Transcript CCNA 3
CCNA 3/Module 1
Introduction to Classless Routing
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Overview: Classful/Classless Routing
•
Classful routing
- a network must use the same subnet mask for the entire network Network IP Network Subnet Mask 192.168.187.0
255.255.255.0
Classless routing –
network address using more than one subnet mask for a
• “subnetting a subnet”
Network IP Network Subnet Masks 192.168.187.0
255.255.255.252
255.255.255.0
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Overview: (Classful) IPv4 Addressing Limits
• •
IPv4 – 20 years old IPv4 – even with subnetting, couldn’t handle the global demand for Internet connectivity
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Class B space was on the verge of depletion.
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Rapid and substantial increase in the size of the Internet's routing tables.
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As more Class C's came online, the flood of new network information threatened Internet routers' capability to cope.
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Overview: (Classful) IPv4 Addressing Limits
• •
Provides IP scheme with limitations:
•
Class A – 126 networks: 16,777,214 hosts each
• •
Class B Class C – 65,000 networks: 65,534 hosts each – 2 million networks: 254 hosts each While available addresses were running out, only 3% of assigned addresses were actually being used!
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Subnet zero, broadcast addresses, pool of unused addresses at Class A and B sites, etc.
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Overview: Scalability & Routing Tables
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Maximum theoretical routing table size is 60,000 entries.
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Classful addressing would have hit this capacity by mid-1994.
•
Internet growth would have ended .
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1.1.1 What is VLSM and why is it used?
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The purpose of VLSM is to alleviate the shortage of IP addresses
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VLSM allows:
• •
More than one subnet mask within the same network Or . . .
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Multiple SNMasks with ONE IP Address Use of long mask on networks with few hosts
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Use of short mask on networks with many hosts
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In order to use VLSM, the routing protocol must support it.
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Cisco routers with the following routing protocols support VLSM:
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OSPF (Open Shortest Path First)
• • • •
IS-IS (Integrated Intermediate System to Intermediate System) EIGRP (Enhanced Interior Gateway Routing Protocol) RIP v2 Static Routing
No RIP v1 6
1.1.1 What is VLSM and why is it used?
Classful routing protocols use one subnet mask for a single network
•
Ex: 192.168.187.0, must use subnet mask 255.255.255.0
VLSM allows a single autonomous system to have networks with different subnet masks , for example:
•
Use a
•
( 30-bit subnet mask on network connections 255.255.255.
252
) •
Use a 24-bit subnet mask for user networks up to 250 users
•
( 255.255.255.
0
) •
Use a
•
( 22-bit 255.255.
subnet mask for user networks up to 1000 users 252.0
)
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1.1.2 A waste of space
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In classless routing, it was recommended that first and last subnet not be used
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First ( SN 0 ) had same address for the network and subnet
•
Last subnet ( all 1’s ) was the broadcast
• • •
Always could have been used, was not recommended practice Address depletion has lead to use of these subnets Now acceptable practice to use conjunction with VLSM the first and last subnets in
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1.1.2 A waste of space
Network Address Borrow 3 bits = SNM Subnets = 192.168.187.0
255.255.255.224
0, 32, 64, 96, 128, 160, 192, 224
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1.1.2 A waste of space
Network Address Borrow 3 bits = SNM Subnets = 192.168.187.0
255.255.255.224
0, 32, 64, 96, 128, 160, 192, 224 If subnet zero is used, there are 8 useable subnets
• •
Each subnet can support 30 hosts Cisco routers use subnet zero by default IOS v. 12.0+ If no ip subnet-zero command is used on the router, there are 7 useable subnets with 30 hosts per subnet
•
If supporting
4
routers (1 subnet each) that need 3 WAN links to each other, all subnets are used
• •
No room for growth Waste of 28 host addresses for each WAN (point-to point) links or 1/3 of potential address space
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1.1.2 A waste of space FOSTER(config)# no ip subnet-zero
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Disables the capability to use subnets that include the network address of the unsubnetted network
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1.1.3 When to use VLSM
• •
Design addressing scheme that allows: Growth Doesn’t waste addresses on point-to-point links VLSM addressing applied instead results in:
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Variable sized subnets
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Take
1
of the
3
subnets and
subnet it again •
Example 192.168.187.224
(last subnet)
•
Apply a 30 bit mask (225.225.225.252)
• •
Creates a possible 8 ranges of addresses with 30 bits Best solution for instead of 30 point-to-point links – use 2 host addresses
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1.1.4 Calculating subnets with VLSM
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VLSM helps to manage IP addresses VLSM can use one SNM for a point-to-point link and one SNM for a LAN 0
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1.1.4 Calculating subnets with VLSM
• • F o s t e r ’ s
2 routers
• • • F a b u l o
1 in Ravenna (50 hosts)
u s
1 in Hollywood (100 hosts) 1 WAN link (2 needed) IP/NW Address: 192.16.10.0
•
Class C
F i l m s Use the BIGGEST first: 100 50 2 14
1.1.4 Calculating subnets with VLSM
• • F o s t e r ’ s
2 routers
• • • F a b u l o
1 in Ravenna (50 hosts)
u s
1 in Hollywood (100 hosts) 1 WAN link (2 needed) IP/NW Address: 192.16.10.0
•
Class C
F i l m s Use the BIGGEST first: 100 /25 126 usable hosts 50 /26 2 /30 62 usable hosts 2 usable hosts 15
1.1.4 Calculating subnets with VLSM
If VLSM were used instead of classful routing:
•
A 24-bit mask
hosts
could be used for LAN segments for 250
• •
A 172.16.32.0
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30-bit mask /20 could be used for WAN segments for (would accommodate 4094 hosts) 2 hosts Binary = 10101100.00010000.00100000.00000000
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SNM = 11111111.11111111.11110000.00000000
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VLSM address172.16.32.0
• • /26
(needed for
62
hosts) Binary = 10101100.00010000.00100000.00000000
SNM = 11111111.11111111.11111111.11000000
•
If 172.16.32.0
/20 used, but only provide
4094 hosts
and
10 hosts waste 4084
on segment, would addresses
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By further subnetting /20 to /26, gain 64 subnets (2 supporting 62 hosts 6 ) each
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1.1.4 Calculating Subnets w/VLSM
Procedure to subnet a subnet /20 to /26 using VLSM: 1. Write 172.16.32.0 in binary form
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Binary = 10101100.00010000.00100000.00000000
2.
Draw a vertical line between the 20 th and 21 st bits (the original subnet boundary) 3. Draw a vertical line between the 26 th and 27 th to segment/host needs bits extending the bits 4. Calculate the number of subnet addresses between the two vertical lines (lowest to highest) in value
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1.1.4 Calculating Subnets w/VLSM
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Keep in mind that only unused subnets can be further subnetted
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If any address for a subnet is used cannot be further subnetted
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1.1.5 Route Aggregation w/VLSM
• • • •
Every network needs a separate entry in routing table Each subnet needs a separate entry Aggregation will reduce routing table size When using VLSM keep subnetwork numbers grouped together in the network to allow for aggregation by using C lassless I nter D omain R outing (CIDR)
• •
172.16.
172.16.
14 15
.
0 .0
•
Router needs to carry only one route 172.16.
14.
0/23
14 in Binary = 00001110 15 in Binary = 00001111
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1.1.5 Route Aggregation w/VLSM
• • • • •
Using CIDR and VLSM
prevents address waste
and
promotes route aggregation
or summarization
•
Without summarization, Internet would collapse Summarization
reduces burden
on upstream routers This process of summarization continues until
entire network
is advertised as a
single aggregate
route Summarization is also called supernetting Possible if the routers of a network run a
classless routing protocol
such as
OSPF or EIGRP • •
Consists of organization IP address The summary route and bit mask uses prefix in routing updates common to all addresses of
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1.1.5 Route Aggregation w/VLSM
• •
Carefully
assign addresses
in a
hierarchical high
-order
bits for summarization
fashion to share
same • • •
A router must know subnets attached A router does A router using table
not
need to
aggregate tell
other routers about routes has in detail
fewer subnets
entries in routing VLSM allows for summarization of routes
•
Works even if networks are not contiguous VLSM increases flexibly by summarization on higher-order bits
• •
Used to calculate the network number of the summary route Uses only shared highest-order bits
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1.1.6 Configuring VLSM
• •
If VLSM is chosen, it must be configured correctly Example: 192.168.10.0 (Class C)
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One router has to support portion of address to provide 62 possible address
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(2 6 = 64 – 2 = 62) 60 hosts , needs 6 bits in host
192.168.10.0
/26 (leaves 6 bits for hosts)
•
One router has to support portion of address to provide 30 possible hosts
• (2 5 = 32
28 hosts
– 2 = 30) 192.168.10.64
, needs 5 bits in host /27 (leaves 5 bits for hosts)
•
Two routers have to support 12 hosts each, needs 4 bits in host portion of address to provide 14 possible hosts
(2 4 = 16 – 2 = 14) 192.168.10.96
/28 (leaves 4 bits for hosts)
192.168.10.112/28
(leaves 4 bits for hosts)
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1.1.6 Configuring VLSM
•
Point-to-point connections are:
• 192.168.10.128/30
(2 address required, 2 bits = 2 host addresses)
• • 192.168.10.132/30 192.168.10.136/30 •
(2 address required, 2 bits = 2 host addresses) (2 address required, 2 bits = 2 host addresses) Choices = .136 .137 .138
.139
•
Configuration as follows for the 192.168.10.136/30 network (.
136/
30 -
network
address;.
139/
30 -
broadcast
address; .
137/
30 and
138/
30 –
host
addresses:
• •
(config)#interface serial 0 (config-if)#ip address 192.168.10.137 255.255.255.252
• •
(config)#interface serial1 (config-if)#ip address 192.168.10.138 255.255.255.252
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1.2.1 RIP History
Internet is a collection of autonomous systems (AS)
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Each AS is administered by a single entity
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Each AS has its own routing technology Routing protocol used within AS is Interior Gateway Protocol Routing protocol used between Protocol Autonomous Systems is an Exterior Gateway RIP v1:
• • •
is an IGP that is classful was designed to work within moderate-sized AS is a distance vector routing protocol
• • • • •
by default, broadcasts entire routing table every 30 seconds uses hop count as metric (16 max) is capable of load balancing 6 equal-cost paths (4 default) Does not send subnet mask information in its updates Is not able to support VLSM or CIDR
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1.2.1 RIP History
If the router receives information about a network, and the receiving interface belongs to same network but is on a different subnet , the router applies the one subnet mask configured on the receiving interface
• • •
Class A default classful mask is 255.0.0.0
Class B default classful mask is 255.255.0.0
Class C default classful mask is 255.255.255.0
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1.2.2 RIP v2 Features
RIP v2 is an Improved version of RIP v1 with following features:
•
Distance vector protocol
• •
Uses hop count as metric Uses hold-down timers (prevent routing loops), default 180 sec.
• • • • • •
Uses split horizon to prevent routing loops Uses 16 hops as infinite distance Provides prefix routing (sends subnet mask with route update) Supports use of classless routing (VLSM) Multicasts updates using 224.0.0.9 address for better efficiency Provides authentication in updates
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Clear text - default
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MD5 encryption – typically used to encrypt enable secret passwords (Message-Digest 5)
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1.2.3 Comparing RIP v1 & v2 RIP v1
Easy to configure
RIP v2
Easy to configure Supports
classful
routing No subnet info sent with routing updates (considered a limitation of v1)
No
authentication Supports
classless
routing Sends subnet mask with routing update Provides for authentication Uses hop count Uses hop count 16 hops as metric for infinite distance 16 hops as metric for infinite distance Broadcasts routing table updates 255.255.255.255
Does not support prefix routing (all devices in same network must use same subnet mask) Multicasts updates 224.0.0.9
Supports prefix routing (VLSM, different subnet masks can be used in same network)
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1.2.4 Configuring RIP v2
To enable a dynamic routing protocol: 1. Select routing protocol
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FOSTER(config)# router rip
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FOSTER(config-router)# version 2 2. Configure routing protocol with the network IP address (identify physically connected network that will receive routing tables)
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FOSTER(config-router)# network 10.0.0.0
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FOSTER(config-router)# network 172.16.0.0
3. Assign IP/SNM to interfaces
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1.2.5 Verifying RIP v2
FOSTER# FOSTER# FOSTER#
show ip interface brief
FOSTER#
show ip protocols show ip route show running-config
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Shows protocol name
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Tells when updates are sent and when the next is due
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Tells if routers have learned about a newly added network
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Displays IP routing table
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Summary of information
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status of interface Checks for a misconfigured routing protocol
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1.2.5 Verifying RIP v2
• • •
RIP updates table every 30 seconds If no update received in 180 seconds, route marked as down If no update after 240 seconds, removes from routing table entry
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1.2.6 Troubleshooting RIP v2
Foster# debug ip rip Foster# no debug all Foster# undebug all Displays RIP routing updates as they are sent and received Turns off all debugging
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1.2.7 Default Routes
Three ways a router learns about paths: 1. Static routes – manual configuration of routes (next hop)
•
Uses ip route command 2. Default routes – manually defined path to take when there is no known route to a destination 3. Dynamic routes – routers lean paths by receiving updates from other routers
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1.2.7 Default Routes
Static Route Command: FOSTER(config)# ip route 172.16.1.0
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1.2.7 Default Routes
DYNAMIC PROTOCOL Default Route Command FOSTER(config)# ip default-network 192.168.20.0
Default NW Used to: 1. Give packets that are not ID’d in the routing table a place to go
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Usually a router that connects to the Internet 2. Connect a router with a static default route
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