02.1 BGP Routing Protocol ISSUE1

7/22/2019 02.1 BGP Routing Protocol ISSUE1

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ODA030010 BGP RoutingProtocol

ISSUE 1.4

Page 1HUAWEI 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


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ChapterChapter 11 Principle of BGPPrinciple of BGP

ChapterChapter 22 BGP Route AttributesBGP Route Attributes


Overview Of BGP

BGP is an exterior 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 policies


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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 aremanaged by the same technical management organization and adopt

the unified routing strategy. Each AS has a unique AS number, whichis allocated by the management organization authorized by theInternet.

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

BGP only


Working Mechanism Of BGP

AS1

AS7

AS6

AS5

AS4

AS3

AS2


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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


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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


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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

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!


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 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)


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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)

Header

error

1

OPEN

Massage

error

2

UPDATE

Massage

error

3

Hold

time

expiry

4

State

Machine

error

5

ExitError Type

6Error Code


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Finite State Machine of BGP

Active

Open-sent

Open-confirm Established

Idle

Connect

Connect-Retrytimer expiry

TCP connection fails

Connect-Retry

timer expiry

Start

Others

TCP connection fails

Error

Error Error

KeepAlivetimer expiry

KeepAlive packet

received

1. KeepAlivetimer expiry

2. Update received

3. KeepAlive received

Correct OPEN

packet 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


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ChapterChapter 11 Principle of BGPPrinciple of BGP

ChapterChapter 22 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


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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 viathe 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


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AS_PATH Attribute

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

path or route to the destination specified by the NLRI.

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

most 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

AS100AS500

RTA

RTB

30.0.0.1

30.0.0.2D (400 300 200)

D (500 200,200,200)

D (200 200 200)


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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 10.0.0.2I 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

RTCI 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 routers 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 samedestination, 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.


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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

another AS of its preferred ingress points. It is meant only for a

single AS to demonstrate a degree of preference when it has

multiple ingress points.

MED attribute affects only the incoming traffic to the AS.

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

destination compare the MED of the routes. The lowest MED value

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

destination 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

influence traffic between two directly connected AS.


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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


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(32 bits)

AA:NN. AA is AS number. NN is an administratively defined identifier.


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Well-known Community

NO_EXPORT

Routes received carrying this value cannot be advertised to real

EBGP peers (still can be advertised within a 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 and other AS in conferation

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 .


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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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02.1 BGP Routing Protocol ISSUE1