BGP Is The De Facto Standard For Routing Between Service Providers On The Internet Because Of Its Rich Features. You Can Also Use It To Exchange Routes In Large Internal Networks. The Internet Assigned Numbers Authority (IANA) Reserved TCP Port 179 To Identify The BGP Protocol. Bgpv4 Was Created To Provide CIDR, A Feature That Was Not Present In The Earlier Versions Of BGP. BGP Is A Path-Vector Routing Protocol; It Is Neither A Distance-Vector Nor Link-State Routing Protocol.
Note: RFC 1519 Describes CIDR, Which Provides The Capability To Forward Packets Based On IP Prefixes Only, With No Concern For IP Address Class Boundaries. CIDR Was Created As A Means To Constrain The Growth Of The Routing Tables In The Internet Core Through The Summarization Of IP Addresses Across Network Class Boundaries. The Early 1990s Saw An Increase In The Growth Of Internet Routing Tables And A Reduction In Class B Address Space. CIDR Provides A Way For Service Providers To Assign Address Blocks Smaller Than A Class B Network But Larger Than A Class C Network.
BGP NEIGHBORS: BGP Is Usually Configured Between Two Directly Connected Routers That Belong To Different Autonomous Systems. Each Autonomous System Is Under Different Technical Administration. BGP Is Frequently Used To Connect The Enterprise To Service Providers And To Interconnect Service Providers. The Routing Protocol Within The Enterprise Could Be Any Interior Gateway Protocol (IGP). Common IGP Choices Include Ripv2, EIGRP, Open Shortest Path First (OSPF), And Intermediate System-To-Intermediate System (IS-IS). BGPv4 Is The Only Deployed Exterior Gateway Protocol (EGP). AS Numbers Are A Managed Resource Allocated By The American Registry Of Internet Numbers (ARIN). In IP, The AS Numbers 64,512 Through 65,535 Are Allocated To IANA And Are Designated For Private Use. Before Two BGP Routers Can Exchange Routing Updates, They Must Become Established Neighbors.
After BGP Routers Establish A TCP Connection, Exchange Information, And Accept The Information, They Become Established Neighbors And Start Exchanging Routing Updates. If The Neighbors Do Not Reach An Established State, They Do Not Exchange BGP Updates. The Information Exchanged Before The Neighbors Are Established Includes The BGP Version Number, AS Number, BGP Router ID, And BGP Capabilities.
EXTERIOR AND INTERIOR GATEWAY PROTOCOLS: In Order To Understand Why BGP Is Designed The Way It Is, You First Need To Understand Where It Fits In The World Of Routing Protocols. Routing Protocols Can Be Divided Along Several Axes, The First Being Interior Gateway Protocols (IGPS) Versus Exterior Gateway Protocols (EGPs). The Primary Difference Between Egps And Igps Is The Place In The Network Where They Provide Reachability Information; That Is, Within An Administrative Routing Domain (Intradomain) Or Between Administrative Routing Domains (INTERDOMAIN).
◙ - ➤ A STUB AS Is A Single-Homed Network With Only One Entry And Exit Point. This Type Of AS Can Be Connected To The External World Through The Use Of A Statically Configured Route.
◙ - ➤ TRANSIT AS: Data From One AS Need To Reach A Remote AS, Then It Has To Travel Through Intermediate AS. The AS Or Autonomous Systems Which Carry The Data From One AS To Another AS Is (Are) Called Transit AS (Es).
◙ - ➤ EBGP: External BGP Is Used Between Two Or More Autonomous Systems.
◙ - ➤ IBGP: Internal BGP Is Used Within An AS.
BGP DEALS WITH LARGE BGP NETWORKS USING TWO METHODS:
◙ Route Reflectors
◙ Confederations
Route Reflectors Are Used To Address The Scalability Issues In Large IBGP Networks. A Route Reflector Is A BGP Router Configured To Forward Routing Updates To BGP Peers Within The Same Autonomous System (AS). Route Reflectors Are Not Used In External BGP (EBGP) Sessions.
THE FOLLOWING ARE THE CHARACTERISTICS OF ROUTER REFLECTORS:
◙ Route Reflector Configuration Is Enabled Only On The Route Reflector; Clients Are Configured Normally As IBGP Peers.
◙ The Usual BGP Routing Algorithm Is Applied To All BGP Routes To Ensure A Loop-Free Topology.
◙ Route Reflectors Preserve All BGP Attributes.
◙ Updates Are Sent From The Route Reflector To All Clients.
◙ Clients Receive All Updates From The Route Reflector Only.
◙ In Any Cluster, There Must Be At Least One Route Reflector.
◙ Nonclients (Not Part Of A Cluster) Must Still Be Fully Meshed To Maintain Full Connectivity.
◙ All Updates Contain The Originator-ID Attribute, Which Ensures A Loop-Free Topology, In Which The Route Reflector Ignores Any Update It Receives With Its Own Originator-ID.
THE BENEFITS OF USING ROUTE REFLECTORS INCLUDE THE FOLLOWING:
◙ Addressing Of Scalability Issues
◙ Enables A Hierarchical Design
◙ Reduces The Number Of TCP Peers And, Therefore, The Amount Of Traffic Across WAN Circuits
◙ Fast Convergence In Propagation Of Information
◙ Provides Easier Troubleshooting As The Information Is Typically Sent From One Source.
FILTERING IS VITAL TO ANY LARGE BGP NETWORK, AND TO ALLOW THE NETWORK DESIGNER FLEXIBILITY, BGP CAN BE FILTERED USING THE FOLLOWING METHODS:
◙ ACCESS LISTS: Used When Configuring Route Maps And Filtering Networks Based On IP Networks Using Filter-Based Lists
◙ DISTRIBUTE LISTS: Filter Incoming Or Outgoing IP Networks
◙ PREFIX LISTS:— Filter Information Based On The Prefix Of Any Address.
Prefix Lists Are A New And A More Efficient Way Of Identifying Routes For Matching And Filtering BGP Information. Prefix Lists Are Efficient Because BGP Routers Perform Lookups On Only The Prefix (Beginning) Address And Can Make Faster Routing Decisions.
1 ◙ - ➤ For More About - > BGP Quick References:
2 ◙ - ➤ For More About - > BGP Neighbors Concepts:
3 ◙ - ➤ For More About - > BGP Lab Virtual Link:
4 ◙ - ➤ For More About - > BGP Lab For Route Reflectors Client:
5 ◙ - ➤ For More About - > BGP Lab Redistribute Versus OSPF:
6 ◙ - ➤ For More About - > Redistribution BGP Vs OSPF And EIGRP:
7 ◙ - ➤ For More About - > BGP MD5 Authentication:
◙ - ➤ OFFSET LIST:
✓ Offset-List Command Adds An Offset To Incoming And Outgoing Metrics To Routes Learned Via EIGRP Or RIP
✓ The Offset Value Is Added To The Routing Metric
✓ Router5(Config-Route-Map)# Offset-List 23 In 2 Serial 0/0
✓ The Router Applies An Offset Of 2 To Routes Learned From The S0/0 Interface With The Match On ACL 23
◙ - ➤ IP SLAS (SERVICE LEVEL AGREEMENTS):
✓ Perform Network Performance Measurement
✓ Configuration:
• Define One Or More Probes
• Define One Or More Tracking Objects
• Define The Action On The Tracking Object
◙ - ➤ POLICY BASED ROUTING (PBR):
✓ Allows Policies To Be Implemented That Selectively Cause Packets To Take Different Paths
• IP Routing Is Destination-Based
• PBR Avoids Destination-Based Routing
✓ Applied To Incoming Packets
✓ Requires A Route Map To Implement The Policy
◙ - ➤ ENTERPRISE TO ISP CONNECTION:
✓ SINGLE –HOMED ISP CONNECTION – OPTION 1:
• Customer Uses Default Route To ISP (With Single ISP)
• Service Provider Uses Static Route(S) For Customer Public Network
• No Automatic Adjustment To Any Changes In Network
✓ SINGLE-HOMED ISP CONNECTION – OPTION 2:
• Customer Deploys BGP To Announce Its Public Network
• ISP Announces A Default Route, A Subset Of Internet Routes Or A Complete Internet Routing Table
✓ DUAL-HOMED ISP CONNECTIONS
• Connected With Two Links To The Same ISP
• Can Use A Single Router Or Two Edge Routers
• Can Use Static Routes Or BGP
✓ MULTIHOMED ISP CONNECTIVITY:
• Connected To Two Or More Different ISPs
• Can Use Single Router Or Multiple Edge Routers
• Default Route From All Providers
• Default Route And Partial Internet Routing From Providers
• Dynamic Routing With BGP
• Dual Multihomed ISP Connections
• Connected To Two Or More DIFFERENT ISPs With Two Links Per ISP
• Typically Uses Multiple Edge Routers (One Per ISP)
• Dynamic Routing With BGP
◙ - ➤ BGP:
◙ IBGP – INTERNAL BGP - Intra-As Peering
◙ EBGP – EXTERNAL BGP – Inter-As Peering
◙ BGP Does Not Consider Speed To Determine The Best Path
◙ BGP Is Policy-Based Routing Protocol
◙ BGP Allows An As To Control Traffic Flow Using Multiple Bgp Path Attributes
◙ BGP Allows A Provider To Use All Paths By Manipulating Path Attributes
◙ BGP Uses TCP Port 179 To Establish A Session
◙ - ➤ WHEN TO USE BGP:
◙ If As Is A Transit As
◙ An As Is Multihomed
◙ Inter-As Routing Policy Must Be Manipulated
◙ Sufficient Memory And Processor Resources Are Necessary To Handle Bgp Routing
◙ - ➤ BGP MESSAGE TYPES:
✓ OPEN MESSAGE – CONTAINS THE FOLLOWING INFORMATION:
• BGP Version Number
• As Number
• Hold Time
• BGP Router ID
• Optional Parameters (Ex: Authentication)
✓ KEEPALIVE MESSAGE:
• Exchanged Between BGP Peers Often Enough To Keep The Hold Timer From Expiring
• Keepalive Message Consists Of Only A Message Header
✓ UPDATE MESSAGE:
◙ A BGP Update Message Has Information On One Path Only
◙ Update Message Can Include The Following Fields:
• Withdrawn Route
• Path Attributes
• Network Layer Reachability Information
✓ NOTIFICATION MESSAGE:
• Sent When An Error Condition Is Detected
• BGP Session Is Closed Immediately After This Is Sent
◙ - ➤ REQUIREMENTS FOR EBGP NEIGHBORS:
◙ Peers Must Be In Different As
◙ Neighbors Must Be Defined On Both Sides Of The Peering
◙ Neighbor Must Be Directly Connected And IP Addresses Must Be Reachable
◙ - ➤ REQUIREMENTS FOR IBGP NEIGHBORS:
◙ Peers Must Be In The Same As
◙ Neighbors Must Be Defined On Both Sides Of The Peering
◙ Neighbor Need Not Be Directly Connected But IP Address Of Peer Must Be Reachable
◙ - ➤ BGP STATES:
◙ IDLE: Router Is Searching The Routing Table To See Whether A Route Exists To Reach The Neighbor
◙ CONNECTED: Router Found A Route To The Neighbor And Has Completed The 3-Way TCP Handshake
◙ OPEN SENT: Open Message Sent, With The Parameters For The BGP Session
◙ OPEN CONFIRM: Router Received An Agreement On The Parameters For Establishing A Session
◙ ESTABLISHED: Peering Is Established; Routing Begins
◙ - ➤ BGP PATH SELECTION:
◙ 1. Prefer Highest Weight (Local) To Router (Cisco Proprietary)
◙ 2. Prefer Highest Local Preference (Global Within As)
◙ 3. Prefer Route Originated By The Local Router (Next Hop = 0.0.0.0)
◙ 4. Prefer Shortest As Path
◙ 5. Prefer Lowest Origin Code
◙ 6. Prefer Lowest MED
◙ 7. Prefer An EBGP Path Over An IBGP Path
◙ 8. Prefer The Path Through The Closes IGP Neighbor
◙ 9. Prefer The Oldest Route For EBGP Paths
◙ 10. Prefer The Path With The Lowest Neighbor BGP Router ID
◙ 11. Prefer The Path With The Lowest Neighbor IP Address
◙ - ➤ SOME BGP ATTRIBUTES:
✓ WELL KNOWN MANDATORY ATTRIBUTES – Must Be Recognized By All BGP Routers And Must Be Present In Any BGP Update
• As Path
• Next Hop
• Origin Code
✓ WELL KNOWN DISCRETIONARY ATTRIBUTES – Must Be Recognized By All BGP Routers But Need Not Be Present In Bgp Advertisements
• Local Preference – Only Used In Updates Between IBGP Peers
✓ Optional Non-Transitive Attributes – Do Not Have To Be Recognized By All BGP Routers And BGP Peers Can Ignore The Update And Not Advertise The Path To Its Other Peers
• Multi Exit Discriminator (MED) – To Influence Incoming Traffic, The Med Is Used To Inform EBGP Peers Of Its Preferred Ingress Point.
BGP QUEST
A. BGP Has Its Own Transport Mechanism To Deliver BGP Packets To Its Neighbors.
B. UDP Is A Preferred Method Because BGP Neighbors Are In Most Cases Directly Connected And The Loss Of Packets Is Unlikely.
C. BGP Uses TCP As Its Transport Mechanism.
Answer: C. BGP Uses TCP As Its Transport Mechanism.
2: ASSUMING NO ROUTE-REFLECTION OR CONFEDERATIONS ARE USED, WHAT PROBLEMS MIGHT OCCUR IF IBGP NEIGHBORS ARE NOT FULLY MESHED?
A. An IBGP Update Will Not Be Propagated To BGP Routers In The AS Because The IBGP Learned Update Is Not Announced To Other IBGP Neighbors.
B. Everything Will Run Fine.
C. Only External BGP Neighbors Won't Receive The BGP Updates.
Answer: A. An IBGP Update Will Not Be Propagated To BGP Routers In The AS Because The IBGP Learned Update Is Not Announced To Other IBGP Neighbors.
3: WHAT BGP TECHNIQUE IS USED TO PENALIZE FLAPPING OF BGP ROUTES IN SOME OTHER AS?
A. Route-Reflection
B. Dampening
C. Peer Groups
Answer: B. Dampening.
4: THE BGP PROCESS CAN EXCHANGE UPDATES WITH ITS NEIGHBORS AFTER PASSING WHICH NEIGHBOR STATE?
A. Established
B. Opensent
C. Active
Answer: A. Established.
5: WHICH OF THE FOLLOWING TECHNIQUES ARE USED IN SOLVING THE IBGP FULL MESH REQUIREMENT?
A. Dampening
B. Aggregation
C. Route Reflection And Confederation
Answer: C. Route Reflection And Confederation.
6: WHAT ARE THE APPLICATIONS OF A ROUTE MAP IN BGP?
Answer: Route Maps Can Be Applied On Incoming Or Outgoing Routing Information To Or From A Neighbor. They Also Influence Route Redistribution Decisions. Route Maps Are Used To Permit Or Deny A Route And Also To Alter Attributes Of A Route With Set Statements.
7: WHAT IS THE PURPOSE OF MD5 AUTHENTICATION IN BGP?
Answer: MD5 Authentication In BGP Can Be Used To Protect Against Spoofing, Denial-Of-Service Attacks, And Man-In-The-Middle Attacks. When MD5 Is Implemented, The BGP Peer Knows That Information It Received Is From The Correct Source And Was Not Altered In Transit.
8: WHY IS THE MAXIMUM-PREFIX COMMAND NEEDED?
Answer: To Prevent An Autonomous System From Being Bombarded With A Multitude Of Networks From An External Neighbor, You Can Limit The Number Of Networks That Your Autonomous System Is Willing To Receive. This Command Is Useful When Preventing An ISP From Being Flooded With Unnecessary Routes From A Customer Or Vice Versa.
9: WHY SHOULD PRIVATE AUTONOMOUS SYSTEM FILTERING BE USED?
Answer: Networks That Aren't Meant To Be Seen On The Internet Can Be Assigned An Autonomous System Number From The Reserved Range Of 64512 To 65535 And Are Considered Private. Therefore, They Should Never Be Seen Outside The Customer's Realm. Private Autonomous System Filtering Prevents Private Autonomous System Information From Being Propagated To The Service Provider And Beyond.
10: WHERE DO YOU USE THE NEXT-HOP-SELF COMMAND?
Answer: Next-Hop-Self Changes BGP Default Behavior, Because Sometimes Routers Don't Know How To Reach The Router That Is Automatically Advertised As The Next Hop. To Solve This Problem, A Router May Specify Its Own IP Address As The Next-Hop Attribute Instead Of The One Automatically Selected. These Situations Include The Following:
✓ In IBGP Or IBGP-Like Confederation Scenarios, The Next Hop Is The EBGP Router That Injected The Route Into The Autonomous System.
✓ When A Route Is Announced On A Multiaccess Medium (Such As Ethernet Or Frame Relay), The Next Hop Is Usually The IP Address Of The Router Interface Connected To That Medium, Which Originated The Route.
11: WHAT IS THE RANGE OF AUTONOMOUS SYSTEM NUMBERS (ASNS) THAT CAN BE ASSIGNED TO A NETWORK USING BGP?
A. 1 to 256
B. 1 to 1024
C.1 to 64,640
D. 1 to 65,535
Answer: D. 1 to 65,535
Explanation: The Assignable BGP Autonomous System Numbers Are From 1 To 65,535 (I.E. 65,535 In Total). Autonomous System Numbers Are Of 16 Bit Length. This 2 ^ 16 = 65536 -1 Possible ASNs, Since ASN Of All 0s Is Not Assigned. Out Of This, The Internet Assigned Numbers Authority (IANA) Has Reserved The Block Of AS Numbers 64512 Through 65535 For Private Use (Not To Be Advertised On The Global Internet).
12: UNDER WHICH OF THE FOLLOWING SCENARIOS BGP IS RECOMMENDED? [CHOOSE 2].
A. When You Need A Default Route To Connect To A Single ISP.
B. When You Need To Connect To Two Or More ISPS.
C. When You Are Sending Traffic Through One As To Get To Another As.
D. When You Need A Simple Routing On Your Intranet.
Answer: B,C
Explanation: Few Recommended Scenarios, Where You Use BGP Are:
1. Connect To Two Or More ISPS
2. The Traffic Flow Out Of Your Network Need To Be Managed To Suit The Requirements (Policy) Of Your Organization.
3. The Traffic Need To Be Sent Through One As To Get To Another As.
13: WHAT KIND OF BGP SESSION IS ESTABLISHED BETWEEN TWO ROUTERS THAT ARE ADJACENT, BUT IN TWO DIFFERENT AUTONOMOUS SYSTEMS?.
A. External BGP (eBGP)
B. Internal BGP (iBGP)
C. Default (dBGP)
D. Direct
Answer: A
Explanation: External BGP (eBGP) Is Used To Establish Session And Exchange Route Information Between Two Or More Autonomous Systems. Internal BGP (iBGP) Is Used By Routers That Belong To The Same Autonomous System (AS).
14: WHICH OF THE FOLLOWING IS TRUE WITH RESPECT TO BGP ROUTING?
A. Routing Decisions Are Made Based On Number Of Hops.
B. Routing Decisions Are Made Based On Metrics
C. Routing Decisions Are Made Based On Tics
D. Routing Decisions Are Made Based On Network Policy.
Answer: D
Explanation: Routers Running BGP In An AS Use Network Policy To Choose The Best Path. Metrics Are Not Used In BGP. Remember That Internet Is Made Of Autonomous Systems (AS) That Are Connected Together Based On Policies Specific To Each AS. Also, AS Numbers (ASN) Are Assigned By AINA And Are Unique Over The Internet. In An Internet (Not Big I) The ASNs Can Be Assigned By The Corporation Itself That Is Implementing Internet.
15: WHICH OF THE FOLLOWING IS NOT A TYPE OF MESSAGE THAT A BGP COMMON HEADER WILL CONTAIN?
A. Open Message
B. Update Message
C. Routebusy Message
D. Keepalive Message
Answer: C
Explanation: The Following Are The Four Possible Message Types In A BGP Header:
✓ TYPE 1- OPEN MESSAGE: This Is The First Message Sent After TCP Session Is Established.
✓ TYPE 2- UPDATE MESSAGE: An UPDATE Message Contains A New Route Or A Route To Be Withdrawn Or Both. Note That Only One New Route Can Be Advertised With One UPDATE Message.
✓ TYPE 3- NOTIFICATION MESSAGE: This Message Is Sent If An Error Occurs During A BGP Session. This Message Can Be Used To Troubleshoot The Problem.
✓ TYPE 4- KEEPALIVE MESSAGE: KEEPALIVE Message Is Used To Confirm That The Connection Between The Neighboring Routers Is Still Active.
16: WHICH OF THE FOLLOWING ARE WELL-KNOWN MANDATORY ATTRIBUTES USED IN BGP? [CHOOSE ALL THAT APPLY].
A. AS_PATH
B. NEXT_HOP
C. ORIGIN
D. COMMUNITIES
Answer: A,B,C
Explanation: Well-Known Mandatory Attributes Must Appear In All BGP Update Messages. The Well-Known Mandatory Messages Are:
✓ 1. AS_PATH: BGP Messages Carry The Sequence Of AS Numbers Indicating The Complete Path A Message Has Traversed.
✓ 2. NEXT_HOP: This Attribute Indicates The IP Address Of The Next-Hop Destination Router.
✓ 3. ORIGIN: This Attribute Tells The Receiving BGP Router, The BGP Type Of The Original Source Of The NLRI Information
17: WHICH OF THE FOLLOWING STATEMENTS IS TRUE WITH RESPECT BGP?
A. Any Two Routers That Have Been Connected And Supposed To Make TCP Connection In Order To Exchange BGP Routing Information Are Called Peers, Or Neighbors.
B. Any Two Physically Adjacent Routers Are Called Peers, Or Neighbors.
C. Any Two Routers That Have Formed A TCP Connection In Order To Exchange BGP Routing Information Are Called Peers, Or Neighbors.
D. Any Routers Belonging To The Same As Are Called Peers, Or Neighbors.
Answer: C
Explanation: Any Two Routers That Have Formed A TCP Connection In Order To Exchange BGP Routing Information Are Called Peers, Or Neighbors. BGP Peers Initially Exchange Their Full BGP Routing Tables.
After This Exchange, Incremental Updates Are Sent As The Routing Table Changes. BGP Keeps A Version Number Of The BGP Table, Which Should Be The Same For All Of Its BGP Peers. The Version Number Changes Whenever BGP Updates The Table Due To Routing Information Changes.
Keepalive Packets Are Sent To Ensure That The Connection Is Alive Between The BGP Peers And Notification Packets Are Sent In Response To Errors Or Special Conditions.
18: WITH MOST BGP IMPLEMENTATIONS, WHAT IS THE SOURCE ADDRESS OF THE TCP PACKETS SENT TO A PEER? HOW CAN THIS CAUSE A PROBLEM FOR EBGP SESSIONS BUILT BETWEEN NON CONNECTED INTERFACES?
Answer: The Source Address Of The TCP Packets Transmitted To A Peer Is Normally The IP Address Of The Interface Used To Reach The Peer's Address (The Destination Address Of The Packets).
This Can Cause Problems For A Pair Of EBGP Peers Communicating Over Non Directly Connected Addresses Because It Can Mean The Expected Source Of The Packets Doesn't Match The Real Source Of The Packets.
19: HOW MANY IP HOPS DOES BGP NORMALLY ASSUME WILL BE BETWEEN TWO EBGP PEERS? HOW IS THIS CONDITION NORMALLY CHECKED WITHIN BGP IMPLEMENTATIONS?
Answer: The BGP Specification States That There Will Only Be One IP Hop Between Two EBGP Peers; BGP Implementations Normally Check This By Making Certain The Peer's IP Address Is On The Same Subnet As The Local Peering IP Address And By Setting The Time To Live On All Packets Sent To An EBGP Peer To 1,
So They Cannot Be Forwarded Beyond Directly Connected Interfaces. Most Implementations Allow These Two Checks To Be Overridden, Allowing EBGP Peers To Be Formed Over Multihop Sessions.
20: WHICH OPEN PARAMETERS CAN CAUSE A BGP SESSION TO FAIL?
Answer:The BGP Version Number, The Autonomous System Number, The Hold Time, And The Route Identifier.
21: WHAT THREE COMMON PROBLEMS CAN CAUSE A PEERING SESSION TO FLAP BETWEEN TWO BGP SPEAKERS?
Answer: Traffic Shaping, Rate Limiting, And Other Forms Of Quality Of Service Configured Between Two Peers That Are Not Configured In Such A Way To Account For BGP Traffic And Maximum Transmission Unit Mismatches.
22: WHAT PROBLEM IS SYNCHRONIZATION BETWEEN BGP AND INTERIOR GATEWAY PROTOCOL LEARNED ROUTES SUPPOSED TO PREVENT? , WHAT METHODS ARE USED TO PREVENT THESE PROBLEMS FROM OCCURRING WITHOUT SYNCHRONIZATION BEING ENABLED?
Answer: Synchronization Between The Interior And Interior BGP Protocol Tables Prevents Routing Black Holes, Where Traffic Is Discarded In The Center Of A Network By A Router With Less Than Full Routing Tables.
Synchronization Isn't Normally Used Any Longer, Because Interior Gateway Protocols Are Not Designed To Handle Efficiently The Number Of Routes Carried Within BGP. Instead Of Using Synchronization, A Full Mesh Of BGP Is Maintained Between All The Routers Over Which The Traffic Could Pass When Transiting The Network.
23: BGP SPEAKERS DO NOT NORMALLY CHANGE THE NEXT HOP WHEN READVERTISING A ROUTE LEARNED FROM AN EXTERNAL PEER TO AN INTERNAL PEER. HOW CAN THIS CAUSE ROUTES TO BE MISSING FROM A BGP SPEAKER'S TABLES?
Answer: If A BGP Speaker Receives A Route With A Next Hop Outside The Autonomous System Doesn't Have A Route To That Next Hop, It Will Consider This Route As Unreachable.
24: WHEN IS THE MULTIPLE EXIT DISCRIMINATOR COMPARED BETWEEN TWO ROUTES?
Answer: Only When The Autonomous Systems The Two Routes Were Learned From Are The Same.
25: CAN THE MULTIPLE EXIT DISCRIMINATOR COMPARISON RULES CAUSE ISSUES WITH ROUTE SELECTION PROCESS?
Answer: It Can Cause The Path Chosen To Forward Traffic Along To Be Chosen In A Different Way At Different Times, Causing Inconsistent Routing Through An Autonomous System. This Inconsistent Routing Can Lead To Route Churn In Some Situations.
26: WHAT RULES CAN YOU FOLLOW IN NETWORK DESIGN WITH REGARD TO THE INTERIOR GATEWAY PROTOCOL METRIC BETWEEN ROUTE REFLECTORS TO PREVENT PERSISTENT ROUTE CHURN?
Answer: You Can Prevent Persistent Oscillation With Route Reflectors By Adjusting The Interior Gateway Protocol Metrics So The Cost Between The Route Reflectors Is Always Higher Than The Cost Of Reaching Any EDGE From The Route Reflectors.
27: BGP CONTAINS TWO PATHS TO A DESTINATION. ASSUMING BOTH ROUTES WERE ORIGINATED LOCALLY AND HAVE AN EQUAL WEIGHT. WHAT WILL BE THE NEXT DETERMINING FACTOR IN CHOOSING THE BEST PATH?
A – Lowest MED
B – Highest Local Preference
C – Lowest Neighbor IP Address
D – Lowest Origin Code
E – Shortest AS-Path
Answer: B
Explanation: Memorizing The BGP Decision Process Steps Is Very Useful And You Should Remember Them. The Table Below Lists The Complete Path Selection Process:
✓ 1. Weight (Bigger Is Better)
✓ 2. Local Preference (Bigger Is Better)
✓ 3. Self Originated (Locally Injected Is Better Than IBGP/EBGP Learned)
✓ 4. AS-Path (Smaller Is Better)
✓ 5. Origin (Prefer ORIGIN Code I Over E, And E Over ?)
✓ 6. MED (Smaller Is Better)
✓ 7. External (Prefer EBGP Over IBGP)
✓ 8. IGP Cost (Smaller Is Better)
✓ 9. EBGP Peering (Older Is Better)
✓ 10. RID (Lower Is Better)
28:WHICH BGP PATH ATTRIBUTE IS CISCO PROPRIETARY?
A. Weight
B. MED
C. Local Preference
D. Origin
E. Next-Hop
F. AS-Path
Answer: A
Explanation: The Weight Attribute Is A Cisco Proprietary Technology. This Attribute Is Local To A Router And Is Not Advertised To Neighboring Routers. If The Router Learns About More Than One Route To The Same Destination, The Route With The Highest Weight Will Be Preferred And Will Be Installed In The Routing Table. By Default, The Value Of Weight Is 0 And The Range Is From 0 To 65535.
The Route With The Highest Weight Will Be Installed In The IP Routing Table.
29: WHICH ATTRIBUTE MUST EXIST IN THE BGP UPDATE PACKET?
A. LOCAL_PREF
B. AGGREGATOR
C. AS_PATH
D. WEIGHT
Answer: C
Explanation: ORIGIN, AS_PATH, NEXT_HOP ARE WELL-KNOWN MANDATORY BGP Attributes That All BGP Updates Must Include.
Note: There Are 4 BGP ATTRIBUTE TYPES:
✓ WELL-KNOWN MANDATORY: Recognized By All Implementations Of BGP And Must Appear In A BGP Update Message. If Missing, A Notification Error Will Be Generated.
✓ WELL-KNOWN DISCRETIONARY: Recognized By All Implementations Of BGP But May Not Be Sent In The BGP Update Message (Include LOCAL_PREF, ATOMIC_AGGREGATOR).
✓ OPTIONAL TRANSITIVE: May Or May Not Be Recognized By All BGP Implementations. Because The Attribute Is Transitive, BGP Accepts And Advertises The Attribute Even If It Is Not Recognized (Include Community Attribte).
✓ OPTIONAL NONTRANSITIVE: May Or May Not Be Recognized By All BGP Implementations. Whether Or Not The Receiving BGP Router Recognizes The Attribute, It Is Nontransitive And Is Not Passed Along To Other BGP Peers (Include MED).
30: WHEN THE BGP PATH SELECTION PROCESS IS BEING PERFORMED ON A CISCO ROUTER, WHICH BGP ATTRIBUTE IS USED FIRST WHEN DETERMINING THE BEST PATH?
A. LOCAL PREFERENCE
B. MED
C. WEIGHT
D. ORIGIN
E. NEXT-HOP
F. AS-PATH
Answer: C
Explanation: Weight Is The First Attribute BGP Uses In The Route Selection Process. Route With A Higher Weight Is Preferred When Multiple Routes Exist To The Same Destination.
31: WHICH BGP ATTRIBUTE WILL NOT BE ADVERTISED IN ROUTING UPDATES TO ITS NEIGHBORING ROUTERS?
A. WEIGHT
B. LOCAL PREFERENCE
C. ORIGIN
D. AS_PATH
E. NEXT HOP
Answer: A
32: WE'LL START WITH A VERY IMPORTANT BGP VALUE - WHICH PORT NUMBER DOES BGP USE TO ESTABLISH ADJACENCIES?
Answer: BGP Uses TCP Port 179 To Create And Maintain Adjacencies.
33: IDENTIFY THE TRUE STATEMENTS?
A. BGP Uses The Same Logical Grouping Concept As EIGRP.
B. BGP Uses The Same Logical Grouping Concept As OSPF.
C. BGP Neighbors Must Be Directly Connected.
D. BGP Neighbors Need Not Be Directly Connected.
Answer: A, D.
Explanation: BGP And EIGRP Use Autonomous Systems To Group Routers Logically. There Is No Requirement For BGP Routers To Be Directly Connected In Order To Become Neighbors.
34: WHAT REQUIRED VALUE IS MISSING FROM THE FOLLOWING BGP COMMAND?
Router1(Config-Router)#Neighbor 172.12.123.3
% Incomplete Command.
Answer: You Must Specify The Potential Neighbor's AS With The Remote-As Portion Of The Command.
Router1(Config-Router)#Neighbor 172.12.123.3
% Incomplete Command.
Router1(Config-Router)#Neighbor 172.12.123.3 Remote-As 200
35: IDENTIFY THE WELL-KNOWN MANDATORY BGP ATTRIBUTES?
A. AS_PATH
B. LOCAL PREF
C. ATOMIC AGGREGATE
D. ORIGIN
E. NEXT-HOP
Answer: A, D, E.
Explanation:The Well-Known Mandatory BGP Attributes Are AS_PATH, Origin, And Next-Hop.
36: IDENTIFY THE WELL-KNOWN DISCRETIONARY BGP ATTRIBUTES?
A. AS_PATH
B. ORIGIN
C. AGGREGATE
D. LOCAL PREF
E. NEXT-HOP
F. ATOMIC AGGREGATE
Answer: D, F.
Explanation: The Well-Known Discretionary BGP Attributes Are Local Preference (LOCAL_PREF) And Atomic Aggregate.
37: GIVE THE THREE POSSIBLE VALUES FOR A BGP ORIGIN CODE AND DESCRIBE THE MEANING OF EACH.
Answer And Explanation: There Are THREE POSSIBILITIES FOR THE ORIGIN CODE:
✓ The Letter "I" Indicates The Path Originated From An IGP And Was Originated With The Network Command.
✓ An “E” Indicates That The Route Originated From An Exterior Gateway Protocol (EGP).
✓ Finally, An Origin Code Of “?” Means The Origin Is Not Quite Clear; Generally, This Means The Route Was Learned Via Redistribution.
38: OF THE BGP VALUES DISCUSSED IN THE PREVIOUS QUESTION, LIST THEM IN ORDER FROM MOST-PREFERRED TO LEAST-PREFERRED?
Answer And Explanation: The Order Of Preference, Most-Preferred To Least-Preferred: IGP, EGP, Incomplete.
39: IN THE BGP BEST PATH SELECTION PROCESS, WHAT ATTRIBUTE IS USED FIRST ON A NON-CISCO ROUTER?
Answer And Explanation: The BGP Attribute Weight Is Used First On Cisco Routers, But This Is A Cisco-Proprietary Attribute. The First Attribute Considered On Non-Cisco Routers Is Local Preference - The Highest LOCAL_PREF Is Preferred.
40: IDENTIFY THE TRUE STATEMENTS REGARDING THE BGP ATTRIBUTE WEIGHT?
A. On Cisco Routers, It's The First Attribute Considered In The Best-Path Selection Process.
B. It Is A Cisco-Proprietary Value
C. It Is Locally Significant Only
D. It Is Advertised To Other Routers As Part Of The BGP Update Process
Answer : A, B, C.
Explanation:Weight Is The First Value Considered In BGP Path Selection Among Multiple Paths - And When The Routers Are Cisco Routers!
There Are Three Major Points To Remember About The Weight Attribute - It's Cisco-Proprietary, It's Locally Significant Only, And It's Never Advertised To Other Routers.
41: IN BGP TERMINOLOGY, WHAT IS A "SOFT RESET"? WHY WOULD WE PERFORM SUCH A RESET, AND HOW IS IT DONE?
Answer And Explanation: BGP Sometimes Requires The Sessions To Be Reset In Order For Configured Changes To Take Effect. Problem Is, That Can Require Tearing Down The Existing BGP Connections - Unless You Use A Soft Set, Which Resets The Sessions Without Tearing The Adjacencies Down.
To Perform A BGP Soft Reset:
R1#Clear IP BGP * Soft
42: On A Cisco Router, What Value Is Preferred, Higher Or Lower Weight, And What Is The Range Of Values For Weight?
Answer And Explanation: Higher Weight Values Are Preferred, And The Range Of Values For Weight Is 0–294967295. The Default Value Is 0.
43: WHAT ARE THE TERMS PEER OR NEIGHBOR USED TO DESCRIBE IN BGP?
Answer And Explanation: A Peer Or Neighbor Indicates A TCP Session Between Two BGP Routers.
44: WHAT IS THE BGP TABLE?
Answer And Explanation: The BGP Table Is A Collection Of Local And Remote Network Entries Describing The Next Hop Address, Local Preference, Weight, And AS Path. Based On These Entries, Networks Are Inserted Into The IP Routing Table.
45: GIVE TWO EXAMPLES OF CLASSFUL PROTOCOLS?
Answer;OSPF, BGP, And IS-IS Are Common Examples.
46: GIVE TWO EXAMPLES OF CLASSLESS PROTOCOLS?
Answer:RIP And IGRP Are Classless Protocols.
47: WHAT DOES A ROUTE REFLECTOR DO TO NONCLIENT IBGP PEER?
Answer And Explanation: A Route Reflector Reflects Information To Only Configured Clients. All Other Peers Must Be Fully Meshed.
48:WHAT IS A BGP CLUSTER?
Answer And Explanation: Cluster Is A Term Used To Describe A Router Reflector And The Configured Route Reflector Clients. Route Reflectors Are Used In IBGP Networks Only.
49: HOW IS A ROUTE REFLECTOR CLIENT CONFIGURED FOR IBGP?
Answer And Explanation: Route Reflector Clients Are Configured For Normal IBGP Peering. The Route Reflector Has Additional Commands To Ensure That Updates Are Reflected From One Route Reflector Client To Another.
50: HOW MANY TCP PEERS ARE REQUIRED IN A 1000 IBGP NETWORK?
Answer And Explanation: The Number Of Peers Without The Use Of Route Reflectors Is N(N-1)/2, Where N Is The Number Of BGP Routers. For Example, With 1000 BGP Routers, The Number Of Peers Is 1000(999)/2 = 499500. With The Use Of Route Reflectors, You Would Only Need 999 Peers (Use The Formulae (N-1) Where N Is The Number Of Routers), Which Is Only 0.2 Percent Of The Same Fully Meshed Network.
51: PROVIDE THE IOS COMMAND SYNTAX TO ENABLE A DEFAULT ROUTE TO BE SENT TO A REMOTE PEER?
Answer:NEIGHBOR PEER IP -ADDRESS DEFAULT-ORIGINATE.
52: TO DISPLAY ROUTE REFLECTOR CLIENTS, WHICH SHOW COMMAND(S) CAN YOU USE, IF ANY?
Answer And Explanation: To View Route Reflectors, You Can Use Two Methods On The Route Reflector:
✓ One Is To Use The IOS SHOW IP BGP NEIGHBORS Command,
✓ And The Second Is To View The Running Configuration With The IOS SHOW RUNNING-CONFIG Command.
53: USING A ROUTE MAP, WHICH IOS COMMAND SETS THE WEIGHT AND LOCAL PREFERENCE ATTRIBUTE TO 100?
Answer And Explanation: First, You Must Define A ROUTE MAP With An Arbitrary Name (Prem In This Example) And Then Complete The Following Set Of Commands:
Router1(config)#route-map ?
WORD Route map tag
Router1(config)#route-map Prem
Router1(config-route-map)#set weight 100
Router1(config-route-map)#set local
Router1(config-route-map)#set local-preference 100
After Defining The Route Map, You Must Apply It To Remote BGP Peers On The Inbound Or Outbound Direction Required.
55: CAN YOU SET THE BGP ATTRIBUTE NEXT-HOP-SELF TO BOTH EBGP AND IBGP PEERS?
Answer And Explanation: No. The Next-Hop-Self Attribute Is Used For IBGP Peers Only. The IOS Command To Set This Attribute To Remote Peers Is Neighbor IP-Address Next-Hop-Self.
56: ON A CISCO ROUTER, WHAT VALUE IS PREFERRED, HIGHER OR LOWER WEIGHT, AND WHAT IS THE RANGE OF VALUES FOR WEIGHT?
Answer And Explanation: Higher Weight Values Are Preferred, And The Range Of Values For Weight Is 0–294967295. The Default Value Is 0. 57: WHAT ARE THE TERMS PEER OR NEIGHBOR USED TO DESCRIBE IN BGP?
Answer And Explanation: A Peer Or Neighbor Indicates A TCP Session Between Two BGP Routers.
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “BGP Just Recap References" And Also We Hope This Guide Will Help Every Beginner 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!Hands - On Experience Is An Invaluable Part Of Preparing For The Lab Exam And Never Pass Up An Opportunity To Configure Or Troubleshoot A Router ( If You Have Access To Lab Facilities, Take Full Advantage Of Them) There Is No Replacement For The Experience You Can Gain From Working In A Lab, Where You Can Configure Whatever You Want To Configure And Introduce Whatever Problems You Want To Introduce, Without Risk Of Disrupting A Production Network. Thank You And Best Of Luck
This Article Written Author By: Premakumar Thevathasan - CCNA, CCNP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+ And Etc.
WARNING AND DISCLAIMER:
Routers Direct And Control Much Of The Data Flowing Across Computer Networks. This Guide Provides Technical Guidance Intended To Help All Network Students, Network Administrators And Security Officers Improve Of Their Demonstrated Ability To Achieve Specific objectives Within Set Timeframes.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, But No Warranty Or Fitness Is Implied.
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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