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
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
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
Routing
Protocol
Is It
Classless?
Sends Mask in
Updates
Supports VLSM Supports Manual Route
Summarization
RIP-1 No No No No
IGRP No No No No
RIP-2 Yes Yes Yes Yes
EIGRP Yes Yes Yes Yes
OSPF Yes Yes Yes Yes
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
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
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
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
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
51. Autosummarization Support and
Configuration
Pg 223
Routing
Protocol
Classless? Supports
Autosummarization?
Defaults to Use
Autosummarization?
Can Disable
Autosummarization?
RIP-1 No Yes Yes No
RIP-2 Yes Yes Yes Yes
EIGRP Yes Yes Yes Yes
OSPF Yes No — —
