BGP Routing Protocol

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ISSUE

Fixed Network Curriculum

Development Section

ODA000011 BGP Routing

Protocol

1.0


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Objectives

Master principle of BGP protocol

Upon completion of this course, you will be able to:


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Course Contents

Chapter 1 Overview of BGP

Chapter 2 BGP route attributes and its

applications


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Overview of BGP

BGP is an external routing protocol, used to transmit routing

information between ASs

It is a kind of distance-vector routing protocol and avoids the

occurrence of loop in design

It provides additional attribute information for the route

Transfer protocol: TCP; port No.: 179

It supports Classless Inter-Domain Routing (CIDR)

Route updating: transmit incremental routes only

Abundant route filtering and routing strategies


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Autonomous System

What is an Autonomous System (AS)?

Why need AS?

The routing protocol inside the AS

The routing protocol between ASs -- BGP


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Working Mechanism of BGP

AS1

AS7

AS6

AS5

AS4

AS3

AS2


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Two Kinds of Neighbors of BGP--IBGP & EBGP

EBGPEBGP

RTB

RTC

IBGP

RTA

RTD

RTE


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Route Advertising Principles of BGP

The BGP Speaker only selects the best one for its own use

The BGP Speaker only advertises the routes used by itself to

its neighbors

For the routes obtained from EBGP, the BGP Speaker will

advertise them to all its neighbors (including EBGP and IBGP)

For the routes obtained from IBGP, the BGP Speaker will not

advertise them to its IBGP neighbors

For the routes obtained from IBGP, whether the BGP Speaker

will advertise them to its EBGP neighbors depends on the

synchronization state of IGP and BGP

Once the connection is established, the BGP Speaker will

advertise all its BGP routes to the new neighbors


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BGP Synchronization

EBGP EBGP

RTB

RTC

IBGP

RTA

RTD

RTE

RTF

S0:1.1.1.2/24

E0:10.1.1.1/24

S0:1.1.1.1/24

S1:2.1.1.2/24 S0:2.1.1.1/24

AS100

AS200

AS300


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One Way to Become the BGP Route -- Full

Dynamic Redistribution

OSPF discovers route 18.0.0.1/8

Dynamically redistribute the route discovered by IGP (OSPF)

into the BGP routing table of RTB

18.0.0.1/8

OSPF

RTB

AS200


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The Second Way to Become the BGP Route:

Semi Dynamic Redistribution

OSPF discovers the route 18.0.0.1/8

Semi-dynamically redistribute the route discovered by IGP

(OSPF) into the BGP routing table of RTB

18.0.0.1/8

RTB

AS200

OSPF


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The Third Way to Become the BGP Route:

Static Redistribution

Manually configure the static route 18.0.0.1/8

Redistribute the static route manually configured into the BGP

routing table of RTB

18.0.0.1/8

AS200

RTB


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BGP Messages

There are four types of BGP messages:

Open: greeting--"hello, let's make friends!"

Keepalive: I'm alive, don't leave me alone

Update: fresh news... Notification: i won't play with you any more!


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BGP Message Header

BGP messages have the same format: "message header +

packet body"

The BGP header is given as follows:

Marker(16 byte)

Length(2 byte) Type(1 byte)


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Open Message

Open Message consists of the Message header and the

structure below

Optional Paramenters

Length

Hold Time

VisionMy Autonomous System

BGP Identifier

Optional Paramenters


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Keepalive Message

Keepalive Message has the header only

Marker(16 byte)

Length(2 byte) Type(1 byte)


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Update Message

An Update Message consists of the header and the following

structure

Withdrawn RoutesUnfeasible Routes Length

Path Attribute Length

Network Layer Reachability Information

Path Attribute


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Notification Message

A Notification packet consists of the header and the structure

below

Data

Error Code Error Subcode

Error

Code

1 2 3 4 5 6

Error

Type

Header

error

OPEN

Massage

error

UPDATE

Massage

error

Hold

time

expiry

State

Machine

error

Exit


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Finite State Machine of BGP

Active

Open-sent

Open-confirm Established

Idle

Connect

Connect-Retry

timer expiry

TCP connection fails

Connect-Retrytimer expiry

Start

Others

TCP connection fails

Error

Error Error

KeepAlive

timer expiry

KeepAlive packet

received

1. KeepAlive

timer expiry

2. Update received

3. KeepAlive received

Correct OPEN

packet received

TCP connection setup

TCP connection setupOthers


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Application of Messages in BGP

The Open message is sent when establishing a BGP

connection via TCP

After the connection is established, the UPDATE message is

sent to notify the peer of the routing information if a routeneeds to be sent or route change occurs

After stabilization, it is necessary to send the KEEPALIVE

message periodically to keep the validity of the BGP

connection

When an error is found during the running of local BGP,

NOTIFICATION message shall be sent to notify the BGP peer


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Course Contents

Chapter 1 Overview of BGP

Chapter 2 BGP route attributes and its

applications


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BGP Route Attributes

Describe the features of a route

Transitional and non-transitional

Mandatory and optional

Easy to expand

Up to now, 16 kinds of attributes are available

Focus of BGP Protocol


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Common BGP Route Attributes

1.Origin

2.AS-Path

3.Next hop

4.MED5.Local-Preference

6.Atomic-Aggregate

7.Aggregator

8.Community

9.Originator-ID

10.Cluster-List

11.Destination Pref (MCI)

12.Advertiser (Baynet)13.Rcid-Path (Baynet)

14.MP_Reach_NLRI

15.MP_Unreach_NLRI

16.Extended_Communities


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Origin Attribute

In a specific implementations, the Origin of one route is

determined as:

If a route is redistributed into the BGP routing table with the

specifically, the origin attribute shall be IGP

If a route is obtained via EGP, the origin attribute shall be EGP

Otherwise, the Origin attribute should be Incomplete


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AS-Path Attribute

D(18.0.0.0/8)

AS200AS300

AS400

AS100AS500

RTA

RTB

30.0.0.1

30.0.0.2D,dl 400 300 200

D,d2 500 200

D,d1 400 300 200

>D,d2 500 200


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Next Hop Attribute

18.0.0.0/8

20.0.0.0/8

RTA

RTC

RTB

RTD19.0.0.0/8

21.0.0.2

21.0.0.1

10.0.0.2

10.0.0.3

10.0.0.1

AS100

AS200

IBGP

IBGPEBGP

RTBI can reach 18.0.0.0/8 via the next hop 10.0.0.2

I can reach 20.0.0.0/8 via the next hop 10.0.0.3

RTA

I can reach 18.0.0.0/8 via the next hop10.0.0.2I can reach 20.0.0.0/8 via the next hop 10.0.0.3

I can reach 19.0.0.0/8 via the next hop 21.0.0.1RTC




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Local Precedence Attribute

AS400

AS100

AS300AS200

RTA

RTB RTC

RTDRTE

RTF

D (18.0.0.0/8)

30.0.0.1

30.0.0.2

20.0.0.1

20.0.0.2

Dlocal-pref1 100 Dlocal-pref2 200

Dlocal-pref1 100>Dlocal-pref2 200 RTA will select local-pref2 that hashigher local preference


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Multi-Exit-DISC (MED) Attribute

D(18.0.0.0/8)

RTA

RTB RTC

30.0.0.1

30.0.0.2

20.0.0.1

20.0.0.2

AS100

AS200

>D,metric1 10

D,metric2 20

D,metric1 10 D,metric2 20

RTA will select the lower metric

IBGP


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Community Attribute

A community is a group of routes that have the samedestination.A community is not limited to a network or an AS

only. It has no physical boundary.Accepted communities:

no-export

Do not notify the BGP neighbors outside the alliance/AS

no-advertise

Do not notify any BGP neighbors

local-AS Do notify the EBGP neighbors

Internet

Notify all other routers


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BGP Route Selection Procedure

In general, the procedure of local BGP route selection is:

1. If the next hop of this route is unreachable, then neglect this

route.

2. Select the route with a higherlocal preference.

3. Select the originated route by the local router (same local

precedence).

4. Select the route whose AS path is shortest.

5. Select the route whose origin type is IGP, EGP, andIncomplete in turn.

6. Select the route whose MED is smallest.

7. Select the route whose Router ID is smallest.


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Documents

BGP Routing Protocol