Interior Gateway Routing Protocol (EIGRP) • It can perform unequal cost load balancing. • It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path. • Routing updates are sent only when there is a change in the topology. Học viện mạng Bách Khoa - Website: www.bkacad.com 9 Distance Vector Technology Học viện mạng Bách Khoa - Website: www.bkacad.com 10 Distance Vector Technology • The Meaning of Distance Vector: a router using distance vector routing protocols knows 2 things: 1. Distance to final destination 2. Vector or direction traffic should be directed • Routers using distance vector routing are not aware of the network topology. • The router knows only: 1. The direction or interface in which packets should be forwarded 2. The distance or how far it is to the destination network Học viện mạng Bách Khoa - Website: www.bkacad.com 11 Characteristics of DV Routing Protocol 1.Periodic updates 2.Neighbors: Routing by rumor 3.Broadcast updates 255.255.255.255 4.Entire routing table is included with routing update Routing by Rumo (hop-by-hop update) Học viện mạng Bách Khoa - Website: www.bkacad.com 12 Học viện mạng Bách Khoa - Website: www.bkacad.com 13 Characteristics of DV Routing Protocol Animation 4.1.2.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 14 Routing Protocol Algorithms Animation 4.1.3 • The algorithm used for the routing protocols defines the following processes: 1.Mechanism for sending and receiving routing information. 2.Mechanism for calculating the best paths and installing routes in the routing table. 3.Mechanism for detecting and reacting to topology changes. Học viện mạng Bách Khoa - Website: www.bkacad.com 15 Routing Protocol Characteristics • Criteria used to compare routing protocols includes: 1. Time to convergence 2. Scalability 3. Classless (use of VLSM) or Classful 4. Resource usage 5. Implementation & maintenance Học viện mạng Bách Khoa - Website: www.bkacad.com 16 Routing Protocol Characteristics Học viện mạng Bách Khoa - Website: www.bkacad.com 17 Distance Vector Routing Protocols Animation 4.1.4.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 18 Network Discovery Học viện mạng Bách Khoa - Website: www.bkacad.com 19 Cold Start Router initial start up
After a cold start and before the exchange of routing information, the routers initially discover their own directly connected networks and subnet masks. Animation 4.2.1.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 20 Exchange of Routing Information Initial Exchange: • If a routing protocol is configured then – Routers will exchange routing information • Routing updates received from other routers.Router checks update for new information. If there is new information: 1. Metric is updated 2. New information is stored in routing table • Router convergence is reached when all routing tables in the network contain the same network information Routers continue to exchange routing information. • If no new information is found then Convergence is reached Animation 4.2.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 21 Initial Exchange of Routing Information 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 10.1.0.0
0 Routing Table 10.2.0.0  0 10.2.0.0
0 Routing Table 10.3.0.0  0 10.3.0.0
0 Routing Table 10.4.0.0  0 10.3.0.0  1 10.1.0.0
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1 10.4.0.0  1 10.4.0.0  2 10.1.0.0  2 Học viện mạng Bách Khoa - Website: www.bkacad.com 22 Convergence • Convergence must be reached before a network is considered completely operable • Speed of achieving convergence consists of 2 interdependent categories 1. Speed of broadcasting routing information update 2. Speed of calculating routes Animation 4.2.4.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 23 Routing Table Maintenance Học viện mạng Bách Khoa - Website: www.bkacad.com 24 Periodic Updates : RIPv1 & IGRP Học viện mạng Bách Khoa - Website: www.bkacad.com 25 Periodic Updates : RIPv1 & IGRP • Periodic updates are time intervals in which a router sends out its entire routing table. • Changes may occur for several reasons, including: 1. Failure of a link 2. Introduction of a new link 3. Failure of a router 4. Change of link parameters Học viện mạng Bách Khoa - Website: www.bkacad.com 26 Periodic Updates : RIPv1 & IGRP Học viện mạng Bách Khoa - Website: www.bkacad.com 27 Periodic Updates : RIPv1 & IGRP •RIP uses 4 timers 1. Update timer 30s 2. Invalid timer 180s 3. Holddown timer 180s 4. Flush timer 240s RIP Timers Update 30 secs – Update and Invalid timers are then reset. Invalid 180 secs - Invalid timer expires, route now “unreachable” but still in routing table until flush timer expires. Holddown timer of 180 sec begins. Holddown 180 secs – Holddown timer expires, will now accept a poorer route to same network. Still in routing table Flush 240 secs – Route will now be removed from the routing table. Periodic Updates : RIPv1 & IGRP • Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing table until the flush timer expires. • Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table. • Holddown Timer. This timer stabilizes routing information and helps prevent routing loops during periods when the topology is converging on new information. Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology to learn about the unreachable network. By default, the holddown timer is set for 180 seconds. Học viện mạng Bách Khoa - Website: www.bkacad.com 29 Học viện mạng Bách Khoa - Website: www.bkacad.com 30 Bounded Updates: EIGRP EIRPG routing updates are 1. Partial updates 2. Triggered by topology changes 3. Bounded: meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated. 4. Non periodic Học viện mạng Bách Khoa - Website: www.bkacad.com 31 Triggered Updates Conditions in which triggered updates are sent: 1. An interface changes state 2. A Route becomes unreachable 3. A Route is placed in routing table Animation 4.3.3 Học viện mạng Bách Khoa - Website: www.bkacad.com 32 Triggered Updates Two problems: 1. Packets containing the update message can be dropped or corrupted by some link in the network. 2. The triggered updates do not happen instantaneously. It is possible that a router that has not yet received the triggered update will issue a regular update at just the wrong time, causing the bad route to be reinserted in a neighbor that had already received the triggered update. Học viện mạng Bách Khoa - Website: www.bkacad.com 33 Random Jitter • Synchronized updates: a condition where multiple routers on multi access LAN segments transmit routing updates at the same time. • Problems: – Bandwidth consumption – Packet collisions • Solution: • used of random variable called RIP_JITTER, 0% to 15% of the specified update interval (25 to 30 seconds for the default 30-second interval) Học viện mạng Bách Khoa - Website: www.bkacad.com 34 Random Jitter Animation 4.3.4.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 35 Routing Loop Học viện mạng Bách Khoa - Website: www.bkacad.com 36 Definitions & Implications • Routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination. Học viện mạng Bách Khoa - Website: www.bkacad.com 37 Definitions & Implications Routing loops may be caused by: 1. Incorrectly configured static routes 2. Incorrectly configured route redistribution 3.Slow convergence 4. Incorrectly configured discard routes Animation 4.4.1.1 2. Incorrectly configured route redistribution Học viện mạng Bách Khoa - Website: www.bkacad.com 38 Học viện mạng Bách Khoa - Website: www.bkacad.com 39 Definitions & Implications Routing loops can create the following conditions: 1.Excess use of bandwidth 2.CPU resources may be strained 3.Network convergence is degraded 4.Routing updates may be lost or not processed in a timely manner Animation 4.4.1.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 40 The Solutions 1.Defining a maximum metric to prevent count to infinity 2.Holddown timers 3.Split horizon 4.Route poisoning or poison reverse 5.Triggered updates Học viện mạng Bách Khoa - Website: www.bkacad.com 41 Definitions & Implications • Lab 4.4.1.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 42 Count to Infinity • This is a routing loop whereby packets bounce infinitely around a network. Animation 4.4.2.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 43 Setting a maximum • Distance Vector routing protocols set a specified metric value to indicate infinity • Once a router “counts to infinity” it marks the route as unreachable Học viện mạng Bách Khoa - Website: www.bkacad.com 44 Setting a maximum Học viện mạng Bách Khoa - Website: www.bkacad.com 45 Setting a maximum Học viện mạng Bách Khoa - Website: www.bkacad.com 46 Preventing Routing Loops with Holddown Timers • Network is unstable: a interface resets as up, then down, then up again in rapid succession. The route is flapping. • Using triggered updates, the routers might react too quickly and unknowingly create a routing loop. • A routing loop could also be created by a periodic update that is sent by the routers during the instability. Học viện mạng Bách Khoa - Website: www.bkacad.com 47 Preventing Routing Loops with Holddown Timers • Holddown timers allow a router to not accept any changes to a route for a specified period of time • Holddown timers prevent routing loops from being created by these conditions. • Holddown timers also help prevent the count to infinity condition. • Point of using holddown timers: Allows routing updates to propagate through network with the most current information. Học viện mạng Bách Khoa - Website: www.bkacad.com 48 Holddown timers 1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible. Refer to 4.4.4 Học viện mạng Bách Khoa - Website: www.bkacad.com 49 Holddown timers 2. The router marks the network as possibly down and starts the holddown timer. Học viện mạng Bách Khoa - Website: www.bkacad.com 50 Holddown timers 3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed. Học viện mạng Bách Khoa - Website: www.bkacad.com 51 Holddown timers 4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated. Học viện mạng Bách Khoa - Website: www.bkacad.com 52 Holddown timers 5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires. Holddown timers Học viện mạng Bách Khoa - Website: www.bkacad.com 53 Học viện mạng Bách Khoa - Website: www.bkacad.com 54 Split Horizon Rule • A router should not advertise a network through the interface from which the update came. Animation 4.4.5.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 55 Split horizon with Poison Reverse or Route Poisoning • Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. • Split horizon with poison reverse: – The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface • Animation 4.4.6.1 • Animation 4.4.6.2 • Disable the split-horizon rule: • RIP • (config-if)# no ip split-horizon • EIGRP • (config-if)# no ip split-horizon EIGRP {as_number} Học viện mạng Bách Khoa - Website: www.bkacad.com 56 Học viện mạng Bách Khoa - Website: www.bkacad.com 57 IP & TTL • Purpose of the TTL field – The TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network Học viện mạng Bách Khoa - Website: www.bkacad.com 58 IP & TTL • How the TTL field works – TTL field contains a numeric value – The numeric value is decreased by one by every router on the route to the destination. – If numeric value reaches 0 then Packet is discarded Học viện mạng Bách Khoa - Website: www.bkacad.com 59 Distance Vector Routing Protocols today Học viện mạng Bách Khoa - Website: www.bkacad.com 60 RIP and EIGRP • Factors used to determine whether to use RIP or EIGRP include 1. Network size 2. Compatibility between models of routers 3. Administrative knowledge Học viện mạng Bách Khoa - Website: www.bkacad.com 61 RIP • Supports split horizon & split horizon with poison reverse • Capable of load balancing • Easy to configure • Works in a multi vendor router environment Học viện mạng Bách Khoa - Website: www.bkacad.com 62 RIP RIPv2 introduced the following improvements to RIPv1: 1. Includes the subnet mask in the routing updates, making it a classless routing protocol. 2. Has authentication mechanism to secure routing table updates. 3. Supports variable length subnet mask (VLSM). 4. Uses multicast addresses instead of broadcast. 5. Supports manual route summarization. Học viện mạng Bách Khoa - Website: www.bkacad.com 63 EIGRP 1. Triggered updates 2. EIGRP hello protocol used to establish neighbor adjacencies 3. Supports VLSM & route summarization 4. Use of topology table to maintain all routes 5. Classless distance vector routing protocol 6. Cisco proprietary protocol Học viện mạng Bách Khoa - Website: www.bkacad.com 64 Distance Vector Routing Protocols Compared Animation 4.5.1.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 65 Summary Q&A • Three routers running a distance-vector routing protocol lost all power, including the battery backups. When the routers reload, what will happen? –They will send updates that include only directly connected routes to their directly connected neighbors. Học viện mạng Bách Khoa - Website: www.bkacad.com 66 Q&A • What does the RIP holddown timer do? –instructs routers to ignore updates, for a specified time or event, about possible inaccessible routes Học viện mạng Bách Khoa - Website: www.bkacad.com 67 Q&A • Which of the following methods does split horizon use to reduce incorrect routing information? –Information learned from one source is not distributed back to that source Học viện mạng Bách Khoa - Website: www.bkacad.com 68 Q&A • Which of the following can exist in a distance vector network that has not converged? –routing loops –inconsistent traffic forwarding –inconsistent routing table entries Học viện mạng Bách Khoa - Website: www.bkacad.com 69 Q&A • If all routers are using RIP, how many rounds of updates will occur before all routers know all networks? Học viện mạng Bách Khoa - Website: www.bkacad.com 70 Học viện mạng Bách Khoa - Website: www.bkacad.com 71