ODA030010 BGP Routing Protocol ISSUE1.4

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ODA030010 BGP Routing Protocol

ISSUE 1.4

Page 2HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

Upon completion of this course, you

will be able to:

Understand the principle of BGP

Understand the features of

Attributes


Chapter 1 Principle of BGPChapter 1 Principle of BGP

Chapter 2 Chapter 2 BGP Route AttributesBGP Route Attributes


Overview Of BGP

BGP is an exterior routing protocol, used to transmit routing inf

ormation between ASs

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

currence of loop in design. It provides additional attribute infor

mation 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 policies


Autonomous System

What is an Autonomous System (AS)?

which routing protocol running inside the AS

Which routing protocol running between ASs

The Autonomous System (AS) refers to a set of routers, which are managed by the same technical management organization and adopt the unified routing strategy. Each AS has a unique AS number, which is allocated by the management organization authorized by the Internet.

IGP routing protocol such as static route, OSPF , IS-IS etc

BGP only


Working Mechanism Of BGP

AS1

AS7

AS6

AS5

AS4

AS3

AS2


IBGP Neighbor & EBGP Neighbor

EBGP

RTB

RTC

IBGP

RTA

RTD

RTE

EBGP

AS100

AS200

AS300


Route Advertising Principles of BGP

BGP Speaker only selects the best one for its own use

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


BGP Synchronization

EBGP EBGP

RTB

RTC

IBGP

RTA

RTD

RTE

RTF

E0:10.1.1.1/24

S0

S1

AS100

AS200

AS300


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

OSPFRTB

AS200


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


Static Redistribution

Manually configure the static route 18.0.0.1/8

Redistribute the static route manually configured into the BGP r

outing table of RTB

18.0.0.1/8

AS200

RTB


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!


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)


Open Message

Open Message consists of the Message header and the

structure below

Optional Parameters Length (1byte)

Hold Time (2byte)

Version(1byte)

My Autonomous System (2byte)

BGP Identifier (4byte)

Optional Parameters


Keepalive Message

Keepalive Message only has the header

Marker(16 byte)

Length(2 byte) Type(1 byte)


Update Message

An Update Message consists of the header and the following

structure

Withdrawn Routes (variable length)

Unfeasible Routes Length (2byte)

Path Attribute Length (2byte)

Network Layer Reachability Information (variable length)

Path Attributes (variable length)


Notification Message

A Notification packet consists of the header and the structure below

Data (variable length)

Error Code(1byte)

Error Subcode(1byte)

Error Code 1 2 3 4 5 6

Error TypeHeader

error

OPEN

Massage

error

UPDATE

Massage

error

Hold

time

expiry

State

Machine

error

Exit


Finite State Machine of BGP

Active

Open-sent

Open-confirm Established

Idle

Connect

Connect-Retrytimer expiry

TCP connection fails

Connect-Retrytimer expiry

Start

Others

TCP connection fails

Error

Error Error

KeepAlivetimer expiry

KeepAlive packet received

1. KeepAlive timer expiry2. Update received3. KeepAlive received

Correct OPENpacket received

TCP connection setup

TCP connection setupOthers


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 route

needs 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


Chapter 1 Principle of BGPChapter 1 Principle of BGP

Chapter 2 Chapter 2 BGP Route AttributesBGP Route Attributes


BGP Path Attributes

A Path Attribute is a characteristic of an advertised BGP route.

Each Path Attribute falls into one of four categories:

Well-known mandatory

Well-known discretionary

Optional transitive

Optional nontransitive


Path AttributeWell-known mandatory

ORIGIN

AS-Path

Next hop

Well-known discretionary

Local-Preference

Atomic-Aggregate

Optional transitive

Aggregator

Community

Optional nontransitive

Multi-Exit-Disc (MED)

ORIGINATOR-ID

Cluster-List

Destination Pref (MCI)

Advertiser (Baynet)

Rcid-Path (Baynet)

MP_Reach_NLRI

MP_Unreach_NLRI

Extended_Communities


ORIGIN Attribute

ORIGIN specifies the origin of the routing update. When BGP has multiple routes, it uses ORIGIN as one factor in determining the preferred route.

IGP NLRI (Network layer Reachability Information) was learned from a protocol internal to the originating AS. BGP routes are given an origin of IGP if they are learned from an IGP routing table via the network statement.

EGP NLRI was learned from the Exterior Gateway Protocol.

Incomplete NLRI was learned by some other means. Incomplete imply that the information for determining the origin of the route is incomplete. Routes that BGP learns through redistribution carry the incomplete origin attribute.

Which one is preferred? IGP > EGP > Incomplete


AS_PATH Attribute

AS-PATH uses a sequence of AS numbers to describe the inter-AS pat

h or route to the destination specified by the NLRI.

AS-PATH describes all AS it has passed through ,beginning with the m

ost recent AS and ending with the originating AS.

D(18.0.0.0/8)AS200

AS300

AS400

AS100 AS500

RTA

RTB

30.0.0.1

30.0.0.2D (400 300 200)

D (500 200)


AS_PATH Attribute

The Function of AS-PATH

AS can influence its incoming traffic by changing the AS_PATH of its advertising route

AS_PATH can be used for loop avoidance

D(18.0.0.0/8)AS200

AS300

AS400

AS100 AS500

RTA

RTB

30.0.0.1

30.0.0.2D (400 300 200)

D (500 200,200,200)

D (200 200 200)


Next Hop Attribute

18.0.0.0/8

20.0.0.0/8

RTARTC

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

IBGPIBGPEBGP

RTBI can reach 18.0.0.0/8 via the next hop 10.0.0.2I can reach 20.0.0.0/8 via the next hop 10.0.0.3

RTAI 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 10.0.0.3I can reach 19.0.0.0/8 via the next hop 21.0.0.1

RTC I can reach 19.0.0.0/8 via the next hop 10.0.0.1I can reach 20.0.0.0/8 via the next hop 10.0.0.3


LOCAL_PREF Attribute

LOCAL_PREF is used to communicate a BGP router’s degree

of preference for an advertised route.

LOCAL_PREF is only in updates between internal BGP peers

and it is not passed to other AS.

If an internal BGP speaker receives multiple routes to the same

destination, it compares the LOCAL_PREF attribute of the

routes. The route with highest LOCAL_PREF is selected.

The LOCAL_PREF attribute affects only traffic leaving the AS.


LOCAL_PREF Attribute

D， local-pref1 100

D， local-pref2 200 RTA will select local-pref2 that has higher local preference

AS400

AS100

AS300AS200

RTA

RTB RTC

RTD RTE

RTF

D (18.0.0.0/8)

30.0.0.1

30.0.0.2

20.0.0.1

20.0.0.2

D， local-pref1 100 D ， local-pref2 200


MULTI-EXIT-DISC (MED) Attribute

MED is carried in EBGP updates and allows an AS to inform anoth

er AS of its preferred ingress points. It is meant only for a single A

S to demonstrate a degree of preference when it has multiple ingre

ss points.

MED attribute affects only the incoming traffic to the AS.

If all else is equal , an AS receiving multiple routes to the same de

stination compare the MED of the routes. The lowest MED value is

prefered. MEDs are not compared if two routes to the same destin

ation are received from two different AS.

The MED is passed between internal peers of the receiving AS but

not passed beyond the receiving AS. MED is used only to influenc

e traffic between two directly connected AS.


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


Community Attribute

Community is designed to simplify policy enforcement. It identifies a

destination as a member of some community of destinations that share

one or more common properties.

The COMMUNITY attribute is a set of four octets values AA:NN. AA is

AS number. NN is an administratively defined identifier.


Well-known Community

NO_EXPORT

− Routes received carrying this value cannot be advertised to

EBGP peers and outside of the confederation

NO_ADVERTISE

− Routes received carrying this value cannot be advertised at all

to either EBGP or IBGP peers.

LOCAL_AS

− Routes received carrying this value cannot be advertised to

EBGP peers including peers in other AS within a confederation.

INTERNET

− All routes belong to this community by default. Received routes

belonging to this community are advertised freely


BGP Route Selection Procedure

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

1. If the next hop of this route is unreachable, this route is not selected.

2. Select the route with a higher local preference.

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

4. Select the route with shortest AS path.

5. Select the route with lowest origin code (IGP lower than EGP, EGP lower than Incomplete ).

6. Select the route with smallest MED .

7.Performing load sharing on multiple routes according to the configured number of routes (in case load sharing is configured and there are multiple external routes to the same AS)

8. Select the route with smallest Router ID .


Summary

Feature of BGP

Difference with the IGP and EGP

Principle of BGP

The route attributes of BGP

Microsoft Excel Worksheet

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ODA030010 BGP Routing Protocol ISSUE1.4