Networking Lab Class #6 VLSM & Route Summarization

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Oct 24, 2013 (3 years and 10 months ago)

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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





4
Mask: 255.255.255.0
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
Support for VLSM
7
Without VLSM Support (RIP V1)
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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)
With VLSM Support (RIP V2)
9
Albuquerque(config-router)#no auto-summary
Albuquerque#show ip rout
….


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
10.0.0.0/24 is subnetted, 6 subnets
R 10.2.1.0 [120/1] via 172.16.2.2, 00:06:42, Serial0/0
R 10.2.2.0 [120/1] via 172.16.2.2, 00:05:15, Serial0/0
R 10.2.3.0 [120/1] via 172.16.2.2, 00:07:19, Serial0/0
R 10.3.4.0 [120/1] via 172.16.3.2, 00:08:31, Serial0/1
R 10.3.5.0 [120/1] via 172.16.3.2, 00:02:44, Serial0/1
R 10.3.6.0 [120/1] via 172.16.3.2, 00:04:17, Serial0/1
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
12
Overlap!
To correct: change 172.16.4.0/23  172.16.4.0/24
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
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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
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Designing a VLSM Subnetting- Example

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
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Designing a VLSM Subnetting- Example

Step 4
19
Designing a VLSM Subnetting- Example

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
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VLSM Design II

Adding a new subnet to an existing design

Refer to Wendell Odom’s book
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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)
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172.16.4.1/24
VLSM Configuration
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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
172.16.4.1/24
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
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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
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Routing Table With Summarization
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Effect of (manual) Summarization
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Discards
unwanted packets
Syntax is protocol
dependent
Effect of (manual) Summarization
30
Syntax is protocol
dependent
Discards
unwanted packets
(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
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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!
Example Best Summary on Yosemite
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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
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
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Autosummarization
36
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.
Autosummarization Example
37
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
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
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Discontiguous Classful Networks
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Autosummarization prevents an
internetwork with a discontiguous
network from working properly
Discontiguous Classful Networks
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Solution:
Disable Autosummarization
Autosummarization disabled
on Yosemite and Seville
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
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Autosummarization Support
42
Support for VLSM
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