VLSM & Route Summarization.pdf

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

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This Class

VLSM Route Summarization

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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

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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!

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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

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Support for VLSM

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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)

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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

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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

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Analyzing an existing design

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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.

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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 3rd 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

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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

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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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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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Step 4

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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

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Route Summarization Concepts

Works better if network is designed with summarization in mind

Example of a good design

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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

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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!

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(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!

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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

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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

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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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Autosummarization prevents an internetwork with a discontiguous network from working properly


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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

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Support for VLSM

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VLSM & Route Summarization.pdf

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