ENSA M1 Single Area OSPFv2 Concepts — Transcript

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1. 0:03[Music]
2. 0:11hi
3. 0:11hello there welcome to single area ospf
4. 0:15version 2 concepts
5. 0:18so imagine that it is time for your
6. 0:19family to visit your grandparents
7. 0:22you pack your bags and load them into
8. 0:24the car
9. 0:25but it takes a bit longer than you
10. 0:27planned for and now you are running late
11. 0:31so you pull out your map okay so there
12. 0:34are three different routes
13. 0:36so one route is no good because there is
14. 0:39a lot of construction on the main road
15. 0:41and it is temporarily closed
16. 0:44so another route is a very scenic for
17. 0:46instance
18. 0:47but it takes an additional r to get to
19. 0:50your destination
20. 0:53so the third route is not as pretty
21. 0:56but it includes a highway which is much
22. 0:59faster so in fact it is so much faster
23. 1:03than
24. 1:03you might actually be on time if you
25. 1:05take it
26. 1:07so in networking packets do not need to
27. 1:10take
28. 1:11the scenic route so the fastest
29. 1:13available route is always the best
30. 1:16so ospf or the open shortest path first
31. 1:20is designed to find the fastest
32. 1:21available path
33. 1:23for a packet from source to destination
34. 1:28so this module covers the basic concepts
35. 1:32of a single area ospf version 2.
36. 1:35right so let's get started
37. 1:42okay so for the module objectives
38. 1:46so the module title is single area ospf
39. 1:48concepts
40. 1:50so the module objective is at the end of
41. 1:52this video lecture
42. 1:54you should be able to explain how a
43. 1:56single area spf operates
44. 1:58in both point-to-point and broadcast
45. 2:01multi-access network so sub-topics
46. 2:05includes the ospf features and
47. 2:07characteristics
48. 2:09the ospf packets and the ospf
49. 2:12operations
50. 2:18all right so let's start with the first
51. 2:20section let's talk about
52. 2:22the ospf features and characteristics
53. 2:28so introduction to ospf
54. 2:31so this topic is a brief overview
55. 2:34of the open source test path first or
56. 2:37ospf
57. 2:38which includes a single area and multi
58. 2:41area
59. 2:43so ospf version 2 is used for ipv4
60. 2:46networks
61. 2:48ospf version 3 is used for ipv6 network
62. 2:52so the primary focus of
63. 2:55this entire module is single area ospf
64. 2:58version 2.
65. 3:00so ospf is a link state routing protocol
66. 3:04that was developed as an alternative
67. 3:08for the distance vector routing
68. 3:09information protocol or the rip
69. 3:12so rip was unacceptable
70. 3:16a routing protocol in the early days of
71. 3:19networking
72. 3:20and the internet so however
73. 3:23the reap reliance on hub count as the
74. 3:26only metric for determining the best
75. 3:28route quickly became problematic so
76. 3:31using the hap count does not scale well
77. 3:34in a large networks with multiple paths
78. 3:37of varying speeds so ospf has
79. 3:40significant advantages over rip
80. 3:43that it has offered faster convergence
81. 3:47and scales to much larger network
82. 3:50implementation
83. 3:53so ospf is a link state routing protocol
84. 3:57that uses the concept of areas
85. 4:00so a network administrator can divide
86. 4:03the routing domain into distinct areas
87. 4:06that help control routing update traffic
88. 4:09so a link state or a link is an
89. 4:12interface on a router
90. 4:14so a link is also a network segment
91. 4:17that connects two routers or a stop
92. 4:20network such as the ethernet lan
93. 4:24that is connected to a single router
94. 4:27so information about the state of the
95. 4:29link
96. 4:30is known as the link state
97. 4:33so all link state information includes
98. 4:36the network prefix
99. 4:38the prefix length and the cost so this
100. 4:42module covers the basic
101. 4:44single area ospf implementations and
102. 4:47configurations
103. 4:52so what are the components of ospf
104. 4:56so all routing protocols share similar
105. 4:58components
106. 5:00they all use routing protocols messages
107. 5:03to exchange route information so the
108. 5:06messages
109. 5:07help build data structures
110. 5:10okay so which are then processed
111. 5:14using a routing algorithm so basically
112. 5:17components of ospf includes routing
113. 5:20control messages
114. 5:21data structures and algorithm
115. 5:27now let's talk about the routing control
116. 5:29messages
117. 5:31okay so routers running ospf exchange
118. 5:34messages to convey routing information
119. 5:36using five types of packets
120. 5:39so these packets as shown here in the
121. 5:41figure
122. 5:42are as follows so you've got the hello
123. 5:44packets
124. 5:46okay so database description packet the
125. 5:49link state request packets
126. 5:51the link state update packets and
127. 5:54the link state acknowledgement packets
128. 5:57okay now these packets are used to
129. 6:00discover
130. 6:01neighboring routers and also
131. 6:04to exchange router information to
132. 6:06maintain accurate
133. 6:08information about the network
134. 6:13so for the data structures so all spf
135. 6:16messages are used to create and maintain
136. 6:19three ospf databases
137. 6:21as follows so you've got the adjacency
138. 6:24database
139. 6:26this creates the neighbor table okay
140. 6:30so you also have the link state database
141. 6:32or lsdb
142. 6:34this creates the topology table and
143. 6:37you've got the forwarding database
144. 6:39this creates the routing table now this
145. 6:42table
146. 6:43here contains a list of neighboring
147. 6:45routers to exchange
148. 6:47routing information the tables are kept
149. 6:50and maintained
150. 6:51in ram so in the following table
151. 6:54take a particular note of the command
152. 6:57used to display each
153. 6:58table so basically if you want to see
154. 7:01the content
155. 7:02of the neighbor table so we can view it
156. 7:05by using the command show ipo spf
157. 7:08neighbor
158. 7:10okay now the topology table can be seen
159. 7:14using the show ipo spf database command
160. 7:18and of course the routing table which is
161. 7:20also similar to
162. 7:22static routing we can see the routing
163. 7:24table
164. 7:25via the command show ip route
165. 7:28okay so basically the difference between
166. 7:31these tables are as follows
167. 7:33so when you say neighbor table okay so
168. 7:36it lists down all the neighbor routers
169. 7:39to which a router has established a
170. 7:41bi-directional communication okay
171. 7:44so topology table lists information
172. 7:47about
173. 7:48all the routers in the network so the
174. 7:51database
175. 7:51represents the network lsdp
176. 7:55so all routers within an area have an
177. 7:58identical lsdb
178. 8:01right so the last table is a routing
179. 8:04table
180. 8:05so this are the list of routes generated
181. 8:09when an algorithm is run on the link
182. 8:12state database
183. 8:14so ospf uses the spf or the shortest
184. 8:17path algorithm
185. 8:19or also known as the dixtras algorithm
186. 8:22so each router's routing table is unique
187. 8:25and contains information on how and
188. 8:28where to send packets
189. 8:30to other routers
190. 8:38okay so next would be the algorithm
191. 8:42okay so that's what i'm saying ospf uses
192. 8:46the algorithms
193. 8:47called dextras algorithm or also known
194. 8:49as the spf
195. 8:51or the shortest path first so the spf
196. 8:54algorithm
197. 8:55is based on the cumulative cost to reach
198. 8:57the destination
199. 8:59so the spf algorithm creates an spf
200. 9:033 by placing its router
201. 9:06at the root of the tree and calculating
202. 9:08the shortest path
203. 9:09to each node so the spf3
204. 9:13is then used to calculate the best
205. 9:15routes
206. 9:16so ospf places the best routes into the
207. 9:19for wiring database
208. 9:21which is used to make the routing table
209. 9:25all right now let's talk about the link
210. 9:29state operations
211. 9:31so to maintain information ospf routers
212. 9:36complete a genetic link state routing
213. 9:38process
214. 9:39to reach a state of convergence so when
215. 9:42you say state of convergence
216. 9:43we're talking about the disability of
217. 9:46the
218. 9:46network so the figure here
219. 9:50okay shows a five router
220. 9:54topology it's linked between the router
221. 9:57which are labeled okay
222. 10:02now this includes
223. 10:05okay so establishing a neighbor
224. 10:08adjacencies
225. 10:10right so the exchange of blink state
226. 10:12advertisements
227. 10:14build the link state database execute
228. 10:17the shortest path first
229. 10:19and choosing the best route
230. 10:24okay now let's talk about the first one
231. 10:27so which is establish neighbor
232. 10:29adjacencies
233. 10:31so ospf enabled routers must recognize
234. 10:35each other on the network before they
235. 10:37can share information
236. 10:40so an ospf enabled router sends
237. 10:43hello packets out all ospf enabled
238. 10:47interfaces
239. 10:48to determine okay if
240. 10:52neighbors are present on those links so
241. 10:55if a neighbor is present the ospf
242. 10:58enabled router attempts to establish
243. 11:01neighbor adjacency with that neighbor
244. 11:04okay so each of the router here will
245. 11:07send hello packets to each other
246. 11:09that is establishing a neighbor
247. 11:12adjacency
248. 11:14so the next step is exchange link state
249. 11:17at participants
250. 11:19okay so the link state advertisements
251. 11:22or the lsas okay so after the
252. 11:25adjacencies are established
253. 11:27so routers then exchange link state
254. 11:29advertisements or lsas
255. 11:32okay so lsas contains the state and cost
256. 11:35of each directly connected
257. 11:37link so routers flood their lsas
258. 11:41to adjacent neighbors so adjacent
259. 11:44neighbors
260. 11:44receiving the lsas immediately flood the
261. 11:47lsa
262. 11:48to other directly connected neighbors so
263. 11:51until
264. 11:52all routers in the area have the lsas
265. 11:57all right so the third one would be
266. 12:01building the link state database okay
267. 12:04so after the lsa are received
268. 12:08ospf enabled routers build the topology
269. 12:11table
270. 12:12all right so or the lsdb based on the
271. 12:15received lsas
272. 12:17so this database eventually holds all
273. 12:20the information about the topology
274. 12:22of the area right
275. 12:27so the next step would be router will
276. 12:30execute the spf or the circus pass
277. 12:33algorithm or
278. 12:34also known as the dixor's algorithm so
279. 12:37routers then
280. 12:38execute this spf algorithm okay
281. 12:41so the gears in the figure for this
282. 12:44step are used to indicate the execution
283. 12:47of the spf algorithm so the spf
284. 12:51algorithm creates
285. 12:52the spf3
286. 12:57and then we have now choosing the best
287. 13:00route okay so after the spf3 is built
288. 13:06from the previous step the best path
289. 13:09to each network are offered to the ipo
290. 13:12routing table
291. 13:13so the route will be inserted into the
292. 13:16routing table
293. 13:17unless there is a route source to the
294. 13:19same network
295. 13:20with a lower administrative distance
296. 13:24so such as a static graph right so if
297. 13:27you still remember
298. 13:28static route has a value or an
299. 13:30administrative distance of one
300. 13:32okay so ospf has an administrative
301. 13:36distance of 110
302. 13:39okay so routing decisions are made based
303. 13:42on the entries
304. 13:43in the routing table okay
305. 13:48so numbers here okay so
306. 13:512 20 25 2
307. 13:55those are the cost to reach the
308. 13:56destination
309. 13:58so for instance if our destination is 10
310. 14:005
311. 14:010 0 16 okay so that would be reference
312. 14:05to r1
313. 14:06so 10 5
314. 14:100 0 okay so is located
315. 14:14on r2 okay so it's the land of
316. 14:17r2 here now if that would be our
317. 14:20destination from r1
318. 14:22then the cert the circus path
319. 14:26okay so to get into that destination is
320. 14:28via r2
321. 14:29and the cost is 22. so that would be
322. 14:32the cost between r1 and r2 this figure
323. 14:35is 20.
324. 14:36and you've got from r2 going to the lan
325. 14:39okay you've got one you've got two there
326. 14:42so that means it's 22.
327. 14:45all right
328. 14:48so next would be the single area and the
329. 14:51multi-area
330. 14:52spf so to make an ospf more efficient
331. 14:56and scalable ospf
332. 14:58supports hierarchical routing using
333. 15:00areas
334. 15:02so an ospf area is a group of routers
335. 15:05that share the same link state
336. 15:07information and their
337. 15:08lsdbs so ospf can be implemented
338. 15:12in one of two ways are as follows
339. 15:17okay so you've got the single area ospf
340. 15:21where in all routers are in one area
341. 15:24so best practice is to use area
342. 15:27zero okay so as always if you are not
343. 15:30given
344. 15:31any area on the topology it is
345. 15:32considered to be an area zero
346. 15:35okay if we are working on a single area
347. 15:38it's always an area zero
348. 15:41so the next one is a multi-area ospf
349. 15:45so ospf is implemented using multiple
350. 15:48areas
351. 15:49in a hierarchical fashion
352. 15:53so all the areas must connect to the
353. 15:55backbone
354. 15:56okay so which is area 0. so
355. 15:59routers interconnecting the areas are
356. 16:02referred to as
357. 16:03the area border routers or the abrs
358. 16:06okay so the focus of this module
359. 16:10is on the single area ospf version 2.
360. 16:17okay so how about multi-area ospf
361. 16:21so with multi-area ospf one large
362. 16:24routing domain can be divided into
363. 16:26smaller areas
364. 16:27to support hierarchical routing so
365. 16:30routing is still a course
366. 16:32between the areas you've got the inter
367. 16:34area
368. 16:35routing so while many of the process
369. 16:38source intensive routing operations
370. 16:40such as recalculating the database are
371. 16:44kept
372. 16:44within the area okay
373. 16:47so for instance any time a router
374. 16:51receives
375. 16:51new information about the topology
376. 16:53change
377. 16:55so within the area including the
378. 16:56addition deletion
379. 16:58or modification of a link so the router
380. 17:01must rerun the spf algorithm
381. 17:03creating a new spf tree
382. 17:07and update the routing table so
383. 17:10the spf algorithm is cpu
384. 17:13intensive and the time it takes for the
385. 17:16calculation
386. 17:17depends on the size of the area
387. 17:21okay so routers in other areas
388. 17:24receive updates regarding topology
389. 17:27changes
390. 17:28but these routers only update the
391. 17:31routing table
392. 17:33not run the spf algorithm
393. 17:36so too many routers in one area
394. 17:39would make the lsdb very large and
395. 17:42increase
396. 17:42the load to the cpu so therefore
397. 17:46arranging the routers into areas
398. 17:48effectively partitions
399. 17:50a potentially large database into a
400. 17:53smaller
401. 17:53and more manageable databases
402. 17:57okay now that hierarchical topology that
403. 18:00i'm referring to
404. 18:02okay so share this advantages here
405. 18:06okay so smaller routing tables
406. 18:10reduced link state update overhead and
407. 18:12reduced
408. 18:13frequency of the spf calculations
409. 18:17now in the multi-area ospf so the
410. 18:20network are divided into different areas
411. 18:23but the requirement is all non-area zero
412. 18:27like area 1 here in the figure and area
413. 18:3051
414. 18:31should be connected to the backbone or
415. 18:34area 0. so if you will observe your area
416. 18:371 which is the non-area 0
417. 18:40and area 51 which is also a non-area 0
418. 18:43it should be connected to an asbr
419. 18:47okay which are r1 and r2 of
420. 18:50area 0.
421. 18:56all right so let's talk about ospf
422. 18:58version 3.
423. 19:00so ospf version 3 is the ospf version
424. 19:04equivalent for exchanging ipv6
425. 19:08prefixes so recall that in ipv6
426. 19:12the network address is referred to as
427. 19:14the prefix
428. 19:15and the subnet mask is called the prefix
429. 19:17length
430. 19:19okay so similar to ipv4
431. 19:22okay so this is its counterpart ospf
432. 19:26version 3 exchanges routing information
433. 19:28to populate the ipv6 routing table
434. 19:31with remote prefixes so take note that
435. 19:35with the ospf version 3 address families
436. 19:37feature
437. 19:38the ospf version 3 includes support for
438. 19:41both
439. 19:42ipv4 and ipv6
440. 19:45okay so os address families is beyond
441. 19:48the scope of this curriculum
442. 19:52all right so ospf version 2
443. 19:55runs over the ipv4 network layer
444. 19:59communicating with other ospf ipb for
445. 20:02peers
446. 20:03and advertising on the ipv4 routes
447. 20:07now for ospf version 3 okay
448. 20:10this has the same functionality as ospf
449. 20:13version 2
450. 20:14but uses ipv6 as a network layer
451. 20:17transport
452. 20:18communicating with ospf version 3 peers
453. 20:21and advertising
454. 20:22ipv6 routes
455. 20:25so ospf version 3 also uses the spf
456. 20:29algorithm
457. 20:30or dixor's algorithm as the computation
458. 20:33engine to determine
459. 20:34the best path throughout the routing
460. 20:36domain
461. 20:38ospf version 3 has separate
462. 20:41process from its ipv4 counterpart
463. 20:44so the process and operations are
464. 20:46basically the same as an ipb4
465. 20:49routing protocol but run independently
466. 20:53so ospf version 2 and version 3
467. 20:57each hub separate adjacency tables
468. 21:00ospf topology tables and ip routing
469. 21:04tables
470. 21:05okay now the ospf version 3
471. 21:08configuration and verification commands
472. 21:11are similar to those used in ospf
473. 21:14version 2.
474. 21:18okay so
475. 21:22comparing ospf version 2 and ospf
476. 21:26version 3 data structures okay
477. 21:30so basically both version
478. 21:33has the neighbor table okay
479. 21:36topology table and routing table
480. 21:40okay so it's just that ospf version 2
481. 21:45is applicable only for ipv4 network
482. 21:49and ospf version 3 is applicable for
483. 21:52ipv6 network
484. 21:58okay so on this section we will be
485. 22:01talking about
486. 22:02the ospf buckets
487. 22:08so there are types of ospf packets so
488. 22:12the link state packets are those or are
489. 22:15the tools
490. 22:15used by ospf to help determine the
491. 22:18fastest available route for a packet
492. 22:22so ospf uses the following link state
493. 22:25packets or lsps to establish and
494. 22:28maintain neighbor adjacencies and
495. 22:29exchange routing updates
496. 22:32so each packet serves a specific purpose
497. 22:35in the ospf routing process are as
498. 22:38follows
499. 22:39okay so first you've got type
500. 22:421 okay so the packet name is hello
501. 22:45you've got hello packet
502. 22:47this is used to establish and maintain
503. 22:49adjacency
504. 22:50with other ospf routers so it discovers
505. 22:54neighbors and builds adjacencies between
506. 22:57them
507. 22:58okay so the next one would be
508. 23:01the type 2 okay or also known as the
509. 23:05database description or dvd packet
510. 23:08so this contains an abbreviated list of
511. 23:11the lsdb
512. 23:13of the sending router and is used by the
513. 23:15receiving routers
514. 23:17to check against the local lsdb
515. 23:20so the lsdb must be identical on all
516. 23:23link state routers
517. 23:25within an area to construct an accurate
518. 23:28spf 3
519. 23:30okay so basically it checks for database
520. 23:33synchronization
521. 23:34between routers okay so the third one
522. 23:38is a type 3 or the link state
523. 23:42request or lsr
524. 23:46so receiving routers can then request
525. 23:50more information about any entry in the
526. 23:52air in the dvd
527. 23:54by sending an lsr okay so lsr basically
528. 23:58requests
529. 23:59a specific link state records from
530. 24:02router
531. 24:03to router so the fourth one would be
532. 24:07a type 4 okay or the link state update
533. 24:12so this is used to reply to lsr
534. 24:15or the link state requests and
535. 24:18to announce new information
536. 24:22so lsu's okay or the link state update
537. 24:25contains several different types of
538. 24:27lsas so again lsu's sends
539. 24:30specifically requested link state
540. 24:33records
541. 24:35okay so the last one of the ospf packet
542. 24:38is a type 5 okay so
543. 24:42also known as the link state
544. 24:43acknowledgement or
545. 24:45elsa okay so the elsa packet
546. 24:49so when an lsu or the link state update
547. 24:53is received the router sends
548. 24:56an elsa or acknowledgement to confirm
549. 24:59receipt of the lsu
550. 25:01so the elsa data is
551. 25:04or the lsec data field is empty
552. 25:07okay
553. 25:10okay so the next one is the link state
554. 25:12updates
555. 25:14okay so routers initially exchange
556. 25:17type 2 dbd packets which is an
557. 25:20abbreviated list of the lsdb
558. 25:23of the sending router so it is used by
559. 25:26the receiving routers to check against
560. 25:29the local
561. 25:29lsdb okay
562. 25:33so a type 3 lsr packet is used by the
563. 25:37receiving routers
564. 25:38to request more information about an
565. 25:40entry
566. 25:41in the dvd okay so a
567. 25:44type four lsu okay
568. 25:48so lsu packet is used to reply to an lsr
569. 25:52packet
570. 25:53so a type 5 okay
571. 25:56or the elsa packet is used to
572. 25:58acknowledge the receipt of
573. 26:00a type 4 lsu so lsus
574. 26:03are used to forward ospf routing updates
575. 26:07such as link changes okay so
576. 26:10specifically
577. 26:11an lsu packet can contain 11 different
578. 26:13types of ospf version 2 lsas
579. 26:16with some of the more common ones
580. 26:20shown in the figure okay so
581. 26:24ospf version 3 renamed several of these
582. 26:27lsas
583. 26:28and also contains two additional lsas
584. 26:32okay so these are the lsas here okay so
585. 26:35with
586. 26:36six seven 8 and so on
587. 26:39all right so note that the difference
588. 26:42between the lsu
589. 26:44and lsa's terms can sometimes be
590. 26:47confusing because
591. 26:48these terms are often used
592. 26:51interchangeably
593. 26:52however an lsu contains one or more
594. 26:56lsas just have to remember that okay
595. 26:59an lsu contains one or more
596. 27:02lsas all right
597. 27:10okay so let's get into the detail about
598. 27:13the hello packet
599. 27:15okay so the ospf type one packet
600. 27:18is the hello packet hello packets are
601. 27:20used
602. 27:21to do the following okay so first
603. 27:25it allows discover ospf neighbors and
604. 27:29establish
605. 27:30neighbor adjacencies so advertise
606. 27:34parameters on which two routers must
607. 27:37agree
608. 27:37to become neighbors so elect
609. 27:41designated router or dr and backup
610. 27:44designated router or bdr
611. 27:46on a multi-access networks like ethernet
612. 27:49so take note that point-to-point links
613. 27:52do not require
614. 27:53dr or pdr okay
615. 27:56now the figure here displays the fields
616. 27:59contained
617. 28:00in the ospf type one hello packet
618. 28:04so we are talking about ospf version two
619. 28:09all right so techno that we have here
620. 28:12type one which pertains to hello packet
621. 28:16right so other fields are shown like
622. 28:19router id
623. 28:20area id so you can understand more of it
624. 28:22on the configuration on the next video
625. 28:24lecture
626. 28:29okay so the next section talks about the
627. 28:31ospf operations
628. 28:35okay so now that you know about the ospf
629. 28:38link state packets
630. 28:40this topic explains how they work
631. 28:43with ospf enabled routers so
632. 28:47when an ospf router is initially
633. 28:49connected to a network
634. 28:50it attempts to first create adjacencies
635. 28:53with neighbors
636. 28:55exchange routing information calculate
637. 28:58the best
638. 28:59routes and reach convergence
639. 29:03so the table here okay details the
640. 29:07states of
641. 29:08ospf progresses
642. 29:11through while attempting to which or to
643. 29:14reach
644. 29:15the level of convergence or the state of
645. 29:17convergence
646. 29:18okay so we have here starting with the
647. 29:21down state
648. 29:22the initialization state the two-way
649. 29:25state
650. 29:26okay the x-start state action state
651. 29:31loading state and the full state
652. 29:34okay so these are the ospf operational
653. 29:37states
654. 29:38let's get into each of that state
655. 29:41so initially the router is on down state
656. 29:45so on down state no hello packets
657. 29:48are received okay so router sends hello
658. 29:52packets
659. 29:53and then transition to initialization
660. 29:56state
661. 29:57so when you just configure when you just
662. 29:59connect two routers
663. 30:01and you have not configured yet the ospf
664. 30:04it is in downstairs right so
665. 30:08after the exchange of hello packets
666. 30:12okay it will transition to
667. 30:14initialization stage so take note that
668. 30:17in a down state
669. 30:18router sends hello packets okay
670. 30:22now in the init state hello packets are
671. 30:25received
672. 30:26from the neighbor router so it contains
673. 30:30the router id
674. 30:31of the sending router okay
675. 30:34and then it will transition to the next
676. 30:37state
677. 30:37which is a two-way state
678. 30:41now in the two-way state communication
679. 30:44between the two routers
680. 30:46is bi-directional on a multi-access
681. 30:49links the routers elect dr and
682. 30:52bdr so afterwards transition to the next
683. 30:57state which
684. 30:57is the x-star state
685. 31:01now on the x-start uh state on
686. 31:04point-to-point networks
687. 31:06the two routers decide which router will
688. 31:09initiate
689. 31:10the dbd packet exchange and decide upon
690. 31:13the initial dvd packet sequence number
691. 31:16okay so the next state would be the
692. 31:19exchange state
693. 31:20so routers exchange dvd packets
694. 31:24so if additional router information is
695. 31:27required
696. 31:27then transition to loading otherwise
697. 31:30transition to full stack
698. 31:33okay so loading state
699. 31:36the lsr okay or the link state requests
700. 31:39and the link state updates are used to
701. 31:41gain additional route information
702. 31:44so routes are processed during the spf
703. 31:47algorithm
704. 31:48transition to the full state
705. 31:51and the last one is full state so full
706. 31:54state
707. 31:54is the link state database of the router
708. 31:57in a fully synchronized form
709. 32:00so with the full state the network now
710. 32:03is in the level of convergence
711. 32:08okay so
712. 32:12how about established neighbor
713. 32:13adjacencies
714. 32:15so when ospf is enabled on an interface
715. 32:18the router must determine if there is
716. 32:21another ospf neighbor
717. 32:23on the link so to accomplish this
718. 32:26the router sends a hello packet that
719. 32:29contains
720. 32:30its router id out all ospf enabled
721. 32:34interfaces
722. 32:35okay now the hello packet is sent to the
723. 32:38reserved
724. 32:39all ospf routers ipb4 multicast address
725. 32:4522405
726. 32:47okay so that's a multicast address two
727. 32:49two four zero zero
728. 32:51five so only ospf version two routers
729. 32:54will process these packets so the ospf
730. 32:58router id
731. 32:59is used by the ospf process to uniquely
732. 33:03identify its router
733. 33:04in the ospf area okay so a router id
734. 33:09is a 32-bit number formatted like an
735. 33:12ipv4 address
736. 33:14and assigned to uniquely identify a
737. 33:16router
738. 33:17among the ospf peers
739. 33:21now when enabling ospf enabled router
740. 33:24receives a hello packet
741. 33:26with a router id that is not
742. 33:29within its neighbor list so the
743. 33:32receiving router attempts to establish
744. 33:34an adjacency
745. 33:35with the initiating router
746. 33:41okay so first
747. 33:45it will okay it will be in down state
748. 33:48to initialize it okay so
749. 33:52when ospf version two is enabled
750. 33:55the enabled giga internet zero zero
751. 33:57interface
752. 33:58okay in this diagram here transitions
753. 34:01from down state
754. 34:03to initialize or in its state
755. 34:06so r1 starts sending hello packets
756. 34:10out all ospf enabled interfaces
757. 34:13to discover ospf neighbors to develop
758. 34:16adjacencies with
759. 34:18okay now during the process r1 here
760. 34:22okay so from down state okay so
761. 34:25it will initialize or it will shift to
762. 34:27the initialization state
763. 34:29and on that so you've got the hello
764. 34:32packet
765. 34:33forwarded by r1 to r2
766. 34:36okay so something like hello my router
767. 34:40id
768. 34:40is 172 16 5.1
769. 34:44is there anyone else on this link it
770. 34:47will be forwarded
771. 34:48on a multicast address two two four zero
772. 34:50zero five
773. 34:52where all the ospf enabled routers
774. 34:55are in there okay
775. 34:59next would be the initialization or the
776. 35:02init state
777. 35:04so r2 receives the hello packet from r1
778. 35:09and adds the r1 router id
779. 35:12to its neighbor list okay
780. 35:15r2 then sends a hello packet to r1
781. 35:20the packet contains the r2 router id
782. 35:23and the r1 router id in its list
783. 35:27of neighbors on the same interface
784. 35:31okay so afterwards
785. 35:34so you've got now the two-way state now
786. 35:37what happens
787. 35:38during the two-way state so take note
788. 35:42that
789. 35:42r1 receives the hello and adds the r2
790. 35:45router id
791. 35:46to its list of ospf neighbors
792. 35:49so it also notices its own router id in
793. 35:53the list of neighbors
794. 35:55of the hello packet so when a router
795. 35:58receives a hello packet with
796. 35:59its router id listed in the list of
797. 36:01neighbors
798. 36:02the router transitions from the init
799. 36:05state
800. 36:07to the two-way state okay
801. 36:10so the action performed in the two-way
802. 36:13state depends on the type of
803. 36:15interconnection
804. 36:16between the adjacent router okay so
805. 36:20it could be for instance if the two
806. 36:23adjacent neighbors are interconnected
807. 36:25using a point-to-point
808. 36:26link or connected via serial cable
809. 36:30then they immediately
810. 36:34okay transition from the two-way state
811. 36:37to the x-start stick okay
812. 36:41now if the routers are interconnected
813. 36:43over a common ethernet network
814. 36:45then a designated router and bdr
815. 36:50or backup designated router must be
816. 36:52elected
817. 36:54okay so the two-way state
818. 36:57would depend on the type of network you
819. 37:00are working
820. 37:01with all right it could be a
821. 37:03point-to-point where in routers are
822. 37:04connected by a serial
823. 37:06link or it could be
824. 37:09a a multi-access
825. 37:14okay network like a common internet
826. 37:17network
827. 37:18wherein there is a need for a designated
828. 37:21router and backup designated router
829. 37:23election
830. 37:25okay so afterwards
831. 37:28okay so elect the dr and bdr
832. 37:32in case of the ethernet network okay
833. 37:35so because r1 and r2 are interconnected
834. 37:39over the ethernet so take note that
835. 37:40these are g00 and g01 so this
836. 37:43is an ethernet network so therefore
837. 37:46a dr and bdr election takes place
838. 37:50now as shown in the figure here so r2
839. 37:54becomes the dr and r1 is
840. 37:57the bdr okay so
841. 38:00take a look at this so r1 has a default
842. 38:02priority of one
843. 38:04and the second highest router id so it
844. 38:07will be the bdr
845. 38:09on this link so r2 has a default
846. 38:12priority of one
847. 38:13so both of them are using the default
848. 38:15priority of one
849. 38:16and has the highest router id
850. 38:19so it will be the uh dr or the
851. 38:22designated router
852. 38:23on this link okay so hello packets are
853. 38:27continually exchanged
854. 38:29to maintain routing information
855. 38:35all right so how about synchronizing
856. 38:40ospf databases okay
857. 38:43so after the two-way state
858. 38:46so routers transition to database
859. 38:49synchronization states
860. 38:51so while the hello packet was used to
861. 38:53establish neighbor adjacencies
862. 38:55the other four types of ospf packets are
863. 38:59used
864. 38:59during the process of exchanging and
865. 39:02synchronizing the lsdbs
866. 39:04so this is a three-step process
867. 39:08right so decide first router
868. 39:12exchange dvds and send
869. 39:15an lsr all right
870. 39:19so decide the first router so the router
871. 39:21with the highest router id
872. 39:23sends its dbd first
873. 39:27so exchange dvds so as many as needed
874. 39:32to convey the database the other router
875. 39:35must acknowledge the dpd with an elsa
876. 39:39packet send an lsr
877. 39:43so each router compares the dvd
878. 39:46information
879. 39:47with the local lsdb so
880. 39:50if the dvd has more current link
881. 39:53information
882. 39:54the router transitions to the loading
883. 39:57stick
884. 39:59all right
885. 40:03all right so let's get into the details
886. 40:07so decide first router so
887. 40:10in the start state the two routers
888. 40:13decide
889. 40:14which router will send the dvd packets
890. 40:17first okay now the router with the
891. 40:20higher router id
892. 40:22will be the first router to send dvd
893. 40:25packets
894. 40:25during the exchange state so in the
895. 40:28figure here
896. 40:30r2 has the higher router id
897. 40:34and sends its dvd packets first
898. 40:38okay next
899. 40:42exchange of dvds
900. 40:45now in the exchange state the two
901. 40:47routers exchange
902. 40:48one or more dvd packets so a dvd packet
903. 40:52includes information
904. 40:54about the lsa entry header that appears
905. 40:57in the lsdb of the router
906. 41:00so the entries can be about a link or
907. 41:03about a network
908. 41:04so each lsa's entry header includes
909. 41:08information about the link say type so
910. 41:11the address of the advertising router
911. 41:14the cost of the link and the sequence
912. 41:17number
913. 41:18right so the router uses the sequence
914. 41:20number to determine
915. 41:22the newness of the receive link state
916. 41:24information
917. 41:26now in this figure here r2 sends a dbd
918. 41:30packet to r1
919. 41:32so when r1 receives the dbd it performs
920. 41:36the following actions
921. 41:37first it has to acknowledge the receipt
922. 41:40of the dvd
923. 41:41using the lsat or link state
924. 41:44acknowledgement packet
925. 41:45okay so r1 then sends dbd packets to r2
926. 41:50and then r2 acknowledges r1
927. 41:54all right okay
928. 41:57so next would be sending an lsr
929. 42:02so r1 okay compares
930. 42:06the information received with the
931. 42:07information it has in its own lsdb
932. 42:11so if the dvd packet has a more
933. 42:15current link state entry the router
934. 42:18transitions
935. 42:18to the loading state okay
936. 42:22so for example in the figure
937. 42:25r1 sends an lsr okay so regarding the
938. 42:29network
939. 42:3017216 6.0
940. 42:34to r2 okay so
941. 42:37r2 will respond
942. 42:40okay on that uh lsr
943. 42:43via the lsu okay
944. 42:47so r2 responds with the complete
945. 42:49information about 172 16 6 the 0
946. 42:53in an lsu packet
947. 42:56right so again when r1 receives an lsu
948. 43:00it sends an acknowledgement that's why
949. 43:02you have here elsa or the link state
950. 43:04acknowledgement
951. 43:05so r1 then adds the new link state
952. 43:08entries
953. 43:09into each lsdb so after
954. 43:12all lsrs have been satisfied for a given
955. 43:15router
956. 43:16the adjacent routers are considered
957. 43:19synchronized
958. 43:20and it is in a full state so updates
959. 43:24okay so lsu's are sent only to neighbors
960. 43:28in the following conditions
961. 43:29first when a change is perceived
962. 43:33we could we call that incremental
963. 43:35updates
964. 43:37all right so it's in here let's go back
965. 43:39to the slides
966. 43:40incremental updates and it is every 30
967. 43:45minutes
968. 43:47all right
969. 43:56okay so next would be
970. 44:00the need for a dr okay
971. 44:03why is a dr bdr election
972. 44:06necessary okay now in a multi-access
973. 44:10network okay so it can create two
974. 44:13challenges for ospf
975. 44:15regarding the flooding of lsas as
976. 44:17follows
977. 44:19okay so first you've got the creation of
978. 44:22multiple adjacencies here
979. 44:24so ethernet networks could potentially
980. 44:27interconnect
981. 44:28many ospf routers over a com common link
982. 44:31like
983. 44:31shown in the diagram here now creating
984. 44:34adjacencies with
985. 44:35every router is unnecessary and
986. 44:37undesirable
987. 44:40so it would lead to an excessive number
988. 44:42of lsas exchanged
989. 44:44between the routers on the same network
990. 44:47okay so next would be an extensive
991. 44:50flooding of
992. 44:51lsas so the ling state routers flood
993. 44:54their lsas anytime
994. 44:55ospf is initialized or
995. 44:58when there is a change in the topology
996. 45:00we call it topological changes
997. 45:03so this flooding can become excessive
998. 45:06okay now to understand the problem with
999. 45:09multiple adjacencies
1000. 45:11we must study the formula okay so for
1001. 45:15any number of routers designated as n
1002. 45:19on a multi-axis network there are n
1003. 45:23okay and then n minus one divided by two
1004. 45:26adjacencies
1005. 45:27so for example the figure shows a simple
1006. 45:30topology of five routers
1007. 45:32all of which are attached to the same
1008. 45:35multi-access
1009. 45:36ethernet network or switch now without
1010. 45:39some type
1011. 45:40of mechanism to reduce the number of
1012. 45:42adjacencies
1013. 45:44collectively these routers would form
1014. 45:4810 adjacencies how do we get that so
1015. 45:50i've got five routers
1016. 45:52all right for n y minus one is four
1017. 45:55five times four is 20 divided by two
1018. 45:58that would yield to 10 adjacencies
1019. 46:02now this may not seem like much but as
1020. 46:05routers are added to the network the
1021. 46:07number of adjacencies increases
1022. 46:09dramatically
1023. 46:10for example a multi-access network
1024. 46:14with 20 routers would create 190
1025. 46:17adjacencies can you imagine that okay
1026. 46:26all right so flooding or lsa flooding
1027. 46:29with
1028. 46:30a designated router now a dramatic
1029. 46:33increase
1030. 46:34in the number of routers also
1031. 46:35dramatically increases
1032. 46:37the number of lsas exchanged between the
1033. 46:39routers
1034. 46:40so this flooding of lsa significantly
1035. 46:43impacts the operation
1036. 46:45of the ospf okay
1037. 46:48so an increase in the number of routers
1038. 46:50on a multi-access network or the network
1039. 46:52that is connected by a switch
1040. 46:54imagine it will be flooded with a lot of
1041. 46:56lsas
1042. 46:57okay so if every router in the
1043. 47:00multi-access network has to plug
1044. 47:03okay and acknowledge all received lsas
1045. 47:06to other routers at the same
1046. 47:08multi-access network so the traffic or
1047. 47:10the natural
1048. 47:11traffic would become quite chaotic
1049. 47:14now on a multi-access networks ospf lx
1050. 47:19dr or designated router
1051. 47:22to be the collective and distribution
1052. 47:24point for lsa's
1053. 47:26sent or received a bdr
1054. 47:29is also elected in case the dr fails
1055. 47:32now all other routers become drawers
1056. 47:36take note that we only have one dr and
1057. 47:39one bdr
1058. 47:41in the segment the rest are considered
1059. 47:43to be
1060. 47:44drawers so a drawer is a router that is
1061. 47:48neither
1062. 47:48a dr nor a pdr
1063. 47:52okay so take note that the dr is
1064. 47:55only used to disseminate lsas
1065. 47:59okay so the router will still use the
1066. 48:01best
1067. 48:02next hub router indicated in the routing
1068. 48:05table
1069. 48:06for the forwarding of all other packets
1070. 48:12okay now to understand the problem of
1071. 48:15extensive
1072. 48:16flooding of lsas okay so please refer to
1073. 48:19this
1074. 48:20animations here okay now in the
1075. 48:23animation
1076. 48:24okay so r2 sends out analysis
1077. 48:29so let's wait for that okay so r2
1078. 48:34sends out analysis this event trigger
1079. 48:37every other router to also send out an
1080. 48:39lsa
1081. 48:42all right so not shown in the animation
1082. 48:45okay so i required acknowledgement sent
1083. 48:48for every else is
1084. 48:50received so if every router in the
1085. 48:54multi-access network
1086. 48:56had to flood and acknowledge all
1087. 48:57received lsas
1088. 48:59to all other routers on that same
1089. 49:01multi-access network
1090. 49:02the network traffic become very chaotic
1091. 49:06okay
1092. 49:11now the solution to managing the number
1093. 49:14of adjacencies
1094. 49:15and the flooding of lsas on a multi-axis
1095. 49:17network
1096. 49:18is the dr okay in this case
1097. 49:22we have r2 here elected as the dr
1098. 49:25and r3 here which was elected
1099. 49:28a bdr okay now on a multi-access
1100. 49:32networks ospf
1101. 49:34elects a dr to be the collection and
1102. 49:37distribution point
1103. 49:38for lsa's send and receive
1104. 49:42now a bdr is also elected in case the dr
1105. 49:46fails now all other routers become
1106. 49:50drawer so a drawder is a router that is
1107. 49:53neither
1108. 49:54again a dr or a pdr so r1
1109. 49:58r5 and r4 here are designated as
1110. 50:01rotors now the dr is only used again
1111. 50:05for dissemination of the lsas the router
1112. 50:08will still
1113. 50:09use the best next hub router indicated
1114. 50:12in the routing table
1115. 50:14for the following or for for the
1116. 50:16forwarding of
1117. 50:18all other packets okay
1118. 50:23so that would be the end of this video
1119. 50:25lecture
1120. 50:26thank you for watching and listening
1121. 50:28have a great day
1122. 50:29see you on the next video lecture