Lab Course RouterLab BGP - Border Gateway Protocol (RFC 4271) - - PowerPoint PPT Presentation

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Lab Course RouterLab BGP - Border Gateway Protocol (RFC 4271) - - PowerPoint PPT Presentation

Sep 28, 2022 465 likes •862 views

Lab Course RouterLab BGP - Border Gateway Protocol (RFC 4271) Some of the slides come from: http://www.ietf.org/proceedings/07dec/slides/IDRTut-0.pdf 1 Miscellaneous Anything that needs discussion? BGP 2 Miscellaneous Anything

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SLIDE 1

Lab Course “RouterLab”

BGP - Border Gateway Protocol (RFC 4271)

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

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SLIDE 2

BGP

Miscellaneous

  • Anything that needs discussion?

2

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SLIDE 3

BGP

Miscellaneous

  • Anything that needs discussion?

2

SUBMIT YOUR CONFIG FILES, TRACES, COMMAND DUMP!!!

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SLIDE 4

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

3

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SLIDE 5

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

4

  • 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
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SLIDE 6

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

5

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SLIDE 7

BGP

IGPs and EGPs

  • IGPs
  • Distance Vector: RIP, IGRP, EIGRP
  • Link State: OSPF, IS-IS
  • EGPs
  • Distance Vector: (EGP, BGPv3) BGPv4

6

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SLIDE 8

BGP

What the Internet looks like!

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AS2 AS1 AS69 AS7 AS3 AS666

OSPF IS-IS/OSPF RIP/OSPF RIP/OSPF RIP/OSPF OSPF IS-IS RIP RIP/OSPF

EGP IGP

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SLIDE 9

BGP

8

Why BGP?

! Simple protocol to implement and

  • perate

! 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

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SLIDE 10

BGP

8

Why BGP?

! Simple protocol to implement and

  • perate

! 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

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SLIDE 11

BGP

8

Why BGP?

! Simple protocol to implement and

  • perate

! 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!!!

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SLIDE 12

BGP

8

Why BGP?

! Simple protocol to implement and

  • perate

! 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.

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SLIDE 13

BGP

8

Why BGP?

! Simple protocol to implement and

  • perate

! 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)

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SLIDE 14

BGP

8

Why BGP?

! Simple protocol to implement and

  • perate

! 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.

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SLIDE 15

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

9

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SLIDE 16

BGP

Prefix Announcements

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AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/OSPF

10.0.0.0/8

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SLIDE 17

BGP

Prefix Announcements

10

AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/OSPF

10.0.0.0/8 AS7 10.0.0.0/8 AS7

10.0.0.0/8

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SLIDE 18

BGP

Prefix Announcements

10

AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/OSPF

10.0.0.0/8 AS7 10.0.0.0/8 AS7 10.0.0.0/8 AS2 AS7 10.0.0.0/8 AS3 AS7

10.0.0.0/8

10.0.0.0/8 AS2 AS7

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SLIDE 19

BGP

Prefix Announcements

10

AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/OSPF

10.0.0.0/8 AS7 10.0.0.0/8 AS7 10.0.0.0/8 AS2 AS7 10.0.0.0/8 AS3 AS7 10.0.0.0/8 AS1 AS2 AS7

10.0.0.0/8

10.0.0.0/8 AS2 AS7

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SLIDE 20

BGP

Prefix Announcements

10

AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/OSPF

10.0.0.0/8 AS7 10.0.0.0/8 AS7 10.0.0.0/8 AS2 AS7 10.0.0.0/8 AS3 AS7 10.0.0.0/8 AS1 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

10.0.0.0/8 AS2 AS7

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SLIDE 21

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
  • pening 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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SLIDE 22

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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SLIDE 23

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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SLIDE 24

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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SLIDE 25

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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SLIDE 26

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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SLIDE 27

BGP

Local_Pref

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AS2 AS1 AS69 AS7 AS3 AS666

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

10.0.0.0/8

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SLIDE 28

BGP

Local_Pref

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AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/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

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SLIDE 29

BGP

Peering & C-P Relationship

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AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/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

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SLIDE 30

BGP

MED Example

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AS2 AS1 AS69 AS7 AS3 AS666

RIP/OSPF RIP/OSPF RIP/OSPF

Text

10.0.0.0/8 MED 10 11.0.0.0/8 MED 20 AS2 10.0.0.0/8 MED 20 11.0.0.0/8 MED 10 AS2

10.0.0.0/8 11.0.0.0/8

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SLIDE 31

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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SLIDE 32

BGP

iBGP

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eBGP iBGP

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SLIDE 33

BGP

iBGP

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eBGP iBGP

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SLIDE 34

BGP

iBGP + RR

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eBGP iBGP Route-Reflector

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SLIDE 35

BGP

Anything Missing?

23

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SLIDE 36

BGP

(Best) Route Selection

  • For a set of received advertisements of the

same prefix the local “best” selection is based

  • n:
  • 1. Highest value Local_Pref
  • 2. Shortest AS Path length
  • 3. Lowest MED
  • 4. Minimum IGP cost to Next_Hop Address
  • 5. eBGP-learned routes preferred to iBGP-learned routes
  • 6. Prefer paths learned from router with smaller ID (selected

in the same way as for OSPF)

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SLIDE 37

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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SLIDE 38

BGP

Any other Question?

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