ODA030010 BGP Routing Protocol ISSUE1_4

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ODA030010 BGP Routing Protocol

ISSUE 1.4

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

[Understand the principle of BGP

[Understand the features of Attributes



ChapterChapter 1 1 Principle of BGPPrinciple of BGP

ChapterChapter 2 2 BGP Route AttributesBGP Route Attributes



Overview Of BGP

l BGP is an exterior routing protocol, used to transmit routing information between ASs

l 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

l Transfer protocol: TCP; port No.: 179

l It supports Classless Inter-Domain Routing (CIDR)

l Route updating: transmit incremental routes only

l Abundant route filtering and routing policies



Autonomous System

l What is an Autonomous System (AS)?

l which routing protocol running inside the AS

l 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, 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

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

l BGP Speaker only advertises the routes used by itself to its neighbors

l For the routes obtained from EBGP, the BGP Speaker will advertise them to all its neighbors (including EBGP and IBGP)

l For the routes obtained from IBGP, the BGP Speaker will not advertise them to its IBGP neighbors

l 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

l 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

l OSPF discovers route 18.0.0.1/8

l 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

l OSPF discovers the route 18.0.0.1/8

l 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

l Manually configure the static route 18.0.0.1/8

l Redistribute the static route manually configured into the BGP routing table of RTB

18.0.0.1/8

AS200

RTB



BGP Messages

l 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

l BGP messages have the same format: "message header + packet body"

l The BGP header is given as follows:

Marker(16 byte)

Length(2 byte) Type(1 byte)



Open Message

l Open Message consists of the Message header and the structure below

Optional ParametersLength (1byte)

Hold Time (2byte)

Version(1byte)

My Autonomous System (2byte)

BGP Identifier (4byte)

Optional Parameters



Keepalive Message

l Keepalive Message only has the header

Marker(16 byte)

Length(2 byte) Type(1 byte)



Update Message

l 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

l 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



Finite State Machine of BGP

Active

Open-sent

Open-confirm Established

Idle

Connect

Connect-Retrytimer expiry

TCP connection fails

Connect-Retrytimer expiry

StartOthers

TCP connection fails

Error

Error Error

KeepAlivetimer expiry

KeepAlive packetreceived

1. KeepAlivetimer expiry

2. Update received3. KeepAlive received

Correct OPENpacket received

TCP connection setup

TCP connection setupOthers



Application of Messages in BGP

l The Open message is sent when establishing a BGP connection via TCP

l 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

l After stabilization, it is necessary to send the KEEPALIVE message periodically to keep the validity of the BGP connection

l When an error is found during the running of local BGP, NOTIFICATION message shall be sent to notify the BGP peer



ChapterChapter 1 1 Principle of BGPPrinciple of BGP

ChapterChapter 2 2 BGP Route AttributesBGP Route Attributes



BGP Path Attributes

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

l 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

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

l Which one is preferred? IGP > EGP > Incomplete



AS_PATH Attribute

l AS-PATH uses a sequence of AS numbers to describe the inter-AS path or route to the destination specified by the NLRI.

l 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

l The Function of AS-PATH

[ AS can influence its incoming traffic by changing the AS_PATH ofits 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

10.0.0.1AS100

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 10.0.0.1

I can reach 20.0.0.0/8 via the next hop 10.0.0.3



LOCAL_PREF Attribute

l LOCAL_PREF is used to communicate a BGP routers degree of preference for an advertised route.

l LOCAL_PREF is only in updates between internal BGP peers and it is not passed to other AS.

l 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.

l The LOCAL_PREF attribute affects only traffic leaving the AS.



LOCAL_PREF Attribute

l Dlocal-pref1 100

l Dlocal-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

Dlocal-pref1 100 Dlocal-pref2 200



MULTI-EXIT-DISC (MED) Attribute

l 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.

l MED attribute affects only the incoming traffic to the AS.

l 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.

l The MED is passed between internal peers of the receiving AS butnot passed beyond the receiving AS. MED is used only to influence 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 10D,metric2 20

D,metric1 10 D,metric2 20

RTA will select the lower metric

IBGP



Community Attribute

l 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.

l 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

l 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

l Feature of BGP

l Difference with the IGP and EGP

l Principle of BGP

l The route attributes of BGP

Microsoft Excel Worksheet


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ODA030010 BGP Routing Protocol ISSUE1_4

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