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  1. Any Questions?

  2. Chapter 5 VLSM and Route Summarization • VLSM • Manual Route Summarization • Autosummarization and Discontiguous Classful Networks

  3. Do I know this? Go through the Quiz- 5 minutes

  4. 1. Which of the following routing protocols support VLSM? a. RIP-1 b. RIP-2 c. EIGRP d. OSPF

  5. 1. Which of the following routing protocols support VLSM? a. RIP-1 b. RIP-2 c. EIGRP d. OSPF Answer B, C, D

  6. 2. What does the acronym VLSM stand for? a. Variable-length subnet mask b. Very long subnet mask c. Vociferous longitudinal subnet mask d. Vector-length subnet mask e. Vector loop subnet mask

  7. 2. What does the acronym VLSM stand for? a. Variable-length subnet mask b. Very long subnet mask c. Vociferous longitudinal subnet mask d. Vector-length subnet mask e. Vector loop subnet mask Answer: A

  8. 3. R1 has configured interface Fa0/0 with the ip address 10.5.48.1 255.255.240.0 command. Which of the following subnets, when configured on another interface on R1, would not be considered to be an overlapping VLSM subnet? a. 10.5.0.0 255.255.240.0 b. 10.4.0.0 255.254.0.0 c. 10.5.32.0 255.255.224.0 d. 10.5.0.0 255.255.128.0

  9. 3. R1 has configured interface Fa0/0 with the ip address 10.5.48.1 255.255.240.0 command. Which of the following subnets, when configured on another interface on R1, would not be considered to be an overlapping VLSM subnet? a. 10.5.0.0 255.255.240.0 b. 10.4.0.0 255.254.0.0 c. 10.5.32.0 255.255.224.0 d. 10.5.0.0 255.255.128.0 Answer: C & D

  10. 4. Which of the following summarized subnets is the smallest (smallest range of addresses) summary route that includes subnets 10.3.95.0, 10.3.96.0, and 10.3.97.0, mask 255.255.255.0? a. 10.0.0.0 255.0.0.0 b. 10.3.0.0 255.255.0.0 c. 10.3.64.0 255.255.192.0 d. 10.3.64.0 255.255.224.0

  11. 4. Which of the following summarized subnets is the smallest (smallest range of addresses) summary route that includes subnets 10.3.95.0, 10.3.96.0, and 10.3.97.0, mask 255.255.255.0? a. 10.0.0.0 255.0.0.0 b. 10.3.0.0 255.255.0.0 c. 10.3.64.0 255.255.192.0 d. 10.3.64.0 255.255.224.0 Answer:C

  12. 5. Which of the following summarized subnets is not a valid summary that includes subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0? a. 10.0.0.0 255.0.0.0 b. 10.1.0.0 255.255.0.0 c. 10.1.55.0 255.255.255.0 d. 10.1.48.0 255.255.248.0 e. 10.1.32.0 255.255.224.0

  13. 5. Which of the following summarized subnets is not a valid summary that includes subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0? a. 10.0.0.0 255.0.0.0 b. 10.1.0.0 255.255.0.0 c. 10.1.55.0 255.255.255.0 d. 10.1.48.0 255.255.248.0 e. 10.1.32.0 255.255.224.0 Answer: C & D

  14. 6. Which of the following routing protocols support manual route summarization? a. RIP-1 b. RIP-2 c. EIGRP d. OSPF

  15. 6. Which of the following routing protocols support manual route summarization? a. RIP-1 b. RIP-2 c. EIGRP d. OSPF Answer: B, C, &D

  16. 7. Which routing protocol(s) perform(s) autosummarization by default? a. RIP-1 b. RIP-2 c. EIGRP d. OSPF

  17. 7. Which routing protocol(s) perform(s) autosummarization by default? a. RIP-1 b. RIP-2 c. EIGRP d. OSPF Answer: A, B & C

  18. 8. An internetwork has a discontiguous network 10.0.0.0, and it is having problems. All routers use RIP-1 with all default configurations. Which of the following answers lists an action that, by itself, would solve the problem and allow the discontiguous network? a. Migrate all routers to use OSPF, using as many defaults as is possible. b. Disable autosummarization with the no auto-summary RIP configuration command. c. Migrate to EIGRP, using as many defaults as is possible. d. The problem cannot be solved without first making network 10.0.0.0 contiguous.

  19. 8. An internetwork has a discontiguous network 10.0.0.0, and it is having problems. All routers use RIP-1 with all default configurations. Which of the following answers lists an action that, by itself, would solve the problem and allow the discontiguous network? a. Migrate all routers to use OSPF, using as many defaults as is possible. b. Disable autosummarization with the no auto-summary RIP configuration command. c. Migrate to EIGRP, using as many defaults as is possible. d. The problem cannot be solved without first making network 10.0.0.0 contiguous. Answer: A

  20. Any Questions?

  21. VLSM • VLSM occurs when an internetwork uses more than one mask in different subnets of a single Class A, B, or C network. • Instead of all subnetworks using the same subnet mask • For example: • 172.16.0.0 can be subnetted to provide multiple different size subnetworks • /30 often used for WAN links that only need 2 addresses Pg 202

  22. VLSM Albuquerque#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 11 subnets, 2 masks D 10.2.1.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.2.2.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.2.3.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.2.4.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.3.4.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 D 10.3.5.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 D 10.3.6.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 D 10.3.7.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 C 10.1.1.0/24 is directly connected, Ethernet0/0 C 10.1.6.0/30 is directly connected, Serial0/1 C 10.1.4.0/30 is directly connected, Serial0/0 Pg 202-203

  23. Any Questions?

  24. Classless and Classful Routing Protocols • For a routing protocol to support VLSM, the routing protocol must advertise not only the subnet number but also the subnet mask when advertising routes. Additionally, a routing protocol must include subnet masks in its routing updates to support manual route summarization. Pg 203

  25. Classless and Classful Routing Protocols Pg 204

  26. Any Questions?

  27. VLSM and overlapping Subnetworks • With regular subnetting-it is easy to find overlaps, regular sizes • With VLSM, you have to be more careful • Two general types of problems exist that relate to overlapping VLSM subnets, both in realjobs and for the exams: • analyzing an existing design to find overlaps • choosing new VLSM subnets so that you do not create an overlapped subnet. Pg 204

  28. VLSM and overlapping subnets Step 1 Calculate the subnet number and subnet broadcast address of each subnet; this gives you the range of addresses in that subnet. Step 2 Compare the ranges of addresses in each subnet and look for cases in which the address ranges overlap. Pg 205

  29. Overlapping routes Pg 205

  30. Designing a Subnetting Scheme Using VLSM • When using VLSM in a design, the design process starts by deciding how many subnets of each size are required. • Router to router links are almost always /30 • They only need 2 addresses Pg 206

  31. VLSM Process • Step 1 Determine the number of subnets needed for each mask/prefix based on the design requirements. • Step 2 Using the shortest prefix length (largest number of host bits), identify the subnets of the classful network when using that mask, until the required number of such subnets has been identified. • Step 3 Identify the next numeric subnet number using the same mask as in the previous step. • Step 4 Starting with the subnet number identified at the previous step, identify smaller subnets based on the next-longest prefix length required for the design, until the required number of subnets of that size have been identified. • Step 5 Repeat Steps 3 and 4 until all subnets of all sizes have been found. Pg 206

  32. VLSM Example • Subnet Class B network 172.16.0.0: • Three subnets with mask /24 (255.255.255.0) • 172.16.0.0/24: Range 172.16.0.1–172.16.0.254 • 172.16.1.0/24: Range 172.16.1.1–172.16.1.254 • 172.16.2.0/24: Range 172.16.2.1–172.16.2.254 • Three subnets with mask /26 (255.255.255.192) • 172.16.3.0/26: Range 172.16.3.1–172.16.3.62 • 172.16.3.64/26: Range 172.16.3.65–172.16.3.126 • 172.16.3.128/26: Range 172.16.3.129–172.16.3.190 • Four subnets with mask /30 (255.255.255.252) • 172.16.3.192/30: Range 172.16.3.193–172.16.3.194 • 172.16.3.196/30: Range 172.16.3.197–172.16.3.198 • 172.16.3.200/30: Range 172.16.3.201–172.16.3.202 • 172.16.3.204/30: Range 172.16.3.205–172.16.3.206 Pg 207-209

  33. Adding a New Subnet to an Existing Design • Avoid Overlap • Step 1 If not already listed as part of the question, pick the subnet mask (prefix length) based on the design requirements, typically based on the number of hosts needed in the subnet. • Step 2 Calculate all possible subnet numbers of the classful network, using the mask determined at Step 1. (If the exam question asks for the numerically largest or smallest subnet number, you might choose to only do this math for the first few or last few subnets.) • Step 3 For the subnets found at Step 2, calculate the subnet broadcast address and range of addresses for each assumed subnet. • Step 4 Compare the lists of potential subnets and address ranges to the existing subnets and address ranges. Rule out any of the potential subnets that overlap with an existing subnet. • Step 5 Pick a subnet number from the list found at Step 2 that does not overlap with any existing subnets, noting whether the question asks for the smallest or largest subnet number. Pg 209

  34. Configuring VLSM • Automatic-Happens when the ip address command is used on an interface of subinterface • Must use a classless routing protocol to support VLSM • See previous tables Pg 211

  35. Any Questions?

  36. Manual Route Summarization • Method of limiting entries in a routing table • Route summarization reduces the size of routing tables while maintaining routes to all the destinations in the network. • routing performance can be improved • memory can be saved inside each router. • improves convergence time, by advertising only that the entire summary route is either up or down, the routers that have the summary route do not have to reconverge every time one of the component subnets goes up or down. Pg 211

  37. Route Summarization • Route summarization works much better when the network was designed with route summarization in mind. For example, Figure 5-1, earlier in this chapter, shows the results of good planning for summarization. In this network, the engineer planned his choices of subnet numbers relative to his goal of using route summarization. All subnets off the main site (Albuquerque), including WAN links, start with 10.1. All LAN subnets off Yosemite start with 10.2, and likewise, all LAN subnets off Seville start with 10.3. Pg 212

  38. Summarizing Albuquerque Pg 202

  39. Albuquerque-before summary Albuquerque#show ip route ---SNIP---- Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 11 subnets, 2 masks D 10.2.1.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.2.2.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.2.3.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.2.4.0/24 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0 D 10.3.4.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 D 10.3.5.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 D 10.3.6.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 D 10.3.7.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 C 10.1.1.0/24 is directly connected, Ethernet0/0 C 10.1.6.0/30 is directly connected, Serial0/1 C 10.1.4.0/30 is directly connected, Serial0/0 Pg 203

  40. Albuquerque-after Summarizing Albuquerque#show ip route ---SNIP---- Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks D 10.2.0.0/16 [90/2172416] via 10.1.4.2, 00:05:59, Serial0/0 D 10.3.0.0/16 [90/2172416] via 10.1.6.3, 00:05:40, Serial0/1 C 10.1.1.0/24 is directly connected, Ethernet0/0 C 10.1.6.0/30 is directly connected, Serial0/1 C 10.1.4.0/30 is directly connected, Serial0/0 Pg 212-213

  41. Manual Summarization-Commands • Different for each routing protocol • EIGRP Example • Yosemite(config)#interface serial 0/0 • Yosemite(config-if)#ip summary-address eigrp 1 10.2.0.0 255.255.0.0 • Yosemite defines a summary route to 10.2.0.0, mask 255.255.0.0, which defines a route to all hosts whose IP addresses begin with 10.2. Pg 214

  42. Null Routes • Yosemite#show ip route • ---SNIP---- • 10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks • D 10.2.0.0/16 is a summary, 00:04:57, Null0 • D 10.3.0.0/16 [90/2684416] via 10.1.4.1, 00:04:30, Serial0/0 • C 10.2.1.0/24 is directly connected, FastEthernet0/0 • D 10.1.1.0/24 [90/2195456] via 10.1.4.1, 00:04:52, Serial0/0 • C 10.2.2.0/24 is directly connected, Loopback2 • C 10.2.3.0/24 is directly connected, Loopback3 • C 10.2.4.0/24 is directly connected, Loopback4 • D 10.1.6.0/30 [90/2681856] via 10.1.4.1, 00:04:53, Serial0/0 • C 10.1.4.0/30 is directly connected, Serial0/0 Pg 213

  43. Null routes • Yosemite • D 10.2.0.0/16 is a summary, 00:04:57, Null0 • Routes referring to an outgoing interface of the null0 interface mean that packets matching this route are discarded. • EIGRP added this route-The logic works like this: • If a packet destined for one of the four existing 10.2.x subnets arrives, Yosemite has a correct, more specific route to match • Yosemite needs this odd-looking route because now it might receive packets destined for other 10.2 addresses besides the four existing 10.2 subnets. • If a packet whose destination starts with 10.2 arrives, but it is not in one of those four subnets, the null route matches the packet, causing Yosemite to discard the packet—as it should. Pg 215

  44. Any Questions?

  45. Route Summarization Strategies • Best Summarization • The summary should include all the subnets specified in the question but as few other addresses as is possible. • Step 1 List all to-be-summarized subnet numbers in binary. • Step 2 Find the first N bits of the subnet numbers for which every subnet has the same value, moving from left to right. (For our purposes, consider this first part the “in-common” part.) • Step 3 To find the summary router’s subnet number, write down the in-common bits from Step 2 and binary 0s for the remaining bits. Convert back to decimal, 8 bits at a time, when finished. • Step 4 To find the summary route’s subnet mask, write down N binary 1s, with N being the number of in-common bits found at Step 2. Complete the subnet mask with all binary 0s. Convert back to decimal, 8 bits at a time, when finished. • Step 5 Check your work by calculating the range of valid IP addresses implied by the new summary route, comparing the range to the summarized subnets. The new summary should encompass all IP addresses in the summarized subnets. Pg 215-216

  46. Autosummarization and Discontiguous Classful Networks • When a router has interfaces in more than one Class A, B, or C network, it can advertise a single route for an entire Class A, B, or C network into the other classful network. This feature is called autosummarization. • When advertised on an interface whose IP address is not in network X, routes related to subnets in network X are summarized and advertised as one route. That route is for the entire Class A, B, or C network X. Pg 219

  47. Autosummarization Example • Albuquerque#show ip route • ---SNIP--- • 172.16.0.0/24 is subnetted, 2 subnets • C 172.16.1.0 is directly connected, Ethernet0/0 • C 172.16.3.0 is directly connected, Serial0/1 • R 10.0.0.0/8 [120/1] via 172.16.3.3, 00:00:28, Serial0/1 • Albuquerque#debug ip rip • RIP protocol debugging is on • 00:05:36: RIP: received v1 update from 172.16.3.3 on Serial0/1 • 00:05:36: 10.0.0.0 in 1 hops Pg 220

  48. Discontiguous Classful Networks • Contiguous network: A classful network in which packets sent between every pair of subnets can pass only through subnets of that same classful network, without having to pass through subnets of any other classful network. • Discontiguous network: A classful network in which packets sent between at least one pair of subnets must pass through subnets of a different classful network. Pg 220

  49. Discontiguous Network Example • Albuquerque#show ip route • ---SNIP-- • 172.16.0.0/24 is subnetted, 3 subnets • C 172.16.1.0 is directly connected, Ethernet0/0 • C 172.16.2.0 is directly connected, Serial0/0 • C 172.16.3.0 is directly connected, Serial0/1 • R 10.0.0.0/8 • [120/1] via 172.16.3.3, 00:00:13, Serial0/1 • [120/1] via 172.16.2.2, 00:00:04, Serial0/0 Pg 222

  50. Solutions for Discontiguous Networks • The solution to this problem is to disable the use of autosummarization. Because classful routing protocols must use autosummarization, the solution requires migration to a classless routing protocol and disabling the autosummarization feature. Example 5-8 shows the same internetwork from Figure 5-5 and Example 5-7, but this time with (classless) EIGRP, with autosummarization disabled. Pg 222