1 00:00:00,000 --> 00:00:07,000 There are different types of paradigms or algorithms used by routing protocols. 2 00:00:07,000 --> 00:00:13,000 The first type is distance vector also known as routing by rumor. 3 00:00:13,000 --> 00:00:16,000 Distance vector routing protocols will determine 4 00:00:16,000 --> 00:00:20,000 the direction and the distance to a destination. 5 00:00:20,000 --> 00:00:23,000 The direction is also known as a vector 6 00:00:23,000 --> 00:00:30,000 and the distance such as hop count is determined to any destination in the network. 7 00:00:30,000 --> 00:00:39,000 RIP as an example, uses hop count to determine the cost or distance of a remote network. 8 00:00:39,000 --> 00:00:43,000 The advantage of distance vector routing protocols 9 00:00:43,000 --> 00:00:46,000 is that they are very simple to configure. 10 00:00:46,000 --> 00:00:52,000 The major disadvantage of distance vector routing protocols 11 00:00:52,000 --> 00:00:55,000 is that they have limited visibility 12 00:00:55,000 --> 00:00:59,000 they only know what neighbors tell them 13 00:00:59,000 --> 00:01:04,000 and that’s where the concept or analogy of routing by rumor comes from. 14 00:01:04,000 --> 00:01:09,000 It’s possible that routers will learn incorrect information 15 00:01:09,000 --> 00:01:16,000 or make bad choices based on the information that they received from their neighbors. 16 00:01:16,000 --> 00:01:19,000 Distance vector routing protocols 17 00:01:19,000 --> 00:01:23,000 used the Bellman–Ford algorithm to calculate paths. 18 00:01:23,000 --> 00:01:28,000 This requires that a router inform its neighbors of topology changes 19 00:01:28,000 --> 00:01:34,000 periodically and in some cases when changes are detected in the network. 20 00:01:34,000 --> 00:01:41,000 RIP as an example, will advertise its entire routing table every 30 seconds 21 00:01:41,000 --> 00:01:45,000 and will send triggered updates when there’s a change in the topology. 22 00:01:45,000 --> 00:01:51,000 Distance vector means that routers advertise routes 23 00:01:51,000 --> 00:01:54,000 as a vector of the distance and direction. 24 00:01:54,000 --> 00:02:02,000 Direction is represented by the next hop address and local exit interface and distance 25 00:02:02,000 --> 00:02:08,000 uses a metric such as hop count in RIP to determine the cost of a route. 26 00:02:08,000 --> 00:02:13,000 It’s important to understand that distance vector routing protocols 27 00:02:13,000 --> 00:02:19,000 do not have knowledge of the entire path to a destination 28 00:02:19,000 --> 00:02:23,000 hence once again the term routing by rumors 29 00:02:23,000 --> 00:02:29,000 used because the routers are relying on the information that they've received 30 00:02:29,000 --> 00:02:33,000 from other routers and they themselves cannot determine 31 00:02:33,000 --> 00:02:37,000 if the information is actually valid or true. 32 00:02:37,000 --> 00:02:41,000 This can introduce instability and routing loops 33 00:02:41,000 --> 00:02:48,000 and thus multiple features have been introduced to try and combat loops. 34 00:02:48,000 --> 00:02:51,000 As an example, mechanisms or features 35 00:02:51,000 --> 00:02:55,000 used to stop routing loops include split horizon 36 00:02:55,000 --> 00:02:57,000 poisoned reverse and trigger updates. 37 00:02:57,000 --> 00:03:04,000 As an analogy distance vector routing protocols have information available to them 38 00:03:04,000 --> 00:03:10,000 in a similar fashion to the information shown on a road sign. 39 00:03:10,000 --> 00:03:13,000 If you’re on a road going to a destination 40 00:03:13,000 --> 00:03:18,000 and you don’t have a map you must rely on the road signs 41 00:03:18,000 --> 00:03:21,000 to tell you the direction of the destination 42 00:03:21,000 --> 00:03:27,000 and the distance to get to it, in the same way in distance vector routing protocols 43 00:03:27,000 --> 00:03:32,000 a router does not know the entire path to every network segment. 44 00:03:32,000 --> 00:03:38,000 The router only knows the direction or vector in which to send the packet. 45 00:03:38,000 --> 00:03:40,000 The distance vector routing approach 46 00:03:40,000 --> 00:03:43,000 to determine the direction called a vector 47 00:03:43,000 --> 00:03:48,000 and distance such as hop count to any destination network. 48 00:03:48,000 --> 00:03:52,000 So to summarize, distance vector means 49 00:03:52,000 --> 00:03:59,000 that a destination such as 10.1.1.0 is a distance of 5 hops away 50 00:03:59,000 --> 00:04:03,000 in the direction of the next hop router router 2 51 00:04:03,000 --> 00:04:05,000 that statement sums up essentially 52 00:04:05,000 --> 00:04:09,000 how distance vector routing protocol operate. 53 00:04:09,000 --> 00:04:16,000 The destination 10.1.1.0 is a distance of 5 hops away in the direction or router 2 54 00:04:16,000 --> 00:04:24,000 this is similar to our analogy of using a road sign to get to a destination. 55 00:04:24,000 --> 00:04:27,000 Links state routing protocols on the other hand 56 00:04:27,000 --> 00:04:30,000 have a complete map of the area 57 00:04:30,000 --> 00:04:36,000 Every router built its own internal map of the entire network topology 58 00:04:36,000 --> 00:04:40,000 in its link state also called topology database 59 00:04:40,000 --> 00:04:45,000 each router then runs its own shortest path first algorithm 60 00:04:45,000 --> 00:04:50,000 to calculate the shortest path to all known destinations. 61 00:04:50,000 --> 00:04:54,000 An example, of a links state routing protocol is OSPF 62 00:04:54,000 --> 00:04:59,000 which uses cast as its routing metric which is based on bandwidth. 63 00:04:59,000 --> 00:05:03,000 Link state routing protocols cannot be fooled as easily 64 00:05:03,000 --> 00:05:06,000 into making bad routing decisions 65 00:05:06,000 --> 00:05:11,000 because they have a full view of the network topology. 66 00:05:11,000 --> 00:05:16,000 They can make better choices or more informed decisions 67 00:05:16,000 --> 00:05:20,000 because they have a full view of the path 68 00:05:20,000 --> 00:05:22,000 to get to a destination network. 69 00:05:22,000 --> 00:05:27,000 A road sign use by distance vector routing protocols 70 00:05:27,000 --> 00:05:31,000 will just say go left and the network is 5 hops away. 71 00:05:31,000 --> 00:05:34,000 However, a link state routing protocol has a road map 72 00:05:34,000 --> 00:05:38,000 which allows the router to see the path to the destination 73 00:05:38,000 --> 00:05:42,000 and therefore the router can make a better choice 74 00:05:42,000 --> 00:05:46,000 and how to get to that destination network. 75 00:05:46,000 --> 00:05:50,000 The name link state means that each router 76 00:05:50,000 --> 00:05:53,000 originates information about itself 77 00:05:53,000 --> 00:05:59,000 it’s directly connected interfaces or links and the state of those links 78 00:05:59,000 --> 00:06:02,000 in other words other links up or other links down. 79 00:06:02,000 --> 00:06:06,000 This information is then passed between all routers 80 00:06:06,000 --> 00:06:11,000 and each router makes its own copy of that information 81 00:06:11,000 --> 00:06:14,000 but doesn't change the information 82 00:06:14,000 --> 00:06:17,000 That means that all routers end up sharing 83 00:06:17,000 --> 00:06:21,000 the same information within a network or area 84 00:06:21,000 --> 00:06:25,000 and that’s what called a topological database. 85 00:06:25,000 --> 00:06:32,000 In OSPF as an example, you could use a command such as display ospf lsdb 86 00:06:32,000 --> 00:06:37,000 to see the link-state database that’s identical on all routers in the area. 87 00:06:37,000 --> 00:06:42,000 So each router has identical information about the network 88 00:06:42,000 --> 00:06:45,000 and each router can make independent decisions 89 00:06:45,000 --> 00:06:48,000 on how to calculate its best path. 90 00:06:48,000 --> 00:06:52,000 link state routing protocols use an algorithm 91 00:06:52,000 --> 00:06:58,000 called SPF or Shortest Path First which was develop by EW Dijkstra 92 00:06:58,000 --> 00:07:02,000 and is a very powerful algorithm use by protocols 93 00:07:02,000 --> 00:07:07,000 such as OSPF and ISIS, once again links state routing protocols 94 00:07:07,000 --> 00:07:14,000 have a better visibility of the network than distance vector routing protocols do. 95 00:07:14,000 --> 00:07:20,000 So in summary, each router has a copy of the topological database 96 00:07:20,000 --> 00:07:23,000 which is all routes in the entire network or area 97 00:07:23,000 --> 00:07:29,000 and the routers use the Shortest Path algorithm or SPF algorithm 98 00:07:29,000 --> 00:07:32,000 to determine the best part to every destination. 99 00:07:32,000 --> 00:07:37,000 The routers have better visibility than distance vector routing protocols. 100 00:07:37,000 --> 00:07:42,000 However, the disadvantage of link state routing protocols 101 00:07:42,000 --> 00:07:45,000 is that they are more difficult to configure 102 00:07:45,000 --> 00:07:49,000 they require a hierarchical network topology 103 00:07:49,000 --> 00:07:55,000 where you would have a backbone area, area 0 and multiple other areas. 104 00:07:55,000 --> 00:07:59,000 Link state routing protocols also require more memory 105 00:07:59,000 --> 00:08:04,000 because they maintain multiple tables such as a neighbor table 106 00:08:04,000 --> 00:08:08,000 a link state database table and routing table 107 00:08:08,000 --> 00:08:14,000 the SPF or Shortest Past First algorithm can also be process intensive 108 00:08:14,000 --> 00:08:19,000 and thus link state routing protocols require more powerful CPUs 109 00:08:19,000 --> 00:08:24,000 and more memory in routers than distance vector routing protocols do.