ADDRESSING STANDARDS :
The IANA Has Allocated Several Ranges Of IPv6 Addresses For Different Purposes. RFC 4294 Defines These Rules, Updating The Earlier RFC 3513 Document. This Document Defines Three General Classes Of Addresses “UNICAST, MULTICAST AND ANYCAST”.
FIRST UNDERSTAND THE IPv6 ADDRESS
AN INTERNET PROTOCOL VERSION 6 ADDRESS (IPv6 ADDRESSING) :
When The IPv4 Address Has 32 Bits, The Ipv6 Address Is Represented By A (Hopefully) Inexhaustible 128-Bit Number. Is A Numerical Label That Is Used To Identify A Network Interface Of A Computer Or Other Network Node Participating In An IPv6-Enabled Computer Network.
IP Addresses Serve The Purpose Of Uniquely Identifying The Individual Network Interface(S) Of A Host, Locating It On The Network, And Thus Permitting The Routing Of IP Packets Between Hosts. For Routing, IP Addresses Are Present In Fields Of The Packet Header Where They Indicate Source And Destination Of The Packet.
IPv6 Is The Successor To The Internet's First Addressing Infrastructure, Internet Protocol Version 4 (IPv4). In Contrast To Ipv4, Which Defined An IP Address As A 32-Bit Number, Ipv6 Addresses Have A Size Of 128 Bits. Therefore, IPv6 Has A Vastly Enlarged Address Space Compared To IPv4. IPv6 Address Types And Usage The 128-Bit IPv6 Address Is Totally Different From IPv4.
There Are Currently Fourteen Different Types Of Ipv6 Address Layouts, And More May Be Defined In The Future. Each Type Is Used For Different Purposes, And A Single Machine Typically Has Four To Six Types Actively Employed Or Available At Any One Time!
IPv6 ADDRESS NOTATION :
Leading Zeros In Each Field Can Be Omitted As Long As Each Field Is Represented By At Least One Value. The Exception To This Rule Is When There Is An Uninterrupted Series Of Zeros In One Or More Contiguous Fields. In This Case, The Series Of Zeros Can Be Replaced By “::”, With The Restriction That “::” Can Be Used Only Once In A Given Address. Applying This Convention To The Above Examples Results In The Following Address Notations:
2001:Db8::A9:215:60ff:Fe7a:Adc0
2001:Db8:260:0212::01b4
An Ipv6 Address Includes A Network Prefix And An Interface Identifier.
The Use Of "::" Indicates One Or More Groups Of 16 Bits Of Zeros.
The "::" Can Only Appear Once In An Address. The "::" Can Also Be Used To Compress Leading Or Trailing Zeros In An Address.
For Example, The Following Addresses :
2001:DB8:0:0:8:800:200C:417A - > A Unicast Address
FF01:0:0:0:0:0:0:101 - > A Multicast Address
0:0:0:0:0:0:0:1 - > The Loopback Address
0:0:0:0:0:0:0:0 - > The Unspecified Address
May Be Represented As:
2001:DB8::8:800:200C:417A - > A Unicast Address
FF01::101 - > A Multicast Address
::1 - > The Loopback Address
:: - > The Unspecified Address
PREFIX-LENGTH
Is A Decimal Value Specifying How Many Of The Leftmost Contiguous Bits Of The Address Comprise The Prefix.
For Example, The Following Are Legal Representations Of The 60-Bit Prefix 20010DB80000CD3 (Hexadecimal):
2001:0DB8:0000:CD30:0000:0000:0000:0000/60
2001:0DB8::CD30:0:0:0:0/60
2001:0DB8:0:CD30::/60
The Following Are NOT Legal Representations Of The Above Prefix :
2001:0DB8:0:CD3/60 - > May Drop Leading Zeros, But Not Trailing Zeros, Within Any 16-Bit Chunk Of The Address.
2001:0DB8::CD30/60 Address To Left Of "/" Expands To 2001:0DB8:0000:0000:0000:0000:0000:CD30
2001:0DB8::CD3/60 Address To Left Of "/" Expands To 2001:0DB8:0000:0000:0000:0000:0000:0CD3
When Writing Both A Node Address And A Prefix Of That Node Address E.G., The Node's Subnet Prefix), The Two Can Be Combined As Follows:
The Node Address 2001:0DB8:0:CD30:123:4567:89AB:CDEF And Its Subnet Number 2001:0DB8:0:CD30::/60
Can Be Abbreviated As 2001:0DB8:0:CD30:123:4567:89AB:CDEF/60
NETWORK PREFIX :
The Network Prefix (High-Order Bits) In An Ipv6 Address Begins With A Well-Known, Fixed Prefix For Defining The Address Type. Some Examples Of Well-Known, Fixed Prefixes Are:
2000::/3global (Routable) Unicast Address
Fd08::/8 Unique Local Unicast Address
Fe80::/8 Link-Local Unicast Address
Ff00::/8 Multicast Address
The Remainder Of The Network Prefix Depends On The Prefix Type, And Includes Information Such As The Subnet Destination Of Unicast Addresses Or The Flags And Scope Of Multicast Addresses.
In A Given Address, CIDR-Type Notation (Classless Inter-Domain Routing) Is Used To Define The Network Prefix. In The Following Address Example, The 64 Bits Comprising 2001:0db8:0260:0212 Form The Network Prefix:
2001:0db8:0260:0212:0215:60ff:Fe7a:Adc0/64
A Shorter Way To Show This Address Is To Remove The Leading Zeros:
2001:Db8:260:212:215:60ff:Fe7a:Adc0/64
The Full Ipv6 Address Is Written As Eight Blocks, Each Containing Four Hexadecimal Numbers. These Blocks Are Separated By Colons ":". A Hex Number Represents A 4-Bit Field, So A Group Of 4 Hex Numbers Represents 16 Bits. So We Must Have 8 Such Groups Of Numbers To Make Up The Full 128 Bits Of The Ipv6 Address:
x:x:x:x:x:x:x:x
As We Will Discuss In A Moment, Many Types Of Standard Ipv6 Address Implementations Involve A Lot Of Sequential Zeroes In The Address. When This Happens, There Are Some Useful Rules That Allow Us To Simplify The Address.
It's Easiest To Understand These Rules With A Concrete Example. Suppose We Have This Address:
FEC0:0000:0000:0001:0000:0000:0000:0001/64
We Can First Delete All Of The High Order Zeroes In Each Group Of Four Hex Numbers.
FEC0:0:0:1:0:0:0:1/64
Then We Can Replace The Longest Set Of Consecutive Zeroes With Simply "::":
FEC0:0:0:1::1/64
This Object Is Now Much Easier To Work With. Also Note That In The Example, We Have Included The Prefix Size /64, At The End Of The Address. This Convention Is Identical To The Prefix Size Bit Count Used In IPv4 Addresses.
This Is Not Only Similar In Appearance To The IPv4 CIDR Representation Of Addresses, But It Has The Same Function, Allowing Routers To Establish Multiple Hierarchical Address Summarizations In The Network.
IPV6 ADDRESS TYPES
IPV6 DEFINES THREE ADDRESS TYPES :
IPv6 Has Three Types Of Addresses, Which Can Be Categorized By Type And Scope:
Ipv6 Addresses Are Classified By The Primary Addressing And Routing Methodologies Common In Networking: Unicast Addressing, Anycast Addressing, And Multicast Addressing.
Unicast Addresses. A Packet Is Delivered To One Interface.
Multicast Addresses. A Packet Is Delivered To Multiple Interfaces.
Anycast Addresses. A Packet Is Delivered To The Nearest Of Multiple Interfaces (In Terms Of Routing Distance).
Ipv6 Addresses Of All Types Are Assigned To Interfaces, Not Nodes. An Ipv6 Unicast Address Refers To A Single Interface.Since Each Interface Belongs To A Single Node, Any Of That Node's Interfaces'unicast Addresses May Be Used As An Identifier For The Node.All Interfaces Are Required To Have At Least One Link-Local Unicast Address.
NOTE: IPv6 Does Not Use Broadcast Messages.
IPv6 Does Not Implement Broadcast Addressing. Broadcast's Traditional Role Is Subsumed By Multicast Addressing To The All-Nodes Link-Local Multicast Group Ff02::1. However, The Use Of The All-Nodes Group Is Not Recommended, And Most Ipv6 Protocols Use A Dedicated Link-Local Multicast Group To Avoid Disturbing Every Interface In The Network.
Unicast And Anycast Addresses In Ipv6 Have The Following Scopes (For Multicast Addresses, The Scope Is Built Into The Address Structure):
• Link-Local - > The Scope Is The Local Link (Nodes On The Same Subnet).
• Site-Local - > The Scope Is The Organization (Private Site Addressing).
• Global - > The Scope Is Global (Ipv6 Internet Addresses).
UNICAST IPV6 ADDRESSES : Ipv6 Has Several Major Unicast Address Types.
UNICAST GLOBAL ADDRESSES :
IPv6 Unicast Global Addresses Are Similar To Ipv4 Public Addresses. Also Known As Aggregatable Global Unicast Addresses, Global Addresses Are Globally Routable. The Structure Of An Ipv6 Unicast Global Address Creates The Three-Level Topology Shown In The Following Illustration.
INTERFACE (DEVICE) IDENTIFIER :
The Remaining (Low-Order) Bits In The Address Comprise A Unique Interface Identifier In An Ipv6 Address. In The Above Example, The Rightmost 64 Bits (215:60ff:Fe7a:Adc0) Comprise The Interface Identifier. Unlike Ipv4, An Ipv6 Identifier For A Unicast Address Can Be Automatically Generated From The Switch's MAC Address Using EUI-64 (Extended Unique Identifier) Format. Other Methods Include Dhcpv6 Assignments And Static Configuration. Interface Identifiers Are Covered In More Detail In The Later Sections Of This Chapter Describing Different Address Types.
LAB EXAMPLE
LAB FOR GENERATING IPV6 ADDRESSES AUTOMATICALLY EXAMPLE :
There Are Two Methods For Generating IPv6 Addresses Automatically. The First Uses The Autoconfig Command:
Router1#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.
Router1(Config)#IPv6 Unicast-Routing
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address Autoconfig
Router1(Config-If)#End
Router1#
The Second Method Uses The EUI-64 Method To Automatically Generate Only The Host Part Of The Ipv6 Address, Combined With A Defined Network Portion:
Router1#Configure Terminal
Enter Configuration Commands, One Per Line. End With Cntl/Z.
Router1(Config)#Ipv6 Unicast-Routing
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#IPv6 Address AAAA::/64 Eui-64
Router1(Config-If)#End
Router1#
DISCUSSION : Throughout This Chapter You Will See The Following Command Frequently:
Router1(Config)#Ipv6 Unicast-Routing
By Default, The Router Will Not Route Ipv6 Packets. You Can Configure Interfaces With Ipv6 Addresses, And You Can Even Use Ipv6 Applications Such As PING And TELNET To Communicate To And From These Routers.
And, Somewhat Confusingly, You Can Configure Static Ipv6 Routes To Allow The Routers To Do Simple IPv6 Networking. But Without This Command, You Cannot Enable Any Ipv6 Routing Protocols. So, Even Though We Are Not Using Any Routing Protocols In This Recipe, We Have Enabled The Ipv6 Unicast-Routing Command Because It Is A Good Practice And Will Help To Avoid Confusion Later On When You Do Want To Run Routing Protocols.
THE FIRST METHOD: Described In This Recipe Uses The Autoconfig Command. This Does Two Things. First, It Automatically Generates A Link-Local Address For Use On The Local Network Segment. As We Discussed In The Introduction To This Chapter, Link-Local Addresses Are Valid Ipv6 Addresses That Can Be Used To Communicate With Other Devices On The Segment, But That Do Not "Leak" Off Of It. So It Is Not Possible To Route Packets To These Addresses.
IPV6 LINK-LOCAL ADDRESS
WHY WOULD WE WANT TO CONFIGURE A LINK-LOCAL ADDRESS ON A ROUTER :
Well, the short answer is that RFC 4291 Says That Every Interface Must Have One. The Rationale Is That You Don't Need A Globally Scoped Address For Any Situation Where The Interface In Question Is Neither The Source Nor The Destination For A Packet.
FOR EXAMPLE : Suppose You Have Two Routers Connected By An Ethernet Segment. If The First Router Recives A Packet That It Wants Via The Second Router, The Next Hop Address In Its Routing Table Doesn't Need To Be Accessible Off The Segment.
And There Are Many Situations Like This When The Link-Local Address Can Be Used, Either Because The Packets Are Exchanged Purely Between Neighbors Or Because The Address Doesn't Appear In The Packet Header But Is Only Used For Routing Decisions:
Router1#show ipv6 interface FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::20E:D7FF:FED6:4D80
No global unicast address is configured
Joined group address(es):
FF02::1
FF02::1:FFD6:4D80
MTU is 1500 bytes
ICMP error messages limited to one every 100 milliseconds
ICMP redirects are enabled
ND DAD is enabled, number of DAD attempts: 1
ND reachable time is 30000 milliseconds
Router1#
The Address That The Router Generates This Way Uses The Standard Link-Local Prefix FE80::/10 And The EUI-64 Version Of The MAC Address For The Last 64 Bits. We Will Discuss The EUI-64 Encoding In A Moment.
THE SECOND THING : That This Command Does Is Check For Any Available Ipv6 DHCP Servers.
If DHCP Is Available On The Local Network Segment, Then The Router Will Automatically Attempt To Use This Protocol To Acquire A More General Ipv6 Address. If A DHCP Server Is Available, The Router Will Automatically Discover And Download An Appropriate Configuration. For More Information On DHCP For Ipv6.
The Other Method Shown In The Solution Section Of This Recipe Uses The EUI-64 Keyword:
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address AAAA::/64 Eui-64
In This Case, The Router Will Automatically Use The MAC Address Of The Interface To Generate The Host Portion Of The Ipv6 Address. In This Case, The Network Portion Of The Address Is AAAA::/64. Then, If We Look At The Interface With The Show Ipv6 Interface Command, We Can See What The Actual Address Is:
Router1#Show Ipv6 Interface Fastethernet0/0
Fastethernet0/0 Is Up, Line Protocol Is Up
Ipv6 Is Enabled, Link-Local Address Is FE80::20E:D7FF:FED6:4D80
Global Unicast Address(Es):
AAAA::20E:D7FF:FED6:4D80, Subnet Is AAAA::/64
Joined Group Address(Es):
FF02::1
FF02::1:FFD6:4D80
MTU Is 1500 Bytes
ICMP Error Messages Limited To One Every 100 Milliseconds
ICMP Redirects Are Enabled
ND DAD Is Enabled, Number Of DAD Attempts: 1
ND Reachable Time Is 30000 Milliseconds
Router1#
We Can Use The Show Interface Command To See The 48-Bit MAC Address For This Interface :
Router1#Show Interface Fastethernet0/0
Fastethernet0/0 Is Up, Line Protocol Is Up
Hardware Is Amdfe, Address Is 000e.D7d6.4d80 (Bia 000e.D7d6.4d80)
Internet Address Is 192.168.1.3/24
MTU 1500 Bytes, BW 100000 Kbit, DLY 100 Usec,
Reliability 255/255, Txload 1/255, Rxload 1/255
Encapsulation ARPA, Loopback Not Set
Keepalive Set (10 Sec)
Full-Duplex, 100Mb/S, 100basetx/FX
ARP Type: ARPA, ARP Timeout 04:00:00
Last Input 00:00:00, Output 00:00:00, Output Hang Never
Last Clearing Of "Show Interface" Counters Never
Input Queue: 0/75/0/0 (Size/Max/Drops/Flushes); Total Output Drops: 0
Queueing Strategy: Fifo
Output Queue: 0/40 (Size/Max)
5 Minute Input Rate 0 Bits/Sec, 0 Packets/Sec
5 Minute Output Rate 0 Bits/Sec, 0 Packets/Sec
10879 Packets Input, 839782 Bytes
Received 8284 Broadcasts, 0 Runts, 0 Giants, 0 Throttles
0 Input Errors, 0 CRC, 0 Frame, 0 Overrun, 0 Ignored
0 Watchdog
0 Input Packets With Dribble Condition Detected
12137 Packets Output, 908637 Bytes, 0 Underruns
0 Output Errors, 0 Collisions, 2 Interface Resets
0 Babbles, 0 Late Collision, 0 Deferred
0 Lost Carrier, 0 No Carrier
0 Output Buffer Failures, 0 Output Buffers Swapped Out
Router1#
As You Can See, The Interface Is Now Using The Address AAAA:: 20E:D7FF:FED6:4D80. The 48-Bit MAC Address On This Interface Is 000e.D7d6.4d80. There Is A Unique Correspondence Between The Host Portion Of The Ipv6 Address And This Globally Unique Ethernet MAC Address. The EUI-64 Encoding Is Defined In RFC 4291. For The Full Details, We Encourage Readers To Refer To That Document, But We Will Summarize The Scheme Here.
The IEEE 802 Standard 48-Bit MAC Address Used On Ethernet Interfaces Has A Well-Defined Format. The First 24 Bits Specify The Vendor Organizationally Unique Identifier (OUI), Which Identifies The Manufacturer Or The Equipment. The Remaining 24 Bits Specify An Individual Interface. The Vendor Portion Of The MAC Address Has 2 Special Bits In Locations 7 And 8. Bit 7 Is The "Scope" Indicator. If This Bit Is Equal To Zero, Then The MAC Address Has Global Scope, Meaning That It Can Be Used Anywhere. If It Is Equal To One, Then It Has Local Scope, Which Usually Means That It Has Been Redefined For Local Purposes. Bit 8 Is The "Individual/Group" Bit, Which Specifies Whether This MAC Address Is To Be Used By A Single Device Or A Collection Of Devices.
The First Rule For Converting 48-Bit MAC Addresses To Ipv6 EUI-64 Format Host Addresses Is To Flip The Scope Bit. If It Is Zero, Make It One, And Vice Versa. The Second Rule, Which Is Just Required To Pad The 48-Bit Address To 64 Bits, Is To Insert The Hex Value 0xfffe In Between The Vendor And Host Portions Of The MAC Address. So, In Our Case, When The MAC Address Is 000e.D7d6.4d80, The Second Octet Must Change From 0x00 (0000 0000 In Binary) To 0x02 (0000 0010 In Binary). Then, When We Insert The Hex Value 0xfffe, We Get 020e.D7ff.Fed6.4d80. Adding This As The Host Portion Of The Address To The Prefix That We Specified, AAAA::/64, We Get AAAA::020E:D7FF:FED6:4D80/64.
The Great Advantage To This Method Of Addressing Is That You Can Configure Exactly The Same Commands On Every Device On A Segment, And Allow Each Device To Ensure That It Has A Unique Ipv6 Address. In Fact, You Can Keep Your Configurations Very Simple By Just Having To Worry About The Prefixes For Each Network Segment.
The Disadvantage To This Method, However, Is That You Will Not Generally Know Ahead Of Time What The Full Ipv6 Address For Each Device Is, Only That They Will Be Unique. If You Use This Method, You Must Do Your Record Keeping After The Fact Rather Than As Part Of The Design Process. This Implementation Method Is Quite Foreign To The Usual Models Of Network Design For Ipv4 Networks.
IPV6 UNICAST ADDRESS
IPV6 UNICAST ADDRESS :
You Can Configure An Ipv6 Unicast Address On An Interface By Using A Very Similar Process To How We Set Up Ipv4 Addresses In Previous Chapters Of This Book:
Router1#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.
Router1(Config)#Ipv6 Unicast-Routing
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address AAAA::1/64
Router1(Config-If)#Exit
Router1(Config)#End
Router1#
We Can Configure An Ipv6 Anycast Address By Using The Anycast Keyword:
Router1#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.
Router1(Config)#Ipv6 Unicast-Routing
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address AAFF::1/64 Anycast
Router1(Config-If)#Exit
Router1(Config)#End
Router1#
You Can Specify An Ipv6 Link-Local Address By Using The Link-Local Keyword:
Router1#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.
Router1(Config)#Ipv6 Unicast-Routing
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address FE80::1 Link-Local
Router1(Config-If)#Exit
Router1(Config)#End
Router1#
DISCUSSION :
In This Recipe, We Have Manually Configured Three Different Types Of Ipv6 Addresses. The First Example Simply Configures A Standard Globally Accessible Unicast Address. This Is Similar To The Standard Ipv4 Unicast Address:
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address AAAA::1/64
Assigning An Address To An Address Like This Also Enables Ipv6 Functionality For The Interface:
Router1#Show Ipv6 Interface Fastethernet0/0
Fastethernet0/0 Is Up, Line Protocol Is Up
Ipv6 Is Enabled, Link-Local Address Is FE80::20E:84FF:FE24:4E70
Global Unicast Address(Es):
AAAA::1, Subnet Is AAAA::/64
Joined Group Address(Es):
FF02::1
FF02::2
FF02::1:FF00:1
FF02::1:FF24:4E70
MTU Is 1500 Bytes
ICMP Error Messages Limited To One Every 100 Milliseconds
ICMP Redirects Are Enabled
ND DAD Is Enabled, Number Of DAD Attempts: 1
ND Reachable Time Is 30000 Milliseconds
ND Advertised Reachable Time Is 0 Milliseconds
ND Advertised Retransmit Interval Is 0 Milliseconds
ND Router Advertisements Are Sent Every 200 Seconds
ND Router Advertisements Live For 1800 Seconds
Hosts Use Stateless Autoconfig For Addresses.
Router1#
Even Though We Have Only Assigned The Single Global Unicast Address, AAAA::1/64, To The Interface, It Now Has A Link-Local Address As Well, And It Has Joined Several Multicast Groups. Because We Didn't Specify The Link-Local Address, The Router Has Created One For Us Using The Standard FE80::/10 Prefix And The EUI-64 Host Address.
IPV6 ANYCAST ADDRESS
DEFINED AN IPV6 ANYCAST ADDRESS : In The Second Example In The Solution Section, We Have Defined An Anycast Address On This Interface:
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address AAFF::1/64 Anycast
This Command Is Currently Only Available On Certain Higher End Hardware Platforms. After Applying This Command To The Same Interface That We Were Discussing A Moment Ago, We Get The Following Output:
Router1#Show Ipv6 Interface Fastethernet0/0
Fastethernet0/0 Is Up, Line Protocol Is Up
Ipv6 Is Enabled, Link-Local Address Is FE80::20E:84FF:FE24:4E70
Global Unicast Address(Es):
AAAA::1, Subnet Is AAAA::/64
AAFF::1, Subnet Is AAFF::/64 [ANY]
Joined Group Address(Es):
FF02::1
FF02::2
FF02::1:FF00:1
FF02::1:FF24:4E70
MTU Is 1500 Bytes
ICMP Error Messages Limited To One Every 100 Milliseconds
ICMP Redirects Are Enabled
ND DAD Is Enabled, Number Of DAD Attempts: 1
ND Reachable Time Is 30000 Milliseconds
ND Advertised Reachable Time Is 0 Milliseconds
ND Advertised Retransmit Interval Is 0 Milliseconds
ND Router Advertisements Are Sent Every 200 Seconds
ND Router Advertisements Live For 1800 Seconds
Hosts Use Stateless Autoconfig For Addresses.
Router1#
The Line With This Newly Configured Anycast Address Is Indicated By [ANY].
IPV6 ANYCAST ADDRESSES: Can Be Extremely Useful In Allowing Several Different Devices To Fulfill A Single Function. A Typical Expected Use For This Feature Is The Ability To Give Remote Users Automatic Access To Your Backup Site If The Primary Site Fails, Without Having To Rely On DNS To Time Out. In This Case, You Would Distribute The Same Anycast Address Into The Global Routing Tables, But The Backup Site Would Have A Metric Or A BGP AS_PATH That Would Make It Less Desirable From Anywhere On The Network Than The Primary Site. If The Primary Site Becomes Unavailable, The Backup Site Would Automatically Take Over As Soon As The Global Routing Protocol Had Flushed The Primary Site From Its Tables. This Would Likely Take Less Time Than Waiting For DNS To Update Globally.
Another Potential Use For Anycast Addresses Is To Offer Several Equivalent Access Points Into Your Network. Remote Users Will Simply Find The Closest Access Point Through The Global Routing Protocol.
ANYCAST Would Not Function Well As A Replacement For Protocols Like HSRP For Two Reasons. First, When An Ipv6 Device Communicates With Its Next-Hop Router, It Uses Link-Local Addresses, Not A Global Address.
Second, Anycast Addresses Work Best When A Routing Protocol Distributes Them. Then The Network Will Automatically Determine Which Single Device To Send The Packets To. If Two Or More Anycast Devices Were The Same Distance Away, By Virtue Of Being On The Same Physical Segment, And If Both Devices Received The Packet, Both Would Assume That They Were The Only Router Receiving The Packet And Would Forward It Along. This Would Lead To Duplication Of Every Packet, And Result In Both Added Network Congestion And Protocol Confusion.
We Note In Passing That There Is Some Confusion In Some Of Cisco's Documentation Regarding Legitimate Uses For Anycast Addresses. The 12.4 Configuration Guide For Ipv6 States That Anycast Address May Only Be Used By Routers, Not Hosts, And That They May Not Be Used As Source Addresses In Any Packets. These Restrictions Were Present In RFC 3513, But Were Removed In RFC 4291, And Are No Longer Part Of The Ipv6 Standard.
The Final Example In The Solution Section Shows How To Manually Configure A Link-Local Address For The Segment:
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#Ipv6 Address FE80::1 Link-Local
NOTE: That There Is No CIDR Prefix Indicator On Link-Local Addresses. This Is Because All Link-Local Addresses Are Assumed To Be Purely Host Addresses, And Because Link-Local Addresses Are Not Advertised By Routing Protocols, So The Prefix Is Not Relevant.
Now When We Look At The Show Ipv6 Interface Command We See An Interesting Change: Router1#Show Ipv6 Interface Fastethernet0/0
Fastethernet0/0 Is Up, Line Protocol Is Up
Ipv6 Is Enabled, Link-Local Address Is FE80::1
Global Unicast Address(Es):
AAAA::1, Subnet Is AAAA::/64
AAFF::1, Subnet Is AAFF::/64 [ANY]
Joined Group Address(Es):
FF02::1
FF02::2
FF02::1:FF00:1
MTU Is 1500 Bytes
ICMP Error Messages Limited To One Every 100 Milliseconds
ICMP Redirects Are Enabled
ND DAD Is Enabled, Number Of DAD Attempts: 1
ND Reachable Time Is 30000 Milliseconds
ND Advertised Reachable Time Is 0 Milliseconds
ND Advertised Retransmit Interval Is 0 Milliseconds
ND Router Advertisements Are Sent Every 200 Seconds
ND Router Advertisements Live For 1800 Seconds
Hosts Use Stateless Autoconfig For Addresses.
Router1#
Note That The Route Has Replaced The Default Link-Local Address With The New One We Have Defined. These Link-Local Addresses Will Become More Significant In When We Start Talking About Routing Protocols.
We Should Finish This Discussion With A Brief Explanation Of The Information Shown At The End Of The Show Ipv6 Interface Command, The Lines That Begin "ND". This Referes To The Ipv6 Neighbor Discovery Process.
The Ipv6 Protocol Includes Automatic Features That Allow Devices That Share A Network Segment To Discover One Another. The Neighbor Discovery Protocol Is Extremely Simple, And Amounts To Little More Than Simply Sending ICMP Messages. The Values Shown In The Command Output Indicate How Often The Router Will Send These Messages. The Two Most Important Pieces Of Information Here Are The Line That Says "ND DAD Is Enabled", And The Lines That Describe "ND Router Advertisements".
s DAD Stands For Duplicate Address Detection, And Is Exactly That. When This Feature Is Enabled, And It Is Enabled By Default, The Router Will Periodically Send Out Packets Testing To See If Anybody Else On The Segment Has Taken Over Its Address.
ND Router Advertisements (RA) Packets Are ICMP Packets That The Router Sends Out Periodically To Advertise Itself As A Router Uses The Default Parameters, Advertising Itself As A Router Every 200 Seconds. And, By Default, It Will Retain Information About Other Routers Detected This Way For 1,800 Seconds.
For More About - > IPv6 Book - 1
For More About - > IPv6 Book - 2
For More About - > IPv6 Book - 3
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “CISCO IPv6 Configuration". Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts. Thank You And Best Of Luck.
This Article Written Author By: Premakumar Thevathasan. CCNA, CCNP, CCIP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+.
DISCLAIMER:
This Document Carries No Explicit Or Implied Warranty. Nor Is There Any Guarantee That The Information Contained In This Document Is Accurate. Every Effort Has Been Made To Make All Articles As Complete And As Accurate As Possible.
It Is Offered In The Hopes Of Helping Others, But You Use It At Your Own Risk. The Author Will Not Be Liable For Any Special, Incidental, Consequential Or Indirect Any Damages Due To Loss Of Data Or Any Other Reason That Occur As A Result Of Using This Document. But No Warranty Or Fitness Is Implied. The Information Provided Is On An "As Is" Basic. All Use Is Completely At Your Own Risk.
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