INTRODUCTION TO BGP CONNECTION :
They Must Themselves Share The Same Data Link Subnetwork That Their Border Gateways Share. Thus, A BGP Session Between Adjacent AS's Requires No Support From Either Inter-AS Or Intra-AS Routing.
Thus, At Each Connection, Each AS Has One Or More BGP Speakers And One Or More Border Gateways, And These BGP Speakers And Border Gateways Are All Located On A Shared Data Link Subnetwork.
Note: That BGP Speakers Do Not Need To Be A Border Gateway, And Vice Versa. Paths Announced By A BGP Speaker Of One AS On A Given Connection Are Taken To Be Feasible For Each Of The Border Gateways Of The Other AS On The Same Shared Subnetwork, I.E. Indirect Neighbors Are Allowed.
Much Of The Traffic Carried Within An AS Either Originates Or Terminates At That AS (I.E., Either The Source IP Address Or The Destination IP Address Of The IP Packet Identifies A Host Internal To That AS). Traffic That Fits This Description Is Called "Local Traffic". Traffic That Does Not Fit This Description Is Called "Transit Traffic". A Major Goal Of BGP Usage Is To Control The Flow Of Transit Traffic.
BASED ON HOW A PARTICULAR AS DEALS WITH TRANSIT TRAFFIC, THE AS MAY NOW BE PLACED INTO ONE OF THE FOLLOWING CATEGORIES :
Stub AS: An AS That Has Only A Single Connection To One Other AS. Naturally, A Stub AS Only Carries Local Traffic.
Multihomed AS: An AS That Has Connections To More Than One Other AS, But Refuses To Carry Transit Traffic.
Transit AS : An AS That Has Connections To More Than One Other AS, And Is Designed (Under Certain Policy Restrictions) To Carry Both Transit And Local Traffic.
Since A Full AS Path Provides An Efficient And Straightforward Way Of Suppressing Routing Loops And Eliminates The "Count-To-Infinity" Problem Associated With Some Distance Vector Algorithms, BGP Imposes No Topological Restrictions On The Interconnection Of AS's.
BGP NEIGHBOR RELATIONSHIPS :
The Internet Is Viewed As A Set Of Arbitrarily Connected AS's. Routers That Communicate Directly With Each Other Via BGP Are Known As BGP Speakers. BGP Speakers Can Be Located Within The Same AS Or In Different AS's. BGP Speakers In Each AS Communicate With Each Other To Exchange Network Reachability Information Based On A Set Of Policies Established Within Each AS. For A Given BGP Speaker, Some Other BGP Speaker With Which The Given Speaker Communicates Is Referred To As An External Peer If The Other Speaker Is In A Different AS, While If The Other Speaker Is In The Same AS It Is Referred To As An Internal Peer.
There Can Be As Many BGP Speakers As Deemed Necessary Within An AS. Usually, If An AS Has Multiple Connections To Other AS's, Multiple BGP Speakers Are Needed. All BGP Speakers Representing The Same AS Must Give A Consistent Image Of The AS To The Outside. This Requires That The BGP Speakers Have Consistent Routing Information Among Them.
These Gateways Can Communicate With Each Other Via BGP Or By Other Means. The Policy Constraints Applied To All BGP Speakers Within An AS Must Be Consistent. Techniques Such As Using A Tagged IGP May Be Employed To Detect Possible Inconsistencies.
In The Case Of External Peers, The Peers Must Belong To Different AS's, But Share A Common Data Link Subnetwork. This Common Subnetwork Should Be Used To Carry The BGP Messages Between Them. The Use Of BGP Across An Intervening AS Invalidates The AS Path Information. An Autonomous System Number Must Be Used With BGP To Specify Which Autonomous System The BGP Speaker Belongs To.
POLICY MAKING WITH BGP:
BGP Provides The Capability For Enforcing Policies Based On Various Routing Preferences And Constraints. Policies Are Not Directly Encoded In The Protocol. Rather, Policies Are Provided To BGP In The Form Of Configuration Information.
BGP Enforces Policies By Affecting The Selection Of Paths From Multiple Alternatives And By Controlling The Redistribution Of Routing Information. Policies Are Determined By The AS Administration.
Routing Policies Are Related To Political, Security, Or Economic Considerations. For Example, If An AS Is Unwilling To Carry Traffic To Another AS, It Can Enforce A Policy Prohibiting This.
THE FOLLOWING ARE EXAMPLES OF ROUTING POLICIES THAT CAN BE ENFORCED WITH THE USE OF BGP:
1. A Multihomed AS Can Refuse To Act As A Transit AS For Other AS's. (It Does So By Only Advertising Routes To Destinations Internal To The AS.)
2. A Multihomed AS Can Become A Transit AS For A Restricted Set Of Adjacent AS's, I.E., Some, But Not All, AS's Can Use The Multihomed AS As A Transit AS. (It Does So By Advertising Its Routing Information To This Set Of AS's.)
3. An AS Can Favor Or Disfavor The Use Of Certain AS's For Carrying Transit Traffic From Itself.
A NUMBER OF PERFORMANCE-RELATED CRITERIA CAN BE CONTROLLED WITH THE USE OF BGP:
1. An AS Can Minimize The Number Of Transit AS's. (Shorter AS Paths Can Be Preferred Over Longer Ones.)
2. The Quality Of Transit AS's. If An AS Determines That Two Or More AS Paths Can Be Used To Reach A Given Destination, That AS Can Use A Variety Of Means To Decide Which Of The Candidate AS Paths It Will Use. The Quality Of An AS Can Be Measured By Such Things As Diameter, Link Speed, Capacity, Tendency To Become Congested, And Quality Of Operation. Information About These Qualities Might Be Determined By Means Other Than BGP.
3. Preference Of Internal Routes Over External Routes.
For Consistency Within An AS, Equal Cost Paths, Resulting From Combinations Of Policies And/Or Normal Route Selection Procedures, Must Be Resolved In A Consistent Fashion.
Fundamental To BGP Is The Rule That An AS Advertises To Its Neighboring AS's Only Those Routes That It Uses. This Rule Reflects The "Hop-By-Hop" Routing Paradigm Generally Used By The Current Internet.
BEFORE GOING TO BGP MULTIHOMING WE SHOULD UNDERSTAND WHAT IS MULTI-HOMED HOST?
A Multi-Homed Host Is Known As A Computer That Has Multiple Network Connections, Of Which The Connections May Or May Not Be The Same Network. Web Hosts And Application Service Providers Setup A Multihomed Router (Or System) To Connect To Two Or More Upstream Internet Service Providers, And Setup Network Redundancy. The Term, Multihomed, Is Used To Denote That A System Is Connected To Multiple Networks.
In The Multi-Provider Internet, It Is Common For A Service Subscriber Customer) To Have More Than One Service Provider, Or To Have Arrangements For Redundant Connectivity To The Global Connected Internet.
There Are Many Reasons Why One Would Setup A Computer As A Multi-Homed Host. You May Wish To Setup A Network Redundancy For Higher Availability (Or Fault Tolerance), Setup Your Machine As A Network Gateway (A Function Of Router) For Sharing Internet With Multiple Hosts Within The LAN, Or Connect Your Host To Both Internet And Intranet. There May Be Other Reasons, But The Basic Fundamental Is To Connect A Host To Two Separate Network
When Setting Up A Dual NIC Cards (Multi-Homed Host), Only One Default Gateway Should Be Assigned To One Of The Two Network Adapters. If A NIC Is Used To Connect To The Internet, The Default Gateway Should Be Assigned To That NIC Card. For A NIC Card That Is Connected To The LAN, A Static Routing Entry Should Be Entered Into The Computer's Routing Table If The Host Needs To Obtain Network Resources Outside Of Its Subnet.
WHAT IS BGP MULTIHOMING?
Multiple Connections, Known As Multi-Homing, Multi-Homed Networks Are Often Connected To Several Different ISPs (Internet Service Providers). Each ISP Assigns An IP Address (Or Range Of IP Addresses) To The Company. Routers Use BGP (Border Gateway Protocol), A Part Of The TCP/IP Protocol Suite, To Route Between Networks Using Different Protocols. In A Multi-Homed Network, The Router Utilizes IBGP (Internal Border Gateway Protocol) On The Stub Domain Side And EBGP (External Border Gateway Protocol) To Communicate With Other Routers.
When Using NAT With Multi-Homing, The NAT Router Is Configured With Multiple Pools Of Inside Global Addresses Allocated By Different Isps. The Same Inside Local Address Should Be Mapped To More Than One Inside Global Address From The Configured Pools, Depending On The Provider Through Which The Traffic Gets Routed To The Destination. This Is Known As NAT By Destination.
BGP Is One Of The Key Tools For Achieving Internet Connection Redundancy. When You Connect Your Location To Two Different Internet Service Providers, It Is Called Multihoming.
Border Gateway Protocol (BGP) Is One Of The Key Protocols To Use To Achieve Internet Connection Redundancy. When You Connect Your Network To Two Different Internet Service Providers (ISPs), It Is Called Multihoming.
MULTIHOMING Provides Redundancy And Network Optimization. It Selects The ISP Which Offers The Best Path To A Resource
When You Run BGP With More Than One Service Provider, You Run The Risk That Your Autonomous System (AS) Will Become A Transit AS.
This Causes Internet Traffic To Pass Through Your AS And Potentially Consume All Of The Bandwidth And Resources On The CPU Of Your Router.
When You Multihome Your Network To Two Different ISPs, BGP Runs On Your Internet Router(S) And Provides Redundancy And Network Optimization By Selecting Which ISP Offers The Best Path To A Resource.
More Than One Link External To The Local Network Two Or More Links To The Same ISP Two Or More Links To Different ISPs.
Usually Two External Facing Routers One Router Gives Link And Provider Redundancy Only.
BGP Is Only Useful If You Are Multihomed (Have More Than One Internet Connection.) If You Only Have One Internet Connection, You Only Have One Path To The Internet, And BGP Would Only Ever Announce That One Path To Your Network. If That One Link Goes Down, There Is No Failover That Can Be Done Via BGP. A Network Topology With Only One Path Would Be Much Better Served By Using Static IP Routes.
BGP Allows For The Announcement To The Rest Of The Internet That You Have More Than One Path Into Your Network. This Means That Any Traffic Destined For Your Network Has A Redundant Path And Having Redundant Paths Into Your Network Unshackles You From A Single Point Of Network Transit Failure.
This Is Generally Accepted To Be A Very Good Thing Due To The Increased Uptime And Accessibility Of Your Network When Running BGP.
IPv4 MULTIHOMING :
In Order To Be Multihomed, A Network Must Have Its Own Public IP Address Range And An AS Number. Then A Connection To Two (Or More) Separate ISPs Is Established. The Routing Over These Connections Is Normally Controlled By A BGP Enabled Router.
In The Case Where One Outgoing Link From The Multihomed Network Fails, Outgoing Traffic Will Automatically Be Routed Via One Of The Remaining Links. More Importantly, Other Networks Will Be Notified, Through BGP Updates Of The Multihomed Network Routes, Of The Need To Route Incoming Traffic Via Another ISP And Link.
A Key Pitfall In Multihoming Is That Two Apparently Independent Links, From Completely Different ISPs May Actually Share A Common Transmission Line And/Or Edge Router. This Will Form A Single Point Of Failure And Considerably Reduce The Reliability Benefits From Multihoming.
Another Problem To Look Out For Is That Multihoming Too Small A Network May Not Be Effective Since Route Filtering Is Very Common Among BGP Users And Smaller Prefixes May Be Filtered Out. This Will Make Multihoming Fail.
IPv6 MULTIHOMING :
Multihoming In The Next-Generation Internet Protocol (IPv6) Is Not Yet Standardized, As Discussions About The Various Possible Approaches To Multihoming Are Still Unresolved.
Provider Independent Address Space Is Available In IPv6. This Technique Has The Advantage Of Working Like IPv4, Supporting Traffic Balancing Across Multiple Providers, And Maintaining Existing TCP And UDP Sessions Through Cut-Overs.
BPG MULTIHOMING STEPS ARE :
1. Obtain Your ASN.
2. Identify Your Network Block Of IP Addresses. If You Own These, Then You Have The Right To Advertise Them On The Internet Through BGP. If You Are Borrowing These From Your Provider, Then You Must Ask Your Provider For Permission Before Advertising Them Through Another Provider.
3. If You Have A Single Provider, You Are Typically Using A Static Route To Connect To That Provider. That Provider Is Not Sending You Any BGP Routes. Assuming That Is True, You Will Have To Request That Your Provider Send You BGP Routes. (Your Provider Will Need To Know Your ASN And Your Remote Router’s Neighbor Address.
The Neighbor Is The IP Address That Your BGP Process Uses To Communicate With.) Once You Have The Provider's BGP Routes In Your Routing Table And You Are Advertising Your Network To Your Provider Through BGP, You Can Remove Your Static Route And Have Your Provider Remove Their Static Route.
4. Next, Assuming That You Are Multihoming On A Single Router, Bring Up Your Secondary Provider. They Can Set It Up So That They Send You BGP Routes. Again, They Will Need To Know Your ASN And Your Neighbor Address.
5. Within The BGP Table (Database) On Your Router, You Will See The Routes From Each Of Your Providers. The Best Route In BGP Is The Route With The Shortest AS Path. (If The AS Paths Are Identical, There Is A Tiebreaking Procedure, But This Is Normally Not The Case.) The Route That Has The Shortest AS Path Will Be Placed In Your Router’s Routing Table.
If You Lose The Connection To One Of Your Internet Service Providers, BGP’s Keep Alive Packets Will Time Out, And That BGP Neighbor (From That ISP) Will Become Down. Those Routes Will Be Removed From The BGP Table, And Thus, From Your Router’s Routing Table. Then, With Only One Set Of Routes In Your BGP Table, That Provider’s Routes (The Secondary/Redundant Provider) Are Marked As “Best” And Placed In Your Routing Table.
Normally, There Will Be Some Paths From One Provider That Are Shorter Than The Other ISP, And Vice Versa. So Your Traffic Will Be Distributed To The Provider With The Best AS Path For Whatever Networks Are Advertised. However, If You Are Sending More Traffic To A Certain Network (Through One Provider) Than Your Link To That Provider Can Handle, The Extra Traffic Will Not Be Distributed Onto Your Second Link. Using BGP Metrics, You Can Attempt To Do Different Forms Of Load Distribution, But There Is No Real Form Of BGP Load Balancing.
NOTE:In This Network, 1010:1010::/64 And 2020:2020::/64 Is Advertised By AS 101 To The Outside And Network 1212:1212::/64 Is Received From Two Different AS, AS 202 And AS 303.
START CONFIGURATIONS :
This Document Uses These Configurations:
IPv6 CONFIGURATION ON ROUTER-A:
Router-A#
Ipv6 Unicast-Routing
Ipv6 Cef
Interface Serial3/0
Description CONNECTED TO Service Provider - A
Ip Address 192.168.10.1 255.255.255.0
Ipv6 Address 1202:ABCD::/64 Eui-64
Ipv6 Enable
No Fair-Queue
Clock Rate 64000
!
Interface Serial3/1
Description CONNECTED TO Service Provider - B
No IP Address
Ipv6 Address 2303:ABCD::/64 Eui-64
Clock Rate 64000
!
Router Bgp 101
Bgp Router-Id 1.1.1.1
No Bgp Default Ipv4-Unicast
Bgp Log-Neighbor-Changes
Neighbor 1202:ABCD::21B:54FF:FEA9:24B0 Remote-As 202
Neighbor 1202:ABCD::21B:54FF:FEA9:24B0 Ebgp-Multihop 2
Neighbor 2303:ABCD::21B:54FF:FE54:FB10 Remote-As 303
!
Address-Family Ipv6
Neighbor 1202:ABCD::21B:54FF:FEA9:24B0 Activate
Neighbor 2303:ABCD::21B:54FF:FE54:FB10 Activate
Network 1010:1010::/64
Network 2020:2020::/64
Exit-Address-Family
!
CONFIGURATION ON SERVICE PROVIDER - A:
Service Provider A#
Ipv6 Unicast-Routing
Ipv6 Cef
Interface Serial1/0
No Ip Address
Ipv6 Address 1202:Abcd::/64 Eui-64
Ipv6 Enable
No Fair-Queue
!
Router Bgp 202
Bgp Router-Id 2.2.2.2
No Bgp Default Ipv4-Unicast
Bgp Log-Neighbor-Changes
Neighbor 1202:Abcd::21c:58ff:Feed:3e90 Remote-As 101
!
Address-Family Ipv6
Neighbor 1202:Abcd::21c:58ff:Feed:3e90 Activate
Network 1212:1212::/64
Exit-Address-Family
!
CONFIGURATION ON SERVICE PROVIDER - B:
Service Provider B#
Ipv6 Unicast-Routing
Ipv6 Cef
Interface Serial1/0
No Ip Address
Ipv6 Address 2303:Abcd::/64 Eui-64
No Fair-Queue
!
Router Bgp 303
No Synchronization
Bgp Router-Id 3.3.3.3
Bgp Log-Neighbor-Changes
Neighbor 2303:Abcd::21c:58ff:Feed:3e90 Remote-As 101
Neighbor 2303:Abcd::21c:58ff:Feed:3e90 Ebgp-Multihop 5
No Auto-Summary
!
Address-Family Ipv6
Neighbor 2303:Abcd::21c:58ff:Feed:3e90 Activate
Network 1212:1212::/64
Exit-Address-Family
!
TO VERIFY (Show Command):
Show Ipv6 Route
Show Ipv6 Route Bgp
Show Bgp Ipv6 Unicast Summary
Router A peering with Two ISPs
Router-A#show bgp ipv6 unicast summary
BGP Router Identifier 1.1.1.1, Local AS Number 101
BGP Table Version Is 6, Main Routing Table Version 6
3 Network Entries Using 447 Bytes Of Memory
4 Path Entries Using 304 Bytes Of Memory
4/2 BGP Path/Bestpath Attribute Entries Using 496 Bytes Of Memory
2 BGP AS-PATH Entries Using 48 Bytes Of Memory
0 BGP Route-Map Cache Entries Using 0 Bytes Of Memory
0 BGP Filter-List Cache Entries Using 0 Bytes Of Memory
BGP Using 1295 Total Bytes Of Memory
BGP Activity 3/0 Prefixes, 14/10 Paths, Scan Interval 60 Secs
Neighbor V AS Msgrcvd Msgsent Tblver Inq Outq Up/Down State/Pfxrcd
1202:ABCD::21B:54FF:FEA9:24B0 4 202 108 119 6 0 0 00:31:41 1
2303:ABCD::21B:54FF:FE54:FB10 4 303 108 121 6 0 0 00:25:1 1
ROUTER - A LEARNED ROUTES FROM SERVICE PROVIDER - A & SERVICE PROVIDER - B
Router-A#Show Bgp Ipv6 Unicast
BGP Table Version Is 6, Local Router ID Is 1.1.1.1
Status Codes: S Suppressed, D Damped, H History, * Valid, > Best, I - Internal,
R RIB-Failure, S Stale
Origin Codes: I - IGP, E - EGP, ? - Incomplete
Network Next Hop Metric Locprf Weight Path
*> 1010:1010::/64 :: 0 32768 I
* 1212:1212::/64 2303:ABCD::21B:54FF:FE54:FB10 0 0 303 I
*> 1202:ABCD::21B:54FF:FEA9:24B0 0 0 202 I
*> 2020:2020::/64 :: 0 32768 I
ON SERVICE PROVIDER - A:
Sp-A#Sh Bgp Ipv6 Unicast
Bgp Table Version Is 4, Local Router Id Is 2.2.2.2
Status Codes: S Suppressed, D Damped, H History, * Valid, > Best, I - Internal,
R Rib-Failure, S Stale
Origin Codes: I - Igp, E - Egp, ? - Incomplete
Network Next Hop Metric Locprf Weight Path
*> 1010:1010::/64 1202:Abcd::21c:58ff:Feed:3e90 0 0 101 I
*> 1212:1212::/64 :: 0 32768 I
*> 2020:2020::/64 1202:Abcd::21c:58ff:Feed:3e90 0 0 101 I
On Service Provider - B:
Sp-B#Sh Bgp Ipv6 Unicast
Bgp Table Version Is 4, Local Router Id Is 3.3.3.3
Status Codes: S Suppressed, D Damped, H History, * Valid, > Best, I - Internal,
R Rib-Failure, S Stale
Origin Codes: I - Igp, E - Egp, ? - Incomplete
Network Next Hop Metric Locprf Weight Path
*> 1010:1010::/64 2303:Abcd::21c:58ff:Feed:3e90 0 0 101 I
* 1212:1212::/64 2303:Abcd::21c:58ff:Feed:3e90 0 101 202 I
*> :: 0 32768 I
*> 2020:2020::/64 2303:Abcd::21c:58ff:Feed:3e90 0 0 101 I
TROUBLESHOOTING COMMANDS:
Use The Debug BGP IPv6 Update Command In Order To Display Debugging Information On The Updates To Help Determine The State Of The Peering.
IMPORTANT COMMANDS TO MONITOR AND TROUBLESHOOT BGP :
SHOW IP BGP NEIGHBOR IP-ADDRESS - DISPLAYS DETAILED NEIGHBOR INFORMATION
SHOW IP BGP - DISPLAYS ALL THE ROUTES IN THE BGP TABLE
SHOW IP BGP IP-PREFIX [MASK SUBNET-MASK] - DISPLAYS DETAILED INFORMATION ABOUT ALL PATHS FOR A SINGLE PREFIX
DEBUG IP TCP TRANSACTIONS - DISPLAYS ALL TCP TRANSACTIONS
DEBUG IP BGP EVENTS - DISPLAYS SIGNIFICANT BGP EVENTS
DEBUG IP BGP KEEPALIVES - DEBUGS BGP KEEPALIVE PACKETS
DEBUG IP BGP UPDATES - DISPLAYS ALL INCOMING OR OUTGOING BGP UPDATES
DEBUG IP BGP UPDATES ACL - DISPLAYS ALL INCOMING AND SENT UPDATES MATCHING AN ACL
DEBUG UP BGP IP-ADDRESS UPDATE [ACL] - DISPLAYS ALL BGP UPDATES RECEIVED FROM OR SENT TO A SPECIFIC NEIGHBOR
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “CISCO - BGP (IPv6) MULTIHOMING CONFIGURATION EXAMPLE ” .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.
The School Of Cisco Networking (SCN)
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