Networking Lab Class #6 VLSM & Route Summarization Parviz Kermani Spring 2012 UMasss Amhers A mherstt
Acknowledgement
Wendel Odom: CCNA ICND2 : Official Exam Certification 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
Mask: 255.255.255.0
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VLSM
Note: using more than one mask does not constitute VLSM by itself
But using more than one mask in 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 p rotocols: ols: Classless routing protoc
Advertise mask information for each subnet Support VLSM (V ( Variable Length Subnet Mask) Route summarization
An inherent property of a routing protocol
Not configurable 6
Support for VLSM
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Without VLSM Support (RIP V1) Albuquerque#show ip route Codes: C - connected, S - static, I - IGRP , R - RIP, M - mobile, B - BGP D - EIGRP EIG RP,, EX - EIGRP external, O - OSPF OSP F, IA - OSPF inter area a rea 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
C C C R
172.16.0.0/24 is subnetted, 3 subnets 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 10.0.0.0 [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
172.16.0.0/24 is subnetted, 3 subnets 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 10.0.0.0/24 is subnetted, 6 subnets 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 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 only from particular parts of the internet
WRONG
DESIGN 10
Dealing with VLSM Subnets
Two types of problems engineers faced with Analyze:
Design
Analyze a design to detect overlaps Choose/add new VLSM subnet avoiding avoiding overlap
Analyze Calculate range of addresses for each subnet Check for overlap
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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 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 least 10 subnets Largest subnet 200 hosts Choose a design with the larg l argest est number of subnets rd Choose the 3 subnet
Design At least least 4 subnet bit; At least least 8 host bits bit s 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 requirement to determine design goals. Design goals: Use Class Class B network 172.16.0.0 Three subnets subnets with mask/24 (255.255.255.0) (255.255.25 5.0) Three subnets subnets with mask /26 (255.255.255.192) Four subnets with with mask m ask /30 (255.255.255.253)
Point-to-point links
Compare it with a classful 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
n ext-longestt prefix Step 4: Identify the next-longes
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 Range 182.16.2.1-172.16.2.254 182.16.2.1-172.16.2.254
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
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Designing a VLSM Subnetting- Example
Steps 3 & 4 until done Step 5: Repeat Steps
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 Routers configure VLSM by virtue of at least 2 router interfaces On the same router or among all routers IP addresses 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
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VLSM Configuration 172.16.4.1/24
R3#conf conf i R3( con conf i R3( con conf i R3( con conf i R3( con conf i
gur e t er mi nal g) #i nt er f ace Fa0/ 0 g) #i p ad addr ess 172. 16. 5. 1 g) #i nt er f ace S0/ 0/ 1 g) #i p ad addr ess 172. 16. 9. 6
255. 255. 255. 0 255. 255. 255. 252
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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 tim e to route packets Large table more time needed to troubleshoot!
Route 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
Routing protocol advertises just the summary s ummary route, as opposed to the original route
Includes all IP addresses cover covered ed by subnets in the original routes routes
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 d esigned 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 ffect of (manual) Summarization
Syntax is protocol dependent
Discards unwanted packets
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Effect ffect of (manual) Summarization
Syntax is protocol dependent
Discards unwanted packets
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(Manual) Route Summarization Strategies
Best summarization
Should include all desired subnets with as few other addresses, if possible!
Example (Yosemite): (Yosemite):
Subnets Subnets 10.2.1.0, 10.2.2.0, 10.2.3.0, 10.2.4.0 (/24) summarized summarized into into 12.2.0.0 12. 2.0.0/16 /16
Summary includes a lot of IP addresses no in the four subnets
Correct, but not good! 31
(Manual) Route Summarization Strategies to-be-summarized subnets sub nets in binary Step 1: List all to-be-summarized 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, 10.3.5.0, 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 01 00 0000 0000 10 . 3 . 4 . 0 Step 4: (summary) Subnet Mask: 1111 1111 1111 1111 1111 11 00 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 10.3.7.254 (Bcast 10.3.7.255) Perfect!
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Example Best Summary Summar y on Yosemite Subnets: 10.2.1.0, 10.2.2.0, 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 0 000 0000 0000 10 . 2 . 0 . 0 Step 4: (summary) Subnet Mask: 1111 1111 1111 1111 1111 1 000 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 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 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.
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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
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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 Autosummarization prevents an internetwork with a discontiguous network from working properly
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Discontiguous Classful Networks Solution: Disable Autosummarization
Autosummarization disabled on Yosemite and Seville
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Autosummarization Support
Classful routing protocol p rotocolss must use autosummarization.
Some classless routing protocols support autosummarization
Default configuration
Can be disabled
OSPF (classless) DOES NOT support autosummarization 41
Autosummarization Support
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Support for VLSM
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