Networking Lab Class #6 VLSM & Route Summarization - EdLab

Networking Lab 

Class #6 

VLSM & Route Summarization 

Parviz Kermani 

Spring 2012 

UMasss Amherst

Acknowledgement 

• Wendel Odom: CCNA ICND2 : Official Exam 

Certification Guide (Second Edition)- 

Ciscopress.com 

2

This Class 

• VLSM 

• Route Summarization 

3

VLSM (Variable Length Subnet Mask) 

• Using more than one mask in a single classful 

network 

• Benefits: 

Reduce number of wasted IP addresses 

Conserve the address space 

Mask: 255.255.255.0 

4

VLSM 

• Note: using more than one mask does not 

constitute VLSM by itself 

But using more than one mask in a single classful 

network does! 

5

Support for VLSM 

• To be able to use VLSM, an IP routing protocol 

should advertise subnet number as well as 

subnet mask 

• Classless routing protocols: 

Advertise mask information for each subnet 

Support VLSM (Variable Length Subnet Mask) 

Route summarization 

• An inherent property of a routing protocol 

Not configurable 

6


7

Without VLSM Support (RIP V1) 

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 

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, * - candidate default 

U - per-user static route 

Gateway of last resort is not set 

172.16.0.0/24 is subnetted, 3 subnets 

C 172.16.2.0 is directly connected, Serial0/0 

C 172.16.3.0 is directly connected, Serial0/1 

C 172.16.1.0 is directly connected, FastEthernet0/0 

R 10.0.0.0 [120/1] via 172.16.3.2, 00:03:21, Serial0/1 

(Bosom NetSim) 

8

With VLSM Support (RIP V2) 

Albuquerque(config-router)#no auto-summary 

Albuquerque#show ip rout 

…. 

Gateway of last resort is not set 

C 

C 

C 

R 

R 

R 

R 

R 

R 


172.16.2.0 is directly connected, Serial0/0 

172.16.3.0 is directly connected, Serial0/1 

172.16.1.0 is directly connected, FastEthernet0/0 


10.2.1.0 [120/1] via 172.16.2.2, 00:06:42, Serial0/0 

10.2.2.0 [120/1] via 172.16.2.2, 00:05:15, Serial0/0 

10.2.3.0 [120/1] via 172.16.2.2, 00:07:19, Serial0/0 

10.3.4.0 [120/1] via 172.16.3.2, 00:08:31, Serial0/1 

10.3.5.0 [120/1] via 172.16.3.2, 00:02:44, Serial0/1 

10.3.6.0 [120/1] via 172.16.3.2, 00:04:17, Serial0/1 

9

Overlapping VLSM Subnets 

• Subnet should not have overlapping addresses 

Easy to detect in a single mask network 

Very subtle and difficult to detect with VLSM 

• With overlapping addresses subnets 

Routers’ behavior unpredictable 

Some host reachable only from particular parts of 

the internet 

WRONG DESIGN 

10

Dealing with VLSM Subnets 

• Two types of problems engineers faced with 

• Analyze: 

Analyze a design to detect overlaps 

• Design 

Choose/add new VLSM subnet avoiding overlap 

• Analyze 

Calculate range of addresses for each subnet 

Check for overlap 

11

Analyzing an existing design 

Overlap! 

To correct: change 172.16.4.0/23 172.16.4.0/24 

12

Design: Addressing Scheme With Single Mask 

• Determine number of subnet & host bits in 

the largest subnets to meet the requirements 

• Choose a subnet mask 

• For the mask, Identify all subnets of the 

network 

• Choose pick the actual sunet. 

13

Design: Addressing Scheme With Single Mask 

Example: 

• Requirements 

Class B network 172.16.0.0 

At least 10 subnets 

Largest subnet 200 hosts 

Choose a design with the largest number of subnets 

Choose the 3 rd subnet 

• Design 

At least 4 subnet bit; At least 8 host bits 

Mask: 255.255.255.0; 256 subnet, 254 hosts 

Choose 172.16.2.0/24 net 

14

Designing a VLSM Subnetting Scheme 

• Analyze the requirement to determine design 

goals. 

• Design goals: 

Use Class B network 172.16.0.0 

Three subnets with mask/24 (255.255.255.0) 

Three subnets with mask /26 (255.255.255.192) 

Four subnets with mask /30 (255.255.255.253) 

• Point-to-point links 

• Compare it with a classful and/or no VLSM design 

goal 

15

Designing a VLSM Subnetting Scheme 

Step 1: Design goals 

Step 2: Use the shortest prefix (largest # of hosts) to 

identify subnets of the classful network 

 

Apply to all identified networks 

Step 3: Identify the next numeric subnet number 

using the same mask 

Step 4: Identify the next-longest prefix 

 

Complete the number with that size 

Step 5: Repeat step 3 & 4 until complete 

16

Designing a VLSM Subnetting- Example 

• Step 2: The shortest prefix is /24 (longest host) 

Use the first 3 subnets of 172.16.0.0 

• Three subnets with mask/24 

172.16.0.0/24: Range 182.16.0.1-172.16.0.254 

172.16.1.0/24: Range 182.16.1.1-172.16.1.254 

172.16.2.0/24: Range 182.16.2.1-172.16.2.254 

• Step 3: the next numeric subnet (same mask) 

172.16.3.0/24 

• Three subnets with mask /26 

17


• Step 4: start with the unallocated subnet number of 

step 3 

• Use the nest longer prefix (/26, mask 

255.255.255.192) 

The first subnet is the one found in step 3 

172.16.3.0/26: range 172.16.3.1-172.16.3.62 

172.16.3.62/26: range 172.16.3.65-172.16.3.126 

172.16.3.128/26: range 172.16.3.129-172.16.3.190 

18


• Step 4 

19


• Step 5: Repeat Steps 3 & 4 until done 

Step 3: The next subnet, using /26, is 172.16.3.192/26 

Step 4: The next longest prefix is /30 (255.255.255.252) 

• Resulting scheme: 

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 

20

VLSM Design II 

• Adding a new subnet to an existing design 

Refer to Wendell Odom’s book 

21

VLSM Configuration 

• An inherent feature of routing protocol (IP) 

No configuration command on routers 

A side effect of ip address command 

• Routers configure VLSM by virtue of at least 2 

router interfaces 

On the same router or among all routers 

IP addresses in the same classful network but 

different mask 

• R3,Fa0/0 (255.255.255.0) & 

S/0/01 (255.255.255.252) 

172.16.4.1/24 

22

VLSM Configuration 

172.16.4.1/24 

R3#configure terminal 

R3(config)#interface Fa0/0 

R3(config)#ip address 172.16.5.1 255.255.255.0 

R3(config)#interface S0/0/1 

R3(config)#ip address 172.16.9.6 255.255.255.252 

23

Route Summarization 

• Routers have many routes in their tables 

Some Internet routers have more than 100,000! 

• Routing tables become too large in large nets 

Consume more memory 

Take more time to route packets 

Large table more time needed to troubleshoot! 

• Route summarization reduces size of routing 

tables while maintaining all routes 

Reduced convergence time 

No need to announce changes to the status of 

individual subnets 

24

Route Summarization Concepts 

• Number of more-specific routes to be 

replaced with a single route 

Includes all IP addresses covered by subnets in the 

original routes 

• Routing protocol advertises just the summary 

route, as opposed to the original route 

Must be configured by the network engineer 

• Concept similar to static route 

Same basic information 

25

Route Summarization Concepts 

• Works better if network is designed with 

summarization in mind 

• Example of a good design 

26

Routing Table Without Summarization 

27

Routing Table With Summarization 

28

Effect of (manual) Summarization 

Syntax is protocol 

dependent 

Discards 

unwanted packets 

29

Effect of (manual) Summarization 

Syntax is protocol 

dependent 

Discards 

unwanted packets 

30

(Manual) Route Summarization Strategies 

• Best summarization 

Should include all desired subnets with as few 

other addresses, if possible! 

• Example (Yosemite): 

Subnets 10.2.1.0, 10.2.2.0, 10.2.3.0, 10.2.4.0 (/24) 

summarized into 12.2.0.0/16 

Summary includes a lot of IP addresses no in the 

four subnets 

Correct, but not good! 

31

(Manual) Route Summarization Strategies 

Step 1: List all to-be-summarized subnets in binary 

Step 2: Find the left N common bits (in-common 

part) 

Step 3: Summary subnet number: in-common part 

followed by all binary “0” 

Convert back to decimal 

Step 4: Summary subnet mask: N binary “1”s 

followed by 32-N binary “0”s 

Convert back to decimal 

Step 5: Check the result by finding the range! 

32

Example Best Summary on Seville 

Subnets: 10.3.4.0, 10.3.5.0, 10.3.6.0, 10.3.7.0 

Step 1: List in binary 

10.3.4.0 : 0000 1010 0000 0011 0000 0100 0000 0000 

10.3.5.0 : 0000 1010 0000 0011 0000 0101 0000 0000 

10.3.6.0 : 0000 1010 0000 0011 0000 0110 0000 0000 

10.3.7.0 : 0000 1010 0000 0011 0000 0111 0000 0000 

Step 2: Find in-common bits, and N 

in-common: 0000 1010 0000 0011 0000 01, N=22 

Step 3: (summary) Subnet Number: 

0000 1010 0000 0011 0000 0100 0000 0000 

10 . 3 . 4 . 0 

Step 4: (summary) Subnet Mask: 

1111 1111 1111 1111 1111 1100 0000 0000 

255 . 255 . 252 . 0 

Step 5: Range 

10.3.4.0/22 (255.255.252.0): 10.3.4.1 – 10.3.7.254 (Bcast 10.3.7.255) Perfect! 

33

Example Best Summary on Yosemite 

Subnets: 10.2.1.0, 10.2.2.0, 10.2.3.0, 10.2.4.0 

Step 1: List in binary 

10.2.1.0 : 0000 1010 0000 0010 0000 0001 0000 0000 

10.2.2.0 : 0000 1010 0000 0010 0000 0010 0000 0000 

10.2.3.0 : 0000 1010 0000 0010 0000 0011 0000 0000 

10.2.4.0 : 0000 1010 0000 0010 0000 0100 0000 0000 

Step 2: Find in-common bits, and N 

in-common: 0000 1010 0000 0010 0000 0, N=21 

Step 3: (summary) Subnet Number: 

0000 1010 0000 0010 0000 0000 0000 0000 

10 . 2 . 0 . 0 

Step 4: (summary) Subnet Mask: 

1111 1111 1111 1111 1111 1000 0000 0000 

255 . 255 . 248 . 0 

Step 5: Range 

10.2.0.0/21 (255.255.248.0): 10.2.0.1 – 10.2.7.254 The best, but not so perfect! 

Summary route summarizes a larger address set 

34

Autosummarization 

• No advertisement of mask in classful routing 

protocols 

Needed mask information in address class (A, B, C) 

Throughout the inter-network 

Static-Length subnet mask 

• If R1 & R2 have connected network to the same 

single Class A (or B, or C) 

R2 received update from R1 

R2 assumes routes described in R1’s update use the 

same mask as R2 uses 

35

Autosummarization 

When advertized on an interface whose IP address is 

not in network X, routes related to subnets in 

network X are summarized and advertized as one 

route. That route is for the entire class A, B, or C 

network X. 

36

Autosummarization Example 

Classful RIP-1 

protocol in effect 

Note: Albuquerque do 

not have any interface 

in 10.0.0.0, so it 

assumes the mask used 

with 10.0.0.0 (Class A) is 

255.0.0.0.0 

37

Discontiguous Classful Networks 

• Autosummarization works as long as summarized 

networks are contiguous 

• Contiguous Newark 

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 pass through subnets of a different 

classful network 

38


Autosummarization prevents an 

internetwork with a discontiguous 

network from working properly 

39


Solution: 

Disable Autosummarization 

Autosummarization disabled 

on Yosemite and Seville 

40

Autosummarization Support 

• Classful routing protocols must use 

autosummarization. 

• Some classless routing protocols support 

autosummarization 

Default configuration 

Can be disabled 

• OSPF (classless) DOES NOT support 

autosummarization 

41

Autosummarization Support 

42


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Networking Lab Class #6 VLSM & Route Summarization - EdLab

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