Lab Course “RouterLab”

BGP - Border Gateway Protocol(RFC 4271)

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Some of the slides come from: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

BGP

Miscellaneous • Anything that needs discussion?

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BGP

Miscellaneous • Anything that needs discussion?

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SUBMIT YOUR CONFIG FILES, TRACES, COMMAND

DUMP!!!

BGP

Internet Routing• There is no single....

- Routing Protocol- Routing Configuration- Routing State,- Routing Management

• ....... for the entire Internet!• Routing System is a collection of many

components hopefully operating in a consistent manner

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BGP

Internet Routing• Distance Vector

- I tell you all my “best” routes for all destinations that I know and you tell me yours.

- Build simplified topology from local perspective

- E.g. RIP

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• Link State- I announce to everyone

about my links and the addresses I originate on each link and listen to everyoneʼs announcement.

- Build full topology- E.g. OSPF

BGP

Internet Routing• The Routing Architecture uses a 2-level

hierarchy, based on the concept of a routing domain (Autonomous System - AS)

• An AS is an interconnected network with a single exposed topology, a coherent routing policy, and a consistent metric framework

• Within an AS: Interior Gateway Protocols - IGP• Among ASes: Exterior Gateway Protocols - EGP

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BGP

IGPs and EGPs• IGPs

- Distance Vector: RIP, IGRP, EIGRP- Link State: OSPF, IS-IS

• EGPs- Distance Vector: (EGP, BGPv3) BGPv4

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BGP

What the Internet looks like!

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AS2

AS1

AS69AS7

AS3

AS666

OSPF

IS-IS/OSPF

RIP/OSPFRIP/OSPFRIP/OSPF

OSPF

IS-ISRIP

RIP/OSPF

EGPIGP

BGP 8

Why BGP?

! Simple protocol to implement and operate

! Very simple distance metric

! Occludes local policies from external inspection

! Limited inter-SP coordination required

! Mature deployment

Source: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

BGP 8

Why BGP?

! Simple protocol to implement and operate

! Very simple distance metric

! Occludes local policies from external inspection

! Limited inter-SP coordination required

! Mature deployment

Source: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

So simple that to have full knowledge you should read 15 RFCs (~400 Pages!)■ RFC 4271, A Border Gateway Protocol 4

(BGP-4)■ RFC 4456, BGP Route Reflection - An

Alternative to Full Mesh Internal BGP (IBGP)■ RFC 4278, Standards Maturity Variance

Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4 Specification

■ RFC 4277, Experience with the BGP-4 Protocol

■ RFC 4276, BGP-4 Implementation Report■ RFC 4275, BGP-4 MIB Implementation Survey■ RFC 4274, BGP-4 Protocol Analysis■ RFC 4273, Definitions of Managed Objects for

BGP-4■ RFC 4272, BGP Security Vulnerabilities

Analysis■ RFC 3392, Capabilities Advertisement with

BGP-4■ RFC 5065, Autonomous System

Confederations for BGP■ RFC 2918, Route Refresh Capability for BGP-4■ RFC 1772, Application of the Border Gateway

Protocol in the Internet Protocol (BGP-4) using SMIv2

■ RFC 4893, BGP Support for Four-octet AS Number Space

BGP 8

Why BGP?

! Simple protocol to implement and operate

! Very simple distance metric

! Occludes local policies from external inspection

! Limited inter-SP coordination required

! Mature deployment

Source: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

AS-Path, which is a variation of the Distance-Vector Algorithm.

Guess what: it still doesn’t scale!!!

BGP 8

Why BGP?

! Simple protocol to implement and operate

! Very simple distance metric

! Occludes local policies from external inspection

! Limited inter-SP coordination required

! Mature deployment

Source: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

Consequence of the Distance-Vector Algorithm.

BGP 8

Why BGP?

! Simple protocol to implement and operate

! Very simple distance metric

! Occludes local policies from external inspection

! Limited inter-SP coordination required

! Mature deployment

Source: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

BGP sessions are statically set up and manually managed, is this “limited inter-SP” coordination?

Well.... they can do dirty things in the route selection process without the need to talk to each other!(You will do it in the worksheet)

BGP 8

Why BGP?

! Simple protocol to implement and operate

! Very simple distance metric

! Occludes local policies from external inspection

! Limited inter-SP coordination required

! Mature deployment

Source: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf

On this they are right! It is the “De Facto” Standard.

BGP

BGP Basics• BGP obtains route to prefixes from neighboring

ASes

• BGP Propagate “best” routes to other neighbors- “best” depends on policies that depend on neighbors

business relationship (customer-provider model, peering)

• Each BGP routing object is a Prefix and a set of attributes: - <AS Path vector, Origin, Next Hop, Local Pref, …>‣ AS Path Vector is a vector of AS identifiers that form a viable path

- Used for loop prevention and best-path selection

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BGP

Prefix Announcements

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

10.0.0.0/8

BGP

Prefix Announcements

10

AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

10.0.0.0/8AS7

10.0.0.0/8AS7 10.0.0.0/8

BGP

Prefix Announcements

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

10.0.0.0/8AS7

10.0.0.0/8AS7

10.0.0.0/8AS2 AS7

10.0.0.0/8AS3 AS7

10.0.0.0/8

10.0.0.0/8AS2 AS7

BGP

Prefix Announcements

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

10.0.0.0/8AS7

10.0.0.0/8AS7

10.0.0.0/8AS2 AS7

10.0.0.0/8AS3 AS7

10.0.0.0/8AS1 AS2 AS7

10.0.0.0/8

10.0.0.0/8AS2 AS7

BGP

Prefix Announcements

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

10.0.0.0/8AS7

10.0.0.0/8AS7

10.0.0.0/8AS2 AS7

10.0.0.0/8AS3 AS7

10.0.0.0/8AS1 AS2 AS7

Prefix AS Path* 10.0.0.0/8 AS3 AS7 10.0.0.0/8 AS1 AS2 AS7

10.0.0.0/8

10.0.0.0/8AS2 AS7

BGP

BGP = DV Protocol• Maintains a collection of local best paths for all

advertised prefixes• Passes changes to all neighbors containing:

- One single best-path for each prefix- Only incremental updates never a full dump (unless

opening the session or other specific events)

• Changes reflect only events in the local database:- New reachability information to a prefix (update)- Reachability information lost for a prefix (withdraw)

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BGP

BGP Messages• Peers exchange messages over TCP sessions

- Port:179- Can span several physical links

• OPEN- Open TCP Connection- Authenticate peers

• UPDATE- Advertises new paths- Withdraw old paths- Carry all the attributes of the path

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BGP

BGP Messages• KEEPALIVE

- Keeps connection alive in the absence of updates- Otherwise connection is reset due to inactivity

• NOTIFICATION- Reports errors in previous messages- Used to close a session

• ROUTE_REFRESH- Request for full routing information without breaking TCP

connection- There are BGP sessions that have lasted years!

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BGP

BGP Messages over TCP• BGP is not a byte stream protocol like TCP• The byte stream is divided into messages using

BGP-defined markers• Each message is a standalone protocol element• Maximum message size: 4096 bytes

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BGP

Path Attributes (I)• Origin

- How this route was injected into BGP in the first place

• Next_hop- Exit border router

• Multi-Exit-Discriminator- Preference between 2 or more sessions among the same

AS pair

• Local-Pref- Local preference setting

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BGP

Path Attributes (II)• Atomic Aggregate

- The path is the result of aggregation

• Aggregator- ID of proxy aggregator

• Community- Locally defined information field

• Destination-Pref- Preference setting for remote AS

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BGP

Local_Pref

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

Prefix AS Path* 10.0.0.0/8 AS3 AS7 10.0.0.0/8 AS1 AS2 AS7

10.0.0.0/8

BGP

Local_Pref

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

Prefix AS Path* 10.0.0.0/8 AS3 AS7 10.0.0.0/8 AS1 AS2 AS7

10.0.0.0/8

Prefix AS Path LP 10.0.0.0/8 AS3 AS7 10* 10.0.0.0/8 AS1 AS2 AS7 20

BGP

Peering & C-P Relationship

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

10.0.0.0/8

Prefix AS Path LP 10.0.0.0/8 AS3 AS7 10* 10.0.0.0/8 AS1 AS2 AS7 20

$$

$$$$

$$

$$

=

$$ =

Customer- Provider

Peering

BGP

MED Example

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AS2

AS1

AS69AS7

AS3

AS666

RIP/OSPFRIP/OSPFRIP/OSPF

Text

10.0.0.0/8 MED 1011.0.0.0/8 MED 20AS2

10.0.0.0/8 MED 2011.0.0.0/8 MED 10AS2

10.0.0.0/8

11.0.0.0/8

BGP

eBGP and iBGP• eBGP is used across AS boundaries• To synchronize all eBGP speakers of an AS

iBGP is used• iBGP sessions are manually configured• iBGP needs a full mesh (or at least a flooding

hierarchy - Route Reflectors)• iBGP does not detect loops• iBGP does not prepend AS number

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BGP

iBGP

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eBGPiBGP

BGP

iBGP

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eBGPiBGP

BGP

iBGP + RR

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eBGPiBGP

Route-Reflector

BGP

Anything Missing?

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BGP

(Best) Route Selection • For a set of received advertisements of the

same prefix the local “best” selection is based on:

1. Highest value Local_Pref 2. Shortest AS Path length3. Lowest MED4. Minimum IGP cost to Next_Hop Address5. eBGP-learned routes preferred to iBGP-learned routes6. Prefer paths learned from router with smaller ID (selected

in the same way as for OSPF)

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BGP

Worksheet 5 • Use New VLANs topology• Target: Configure BGP and learn to use policies• Readings:

‣ Cisco BGP‣ Juniper BGP‣ RFC 4271

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BGP

Any other Question?

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