CISCO – BGP LAB (USING VIRTUAL LINK):

FIRST KNOW WHAT IS BGP?

The Border Gateway Protocol (BGP) Is A Protocol That Is Used Between Routers To Convey This Information. Since The Routers That Talk BGP To Each Other Aren't Owned By The Same Organization (That Would Kind Of Defeat The Purpose Of Creating Global Reachability) This Is Often Called "Inter-Domain" Routing. 

The Border Gateway Protocol (BGP), Defined In RFC 1771, Allows You To Create Loop−Free Inter Domain Routing Between Autonomous Systems (AS). An AS Is A Set Of Routers Under A Single Technical Administration. 

Routers In An AS Can Use Multiple Interior Gateway Protocols To Exchange Routing Information Inside The AS And An Exterior Gateway Protocol To Route Packets Outside The AS. 

BGP Which May Be Used To Create A Confederation Of Autonomous Systems That Is Represented As A Single Autonomous System To BGP Peers External To The Confederation, There By Removing The "Full Mesh" Requirement. 

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HOW BGP SELECTS BEST PATHS

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PATH SELECTION WITH BGP :

A BGP Speaker Builds A Routing Database Consisting Of The Set Of All Feasible Paths And The List Of Destinations (Expressed As Address Prefixes) Reachable Through Each Path. For Purposes Of Precise Discussion, It's Useful To Consider The Set Of Feasible Paths For A Set Of Destinations Associated With A Given Address Prefix. In Most Cases, We Would Expect To Find Only One Feasible Path. However, When This Is Not The Case, All Feasible Paths Should Be Maintained, And Their Maintenance Speeds Adaptation To The Loss Of The Primary Path. 

Only The Primary Path At Any Given Time Will Ever Be Advertised. The Path Selection Process Can Be Formalized By Defining A Complete Order Over The Set Of All Feasible Paths To A Set Of Destinations Associated With A Given Address Prefix. One Way To Define This Complete Order Is To Define A Function That Maps Each Full AS Path To A Non-Negative Integer That Denotes The Path's Degree Of Preference. Path Selection Is Then Reduced To Applying This Function To All Feasible Paths And Choosing The One With The Highest Degree Of Preference. 

HOW BGP SELECTS PATHS :

A Router Running Cisco IOS Release 12.0 Or Later Does Not Select Or Use An Ibgp Route Unless Both Of The Following Conditions Are True: 

The Router Has A Route Available To The Next Hop Router:

The Router Has Received Synchronization Via An IGP (Unless IGP Synchronization Has Been Disabled). BGP Bases Its Decision Process On The Attribute Values. When Faced With Multiple Routes To The Same Destination, BGP Chooses The Best Route For Routing Traffic Toward The Destination.

THE FOLLOWING PROCESS SUMMARIZES HOW BGP CHOOSES THE BEST ROUTE:

1. If The Next Hop Is Inaccessible, Do Not Consider It.

This Decision Is Why It Is Important To Have An IGP Route To The Next Hop.

2. If The Path Is Internal, Synchronization Is Enabled, And The Route Is Not In The IGP, Do Not Consider The Route.

3. Prefer The Path With The Largest Weight (Weight Is A Cisco Proprietary Parameter).

4. If The Routes Have The Same Weight, Prefer The Route With The Largest Local Preference.

5. If The Routes Have The Same Local Preference, Prefer The Route That Was Originated By The Local Router.

For Example, A Route Might Be Originated By The Local Router Using The Network Bgp Router Configuration Command, Or Through Redistribution From An IGP.

6. If The Local Preference Is The Same, Or If No Route Was Originated By The Local Router, Prefer The Route With The Shortest Autonomous System Path.

7. If The Autonomous System Path Length Is The Same, Prefer The Route With The Lowest Origin Code (IGP < EGP < INCOMPLETE).

8. If The Origin Codes Are The Same, Prefer The Route With The Lowest MED Metric Attribute.

This Comparison Is Only Made If The Neighboring Autonomous System Is The Same For All Routes Considered, Unless The BGP Always-Compare-Med Router Configuration Command Is Enabled.

9. Prefer The External BGP (EBGP) Path Over The IBGP Path.All Confederation Paths Are Considered Internal Paths.

10. Prefer The Route That Can Be Reached Through The Closest IGP Neighbor (The Lowest IGP Metric).

The Router Will Prefer The Shortest Internal Path Within The Autonomous System To Reach The Destination (The Shortest Path To The BGP Next Hop).

11. If The Following Conditions Are All True, Insert The Route For This Path Into The IP Routing Table:

– Both The Best Route And This Route Are External.

– Both The Best Route And This Route Are From The Same Neighboring Autonomous System.

– The Maximum-Paths Router Configuration Command Is Enabled.

Note EBGP Load Sharing Can Occur At This Point, Which Means That Multiple Paths Can Be Installed In The Forwarding Table.

12. If Multipath Is Not Enabled, Prefer The Route With The Lowest IP Address Value For The BGP Router ID.

The Router ID Is Usually The Highest IP Address On The Router Or The Loopback (Virtual) Address, But Might Be Implementation-Specific.

BGP MULTIPATH SUPPORT:

When A BGP Speaker Learns Two Identical EBGP Paths For A Prefix From A Neighboring Autonomous System, It Will Choose The Path With The Lowest Route ID As The Best Path. This Best Path Is Installed In The IP Routing Table. If BGP Multipath Support Is Enabled And The Ebgp Paths Are Learned From The Same Neighboring Autonomous System, Instead Of One Best Path Being Picked, Multiple Paths Are Installed In The IP Routing Table.

During Packet Switching, Depending On The Switching Mode, Either Per-Packet Or Per-Destination Load Balancing Is Performed Among The Multiple Paths. A Maximum Of Six Paths Is Supported. The Maximum-Paths Router Configuration Command Controls The Number Of Paths Allowed. By Default, BGP Will Install Only One Path To The IP Routing Table.

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SIMPLE BGP LAB FOR - VIRTUAL LINK CONNECTION

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CISCO – BGP LAB FOR VIRTUAL LINK EXAMPLE :

STEP BY STEP LAB CONFIGURATION AS FOLLOW :

Router> - User Exec Mode

Router# - Privileged Exec Mode

Router(Config)# - Configuration Mode (Notice The # Sign Indicates This Is Only Accessible At Privileged Exec Mode.)

Router(Config-If)# - Interface Level Within Configuration Mode.

Router(Config-Router)# - Routing Engine Level Within Configuration Mode.

SAVING CONFIGURATIONS - ALWAYS ISSUE THIS COMMAND AFTER EACH CONFIGURATION :

Router# Copy Running-Config Startup-Config - Saves Configuration Into NVRAM

A Cisco IOS Router Stores Configurations In Two Locations - RAM And NVRAM. The Running Configuration Is Stored In RAM And Is Used By The Router During Operation. Any Configuration Changes To The Router Are Made To The Running-Configuration And Take Effect Immediately After The Command Is Entered.

The Startup-Configuration Is Saved In NVRAM And Is Loaded Into The Router's Running-Configuration When The Router Boots Up. If A Router Loses Power Or Is Reloaded, Changes To The Running Configuration Will Be Lost Unless They Are Saved To The Startup-Configuration. To Save The Running-Configuration To The Startup Configuration, Type The Following From Privileged EXEC Mode (I.E. At The "Router#" Prompt.)

Router# Copy Running-Config Startup-Config

LAB - CONFIGURATION ON ROUTER A :

STEP1 :

Router>Enable
Router#Conf T
Router(Config)#Hostname Router A

INTERFACE E0 CONFIGURATION ON ROUTER A:

Router A(Config)#int e0
Router A(Config-if)#ip address 10.0.0.1 255.0.0.0
Router A(Config-if)#no shutdown
Router A(Config-if)#no Keeplive
Router A(Config-if)#exit

THEN GO TO INTERFACE S1 CONFIGURATION ON ROUTER A:

Router A(Config)#int s1
Router A(Config-if)ip address 20.0.0.1 255.0.0.0 (Assign Ip Address For The Interfacec S1)
Router A(Config-if)#no shut (Enable An Interface)
Router A(Config-if)# Clock Rate 56000

To Enable The Back-To-Back Serial Connection Between Two Routers, You Need To Configure One Router As DCE Using The Following Command In Interface Configuration Mode For The Serial Connection On Two different Routers:

Router A(Config-if)# Clock Rate 56000

ROUTER - DTE / DCE :

In A Real-Life Network, Your Serial Interfaces Will Almost Certainly Be Configured As DTE Interfaces. Recall That A CSU/DSU Usually Handles The Clocking For A Synchronous Serial Interface.

A DTE (Date Terminating Equipment) Cable Is The Normal Cable You Should Use. Being DTE You Should Expect The Other End To Provide Clocking.

A DCE (Data Communication Equipment) Means That This Device Must Provide The Clocking On The Wire. DTE/DCE Cable To Directly Connect Two Cisco Router Serial Interfaces.

Connecting The Serial Ports Of Two Routers Directly Using What Is Known As A DCE-To-DTE Crossover Cable. These Cables Allow You To Simulate A Serial WAN Connection Without Requiring A CSU/DSU Or Similar Device.

The Main Issue With Connecting Your Serial Interfaces In This Manner Is The Fact That One Of The Devices Will Need To Be Configured As DCE In Order To Provide The Timing Mechanism Required.

The DCE-To-DTE Crossover Cable Will Have Two Different DB-60 Interfaces – One Marked DTE, And The Other Marked DCE. The Router Connected To The DCE End Of The Cable Will Need Its Serial Interface Configured As A DCE Device.

If Your Device Is The DCE, You Must Provide Clocking Using The Clock Rate Command.

Example - > Router A(Config-if)# Clock Rate 56000

TO DISPLAY DTE/DCE (GIVE THIS COMMAND):

#SHOW CONTROLLERS SERIAL 0

#SHOW IP BGP SUMMARY

SET A LOGICAL BANDWIDTH ASSIGNMENT OF 64K TO THE SERIAL INTERFACE :

Router A(Config-if)# bandwidth 64
Router A(Config-if)#exit

STEP 2: BGP CONFIGURATION ON ROUTER A:

Router A(Config)#Router BGP 10
Router A(Config-Router)#neighbor 20.0.0.2 remote-as 20
Router A(Config-Route)#network 10.0.0.0
Router A(Config-Route)#^Z (Ctrl+Z)

THEN CHECK IT ON ROUTER A (USE ALL THESE COMMANDS) :

Router A#CLEAR IP BGP* SOFT
Router A#SHOW IP BGP
Router A#SHOW IP BGP NEIGHBOR
Router A#CLEAR IP ROUTE*
Router A#SH IP ROUTE
Router A#SH IP INT BRIEF

LAB - CONFIGURATION ON ROUTER B :

STEP 1:

Router>enable
Router#Conf T
Router(Config)#Hostname Router B
Router B(Config)#

INTERFACE S0 CONFIGURATION ON ROUTER B:

Router B(Config)#int s0
Router B(Config-if)#ip address 20.0.0.2 255.0.0.0
Router B(Config-if)#no shutdown
Router B(Config-if)# bandwidth 64 (Set A Logical Bandwidth Assignment Of 64K To The Serial Interface)
Router B(Config-if)#exit

INTERFACE S1 CONFIGURATION ON ROUTER B:

Router B(Config)#int s1
Router B(Config-if)#ip address 30.0.0.1 255.0.0.0
Router B(Config-if)#no shutdown
Router B(Config-if)# Clock Rate 56000
Router B(Config-if)# bandwidth 64 (Set A Logical Bandwidth Assignment Of 64K To The Serial Interface)
Router B(Config-if)#exit

STEP 2: BGP CONFIGURATION ON ROUTER B:

Router B(Config)#Router BGP 20
Router B(Config-Router)#neighbor 30.0.0.2 remote-as
Router B(Config-Router)#neighbor 20.0.0.1 remote-as 10
Router B(Config-Router)#^Z (Press @ the same time - > Ctrl+Z) or (EXIT then Exit)

THEN CHECK IT ON ROUTER B (USE ALL THESE COMMANDS) :

Router B#CLEAR IP BGP* SOFT
Router B#SHOW IP BGP
Router B#SHOW IP BGP NEIGHBOR
Router B#CLEAR IP ROUTE*
Router B#SH IP ROUTE
Router B#SH IP INT BRIEF

LAB - CONFIGURATION ON ROUTER C :

STEP 1:

Router>enable
Router#Conf T
Router(Config)#hostname Router C
Router C(config)#

INTERFACE S0 CONFIGURATION ON ROUTER C:

Router C(Config)#int s0
Router C(Config-if)#ip address 30.0.0.2 255.0.0.0 Router C(Config-if)#no shutdown Router C(Config-if)# bandwidth 64 (Set A Logical Bandwidth Assignment Of 64K To The Serial Interface)
Router C(Config-if)#Exit

INTERFACE S1 CONFIGURATION ON ROUTER C:

Router C(Config)#int s1
RouterC(Config-if)#ip address 40.0.0.1 255.0.0.0
RouterC(Config-if)#no shutdown
RouterC(Config-if)#clock rate 56000
RouterC(Config-if)#bandwidth 64
RouterC(Config-if)#Exit

STEP 2: BGP CONFIGURATION ON ROUTER C:

RouterC(Config)#Router BGP 20
RouterC(Config-Router)#neighbor 30.0.0.1 remote-as 20
RouterC(Config-Router)#neighbor 40.0.0.2 remote-as 20

THEN CHECK IT ON ROUTER C (USE ALL THEISE COMMANDS) :

Router C#CLEAR IP BGP* SOFT
Router C#SHOW IP BGP
Router C#SHOW IP BGP NEIGHBOR
Router C#CLEAR IP ROUTE*
Router C#SH IP ROUTE
Router C#SH IP INT BRIEF
Router C#SHOW IP BGP 40.0.0.2

LAB - CONFIGURATION ON ROUTER D :

STEP 1:

Router>enable
Router#Conf T
Router(Config)#hostname Router D
Router D(Config)#

INTERFACE S0 CONFIGURATION ON ROUTER D:

Router D(Config)#int s0
Router D(Config-if)#ip address 40.0.0.2 255.0.0.0
Router D(Config-if)#no shutdown
Router D(Config-if)#bandwidth 64
Router D(Config-if)#Exit

INTERFACE S1 CONFIGURATION ON ROUTER D:

Router D(Config)#int s1
Router D(config-if)#ip address 50.0.0.1 255.0.0.0
Router D(config-if)#no shut
Router D(config-if)#clock rate 56000
Router D(config-if)#bandwidth 64
Router D(config-if)#exit

STEP 2: BGP CONFIGURATION ON ROUTER D:

Router D(config)#Router BGP 20
Router D(Config-Router)#neighbor 40.0.0.1 remote-as 20
Router D(Config-Router)#neighbor 50.0.0.2 remote-as 30
Router D(Config-Router)#Exit
Router D(Config)#^Z

THEN CHECK IT ON ROUTER D (USE ALL THEISE COMMANDS) :

Router D#CLEAR IP BGP* SOFT
Router D#SHOW IP BGP
Router D#SHOW IP BGP NEIGHBOR
Router D#CLEAR IP ROUTE*
Router D#SH IP ROUTE
Router D#SH IP INT BRIEF

LAB - CONFIGURATION ON ROUTER E :

STEP 1:

Router>enable
Router#Conf T
Router(Config)#hostname Router E
Router E(Config)#

INTERFACE S0 CONFIGURATION ON ROUTER E:

Router E(Config)#int s0
Router E(Config-if)#ip address 50.0.0.2 255.0.0.0
Router E(Config-if)#no shut
Router E(Config-if)#bandwidth 64
Router E(Config-if)#Exit

INTERFACE E0 CONFIGURATION ON ROUTER E:

Router E(Config)#int e0
Router E(config-if)#ip address 60.0.0.1 255.0.0.0
Router E(config-if)#no shut
Router E(config-if)#No keeplive
Router E(config-if)#exit

STEP 2: BGP CONFIGURATION ON ROUTER E:

Router E(Config)#Router BGP 30
Router E(Config-Router)#neighbor 50.0.0.1 remote-as 20

Router E(Config-Router)#Network 60.0.0.0

THEN CHECK IT ON ROUTER E (USE ALL THEISE COMMANDS) :

Router E#CLEAR IP BGP* SOFT
Router E#SHOW IP BGP
Router E#SHOW IP BGP NEIGHBOR
Router E#CLEAR IP ROUTE*
Router E#SH IP ROUTE
Router E#SH IP INT BRIEF

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VIRTUAL LINK ON ROUTER B TO ROUTER D & ROUTER D TO ROUTER B

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STEP1 (ON ROUTER B):

Router B(config)#Router BGP 20
Router B(Config-Router)#neighbor 40.0.0.2 next-hop-self
Router B(Config-Router)#no synchronization (This Will Make The Best Perth From Router D To Router B)

STEP 2 (NEXT GOTO ROUTER D):

Router D(config)#Router BGP 20
Router D(config-Router)#neighbor 30.0.0.1 next-hop-self
Router D(config-Router)#no synchronization (This Will Make The Best Perth From Router B To Router D)

STEP 3 (THEN GOTO ROUTER C): 

NOTE:Advertise The Network Which Are Straightly Connected With Router C.

Router C(Config)#Router BGP 20
Router C(Config-Router)#network 30.0.0.0
Router C(Config-Router)#Network 40.0.0.0

THEN GO TO CHECK IT ON (ROUTER B# & ROUTER D# & ROUTER C#) USE ALL THESE COMMANDS AS : 

#CLEAR IP BGP* SOFT
#SHOW IP BGP NEIGHBOR
#SHOW IP BGP

THEN USE:

Router B# SHOW IP BGP 40.0.0.2
Router D#SHOW IP BGP 30.0.0.1

THEN :

Router B & Router D & Router C Use All These Commands:

#CLEAR IP ROUTE*
#SHOW IP ROUTE

NOTE: Now We Got This Reachability Problems Msg.

STEP 4:

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TO SOLVE REACHABILITY PROBLEMS

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SO WHAT IS REACHABILITY ISSUES :

Border Gateway Protocol (BGP), The De-Facto Inter-Domain Routing Protocol, Can Adapt To Failures By Converging To A New Set Of Valid Paths. However, The Routing Adjustment May Take A Long Time Due To Various Delays In The Propagation Of Update Messages And The Exploration Of Alternative Paths.

As A Result, The Period Of Destination Unreachability Can Be Substantially Longer Than The Time Period Of Actual Physical Connectivity Losses. BGP Convergence Time Without Considering The Impact On Packet Delivery. Packet Delivery Performance Is Related, But Not Equivalent, To The Routing Convergence Time.

Path Vector Messages In BGP: The Autonomous System Boundary Routers (ASBR), Which Participate In Path Vector Routing, Advertise The Reachability Of Networks. Each Router That Receives A Path Vector Message Must Verify That The Advertised Path Is According To Its Policy. If The Messages Comply With The Policy, The ASBR Modifies Its Routing Table And The Message Before Sending It To The Next Neighbor. In The Modified Message It Sends Its Own AS Number And Replaces The Next Router Entry With Its Own Identification.

BGP ROUTES INJECTION PROCESS :

The BGP Process Injects Local Routes In Two Different Ways:

Using The Network Configuration Commands. This Command Lists Networks That Are Candidates If They Appear In The Routing Table.

Using Redistribution From Another Routing Protocol.

REACHABILITY ISSUES:

Reachability Issues When Interfaces Other Than Directly Connected Interfaces Are Used While Peering.

If Certain BGP Routes Are Missing From The Routing Table, Determine If They Are Learned Through BGP And Exist In The BGP Table. Routes Existing In The BGP Table Might Be Missing From The Routing Table Due To The BGP Synchronization Rule Or Due To The Non-Availability Of A Valid Route To The Next Hop. If The Routes Are Missing From The BGP Table Altogether, Determine If The Peering Has Been Successful, And Check For The Existence Of Route Filters. If BGP Routes Are Not Being Advertised, Check If The Conditions For Advertising BGP Routes Are Being Satisfied.

An Internal Border Gateway Protocol (IBGP) Learned Route Is Not Advertised To Other IBGP Peers, And Configuring A Route Reflector Might Be Necessary.

Aggregate Routes Are Not Advertised Unless At Least One Component Route Exists In The BGP Table.

Similarly, Routes Advertised By Issuing The BGP Network Command Are Also Subject To The Existence Of Corresponding Routes In The Routing Table.

Another Factor That Could Contribute To Errors In BGP Is The Availability Of Sufficient Resources.

Ensure That BGP Routers Are Able To Cope With The High Amount Of Routing Information That Is Typically Exchanged Between Routers Configured For BGP.

If Resources Are An Issue, Routing Information Learned Through BGP Can Be Controlled By Configuring Route Filters.

Multihomed Networks Could Also Run Into Problems Such As Becoming A Transit Autonomous System (AS) Or Not Being Able To Load Balance Traffic.

If Cannot Be Measured By Examining The Routing Tables At All The Routers At Time, Because It Takes Time For A Packet To Propagate Through The Network And The Routing Tables May Change During This Time Period. 1Minimum Route Announcement Interval, Which Is Used To Space Out Consecutive Route Announcement Messages.

NOTE: In Such Cases, Additional Configuration Might Be Required.

RECHABILITY PROBLEM TO BE SOLVE ON ROUTER B TO ROUTER C:

ROUTER B (30.0.0.1) <---------> (30.0.0.2) ROUTER C (40.0.0.1) <--------->(40.0.0.2) ROUTER D

For Router B Receiving Point From Router C. So giving This Command On Router B. (Neighbor Next-Hop-Self Address Is 30.0.0.2).

Router B(config)#Router BGP 20
Router B(config-Router)#neighbor 30.0.0.2 next-hop-self.
Router B(config-Router)#^z

THEN GO TO ROUTER C AND CHECK IT:

Router C#CLEAR IP BGP* SOFT
Router C#SHOW IP BGP

ROUTER B TO ROUTER C BEST PATH :

So Now Reachability Problem Is Solve In Router C From Router B. But The Best Path Is Still Not So - > Going To Solve This (Give This Command):

Router C(config)#Router BGP 20

Router C(config-router)#no synchronization (Best Path From Router C To Router B)

RECHABILITY PROBLEM TO BE SOLVE ON ROUTER D TO ROUTER C:

ROUTER B (30.0.0.1) <---------> (30.0.0.2) ROUTER C (40.0.0.1) <--------- >(40.0.0.2) ROUTER D

For Router D Receiving Point From Router C. So giving This Command On Router D. (Neighbor Next-Hop-Self Address Is 40.0.0.2).

GO TO ROUTER D :

Router D(config)#Router BGP 20
Router D(config-Router)#neighbor 40.0.0.1 next-hop-self
Router D(config-Router)#^z

THEN GO TO ROUTER C AND CHECK IT:

Router C#CLEAR IP BGP* SOFT (Refresh New Entries)
Router C#CLEAR IP BGP*
Then - > Give This Command

Router C#SHOW IP BGP

So Now Reachability Problem Is Solve In Router D To Router C. But The Best Path (Router D To Router C) Is Still Not So - > Going To Solve This (Give This Command):

Router C(Config)#Router BGP 20
Router C(config-router)#no synchronization

THEN CHECH IN ROUTER C:

#CLEAR IP BGP* SOFT

#SHOW IP BGP

AND ALSO ROUTER B & ROUTER D (USE ALL THESE COMMANDS): 

#CLEAR IP BGP* SOFT
#CLEAR IP BGP
#SHOW IP BGP
#SHOW IP BGP NEIGHBOR
#SHOW IP BGP
#SHOW IP BGP COMMUNITY

STEP 5:

RECHABILITY PROBLEMS WILL BE SOLVING ROUTER B TO ROUTER D:

ROUTER B (30.0.0.1) <---------> ROUTER C <--------->(40.0.0.2) ROUTER D

Router B to --- > Router D 

Router B(config)#Router BGP 20
Router B(config-router)#neighbor 40.0.0.2 next-hop-self
Router B(config-router)^z

Then Go To Check On Router D:

USE ALL THESE COMMANDS :

Router D#CLEAR IP BGP* SOFT
Router D#CLEAR IP ROUTE*
Router D#SHOW IP BGP
Router D#SHOW IP ROUTE
Router D#SHOW RUN
Router D#SHOW IP BGP NEIGHBOR

FOR BEST PATH ON ROUTER D:

Router D(config)#Router BGP 20
Router D(config-router)#no synchronization

RECHABILITY PROBLEMS WILL BE SOLVING ROUTER D TO ROUTER B:

ROUTER B (30.0.0.1) <---------> ROUTER C <--------->(40.0.0.2) ROUTER D

Router D to --- > Router B 

Router D(config)#Router BGP 20
Router D(config-Router)#neighbor 30.0.0.1 next-hop-self
Router D(config-Router)#^z

FOR BEST PATH FROM ROUTER D TO ROUTER B :

Router B(config)#Router BGP 20
Router B(config-router)#no synchronization
Router(config-Router)#^z

Then Go To Check On Router D and Router B:

USE ALL THESE COMMANDS :

#CLEAR IP BGP* SOFT
D#CLEAR IP ROUTE*
#SHOW IP BGP
#SHOW IP ROUTE
#SHOW RUN
#SHOW IP BGP NEIGHBOR

FINALLY RECHABILITY PROBLEM FROM ROUTER C TO ROUTER B AND ROUTER D:

ROUTER B (30.0.0.1) <---------> ROUTER C <--------->(40.0.0.2) ROUTER D

Router C(config)#Router BGP 20
Router C(config-router)#neighbor 30.0.0.1 next-hop-self
Router C(config-router)#neighbor 40.0.0.2 next-hop-self

FOR BEST PATH ROUTER B AND ROUTER D:

Router B(config)#Router BGP 20
Router B(config-Router)#no synchronization
Router B(config-Router)#^z
Router B#

ON ROUTER D:

Router D(config)#Router BGP 20
Router D(config-router)#no synchronization
Router D(config-router)#^z
Router D#

Then Go To Check On Router D and Router B:

USE ALL THESE COMMANDS :

#CLEAR IP BGP* SOFT
D#CLEAR IP ROUTE*
#SHOW IP BGP
#SHOW IP ROUTE
#SHOW RUN
#SHOW IP BGP NEIGHBOR

FINALLY PING FROM ROUTER A :

Router A#ping

-

-

-

Target IP Address 60.0.0.1

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-

-

Source IP Address 10.0.0.1

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CONCLUSION:

The Goal Of This Article Is To Give An Easy Way To Understand The “CISCO – BGP VIRTUAL LINK LAB CONFIGURATION”. Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts.

Some Topics That You Might Want To Pursue On Your Own That We Did Not Cover In This Article Are Listed Here, 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+.