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Copyright © 2010 Internetwork ExpertCopyright © 2010 Internetwork Expert 1

Understanding, Implementing and Understanding, Implementing and troubleshooting BGPtroubleshooting BGP

01

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Introduction

Instructor IntroductionInstructor Introduction

• Keith Barker, CCIE #6783• CCIE Routing and Switching – 2001

CCIE S it 2003• CCIE Security – 2003

• [email protected]

• YouTube - Keith6783

Copyright © Copyright © 2010 2010 Internetwork Expert, Inc Internetwork Expert, Inc www.INE.comwww.INE.com



Keith’s OfficeKeith’s Office

Copyright © 2010 Internetwork Expert, Inc Copyright © 2010 Internetwork Expert, Inc www.INE.comwww.INE.com

Our JourneyOur Journey

• BGP fundamentals– Injecting networks, iBGP, eBGP, Route

Reflectors, Confederations, Peer GroupsReflectors, Confederations, Peer Groups

• Policy Based BGP– Attributes and Filtering

– Manipulating BGP

• Complex Topologies


g– Advanced route maps, community, load

balancing, dampening, troubleshooting



The playing fieldThe playing field


642642--661661




General Q&AGeneral Q&A


BGP OverviewBGP Overview

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What Is BGP?What Is BGP?

• Border Gateway Protocol Version 4

• Standards basedRFC 4271 “A B d G t P t l 4 (BGP 4)”– RFC 4271 “A Border Gateway Protocol 4 (BGP-4)”

• Exterior Gateway Protocol (EGP)– Used for inter-domain routing between Autonomous

Systems

• Path vector routingU lti l “ tt ib t ” f ti d i i


– Uses multiple “attributes” for routing decision

• Classless– Supports VLSM and summarization

InterInter--AS Routing and ASNsAS Routing and ASNs

• Autonomous System (AS)– “…a set of routers under a single technical administration, using

an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an i t AS ti t l t d t i h t t k t tinter-AS routing protocol to determine how to route packets to other ASes.” (RFC 4271)

• Like IP address space, Autonomous System Numbers (ASNs) allocated by Internet Assigned Numbers Authority (IANA)– http://www.iana.org/numbers/

• BGP ASNs originally 2-byte field– Values 0-65535


– Values 0-65535

• RFC 4893 defines 4-byte ASNs– 0.0 – 65535.65535 notation– 0.[0-65535] denote original 2-byte ASNs



ICANNICANN


ICANNICANN




IANAIANA


Address Space RegistryAddress Space Registry




Why Use BGP?Why Use BGP?

• Scalability– IGPs can scale to thousands of routes– BGP can scale to hundreds of thousands of routes– Current Global (Internet) BGP table ~ 300,000 routes

• Stability– Internet routing table never converges– BGP stable enough to handle routing and decision

making at the same time• Enforce routing policy


– IGP uses link cost for routing decision• Effective traffic engineering nearly impossible with IGP

– BGP uses attributes of the route itself• Traffic engineering feasible and simple to implement

Who Needs BGP?Who Needs BGP?

• Transit networks– SPs that sell access or transit bandwidth to

customerscustomers– Need full routing table to make accurate

decisions – Should not use default routing

• Multihomed networksEnterprise networks with two or more


– Enterprise networks with two or more connections to ISPs

– Allows control of inbound and outbound routing policy



Example Transit NetworkExample Transit Network


Example Multihomed NetworkExample Multihomed Network

Internet

ISP 2AS 2000

ISP 1AS 1000


Customer 1AS 100



The actual network is “The actual network is “DownStreamDownStream””


When When notnot To Use BGPTo Use BGP

• Single ISP connectivity– Default routing sufficient

Li it d d/ CPU• Limited memory and/or CPU resources– Global table needs ~ 1GB of memory just for

storage

• Don’t “own” your IPv4 addresses– ISP advertises “their” address space on your

b h lf


behalf– Red tape involved with getting Provider

Independent address space and BGP ASN





BGP Basic BGP Basic CompnentsCompnents

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BGP Building BlocksBGP Building Blocks


BGP Data StructureBGP Data Structure

• Like EIGRP/OSPF/IS-IS, BGP uses a three table data structure

• Neighbor table– List of active adjacencies called “peerings”

• BGP table– All prefixes learned from all peers

IP R ti t bl


• IP Routing table– The “best” routes from the BGP table actually

used for routing (Maybe . AD still used)



How BGP WorksHow BGP Works

• Establish BGP peerings to build neighbor table

• Exchange updates to build BGP table

• Choose BGP bestpaths to build routing table


Won’t you be my neighbor?Won’t you be my neighbor?




Example: BGP NeighborsExample: BGP Neighbors


Example: Global BGP tableExample: Global BGP table




Example: Routing TableExample: Routing Table


Establishing BGP PeeringsEstablishing BGP Peerings

• Like IGP, first step in BGP is to find neighbors to exchange information with

• Unlike IGPUnlike IGP…– BGP does not have its own transport

• Uses TCP port 179

– BGP neighbors are not discovered• Manually configured via neighbor statement

– BGP neighbors do not have to be connected• IGP is always on a link-by-link basis


IGP is always on a link by link basis• BGP is a logical peering over TCP• Implies that BGP always needs IGP underneath

– BGP has different types of neighbors• External BGP vs. Internal BGP



BGP Packet FormatsBGP Packet Formats

• Peering establishment and maintenance uses four types of packets– OPEN

– KEEPALIVE

– UPDATE

– NOTIFICATION


BGP OPEN MessageBGP OPEN Message

• Used to negotiate parameters for peering• Includes…

– BGP version• Should be 4

– Local ASN– Local Router-ID – Hold time

N ti t d t l t t d l


• Negotiated to lowest requested value

– Options• AKA “capabilities”



BGP Open MessageBGP Open Message


BGP KEEPALIVE MessageBGP KEEPALIVE Message

• Used for dead neighbor detection

• If hold time = 0, keepalives disabled




KeepaliveKeepalive


BGP UPDATE MessageBGP UPDATE Message

• Used to advertise or withdraw a prefix

• Includes– Withdrawn routes

• List of routes that should be discarded

– NLRI• Route being advertised

– Path vector attributes


Path vector attributes• Attributes of route being advertised

• Used for bestpath selection



BGP UpdateBGP Update


Withdrawn RouteWithdrawn Route




BGP NOTIFICATION MessageBGP NOTIFICATION Message

• Used to convey error messages

• After notification sent, BGP session closed

• Examples– Unsupported Version Number

– Unsupported Optional Parameter

– Unacceptable Hold Time

H ld Ti E i d


– Hold Timer Expired

NotificationNotification




BGP Peering State MachineBGP Peering State Machine

• BGP state machine tracks peering establishment• Idle

– Waiting to start 3-way handshake

• Connect– Waiting to complete 3-way handshake

• Active– 3-way handshake failed, try again

• Open sent– 3-way handshake complete, OPEN message sent


• Open confirm– OPEN message received, parameters agreed upon

• Established– Peering complete

Debug Debug ipip bgpbgp






Configuring Basic BGPConfiguring Basic BGP

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Configuring BGP, the beginningConfiguring BGP, the beginning




BGP Peering TypesBGP Peering Types

• External BGP (EBGP) Peers– Neighbors outside my Autonomous System

• Internal BGP (iBGP) Peers– Neighbors inside my Autonomous System

• Update and path selection rules change depending on what type of peer a route is being sent to/received from


being sent to/received from

Basic BGP ConfigurationBasic BGP Configuration

• Enable global BGP process– router bgp [ASN]

• Establish BGP peers– neighbor [address] remote-as [remote ASN]




iBGP PeeringsiBGP Peerings

• Peers in the same AS• Many times not directly connected

– Implies IGP needed to provide TCP transport

• Loop prevention via route suppression– Routes learned from an iBGP peer cannot be

advertised on to another iBGP peer

• Implies that all routers running BGP within


the AS must peer with each other– i.e. “iBGP full mesh”– n*(n-1)/2 peerings

Add Add iBGPiBGP neighborneighbor




Idle, connect, open sent/confirm establishedIdle, connect, open sent/confirm established


Verify neighborsVerify neighbors




Verify Verify iBGPiBGP neighbor detailsneighbor details


EBGP PeeringsEBGP Peerings

• Peers in different ASes

• Usually directly connected neighbors– e.g. DS3 Frame Relay link to ISP

• Can be “multihop”, but TTL defaults to 1– neighbor [address] ebgp-multihop [ttl]

U AS P th tt ib t f l ti


• Uses AS-Path attribute for loop prevention– If I receive an update from an EBGP peer with

my own ASN in the AS-Path, discard it



eBGPeBGP NeighborNeighbor


eBGPeBGP NeighborNeighbor




State/State/PfxRcdPfxRcd


Show Show ipip bgpbgp summarysummary




Show Show ipip bgpbgp neighbor (neighbor (eBGPeBGP))


BGP RouterBGP Router--IDID




BGP RouterBGP Router--ID contID cont


Changing the BGP routerChanging the BGP router--idid






Getting routes/Prefixes into BGPGetting routes/Prefixes into BGP

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Network StatementsNetwork Statements




AutoAuto--Summary is OFF by defaultSummary is OFF by default


Effects of enabling AutoEffects of enabling Auto--summarysummary




Without AutoWithout Auto--SummarySummary


Network statement / static routesNetwork statement / static routes




Redistribution into BGPRedistribution into BGP


Routes must be in routing table to be Routes must be in routing table to be injected on the local router into BGPinjected on the local router into BGP




Redistribute Connected and Origin codeRedistribute Connected and Origin code


Setting the metric on redistributionSetting the metric on redistribution




Sourcing a default route, for othersSourcing a default route, for others


Neighbor sees default routeNeighbor sees default route




ORIGIN Path AttributeORIGIN Path Attribute


Additional ways of injecting routes:Additional ways of injecting routes:

• Aggregate (summary) routes– At least 1 more specific route that belongs to

th t b i th BGP t blthe summary must be in the BGP table

• BGP inject-map– Doesn’t require the route to be in the BGP

table, and is conditional based on policy




Aggregation Aggregation

• BGP aggregation, like IGP summarization, is used to reduce resource requirements needed to process the BGP table

C f– Configured as aggregate-address [network] [mask] [summary-only|as-set|route-map|...]

• Can be applied at any point in the network– No hierarchy like OSPF/IS-IS

• Does not automatically stop subnet advertisements


advertisements– summary-only argument

• Can be used for longest match routing traffic engineering

Synchronization RuleSynchronization Rule

• Off by default

• IF on, then all the BGP routes must ALSO appear via an IGP, otherwise we won’t use them.






Configuring Configuring iBGPiBGP using using LoopbacksLoopbacks

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iBGP Full Mesh ExampleiBGP Full Mesh Example




iBGP Full Mesh ScalabilityiBGP Full Mesh Scalability

• n*(n-1)/2 doesn’t scale– 10 routers, 45 peerings

– 100 routers, 4950 peerings

– 1000 routers, 499,500 peerings

• Can be fixed with two exceptions– Route Reflectors

• Same logic as DR/DIS


• Same logic as DR/DIS

– Confederation• Split the AS into smaller Sub-ASes

How many TCP Sessions for Full MeshHow many TCP Sessions for Full Mesh




BGP Peering RedundancyBGP Peering Redundancy

• BGP peering is based on TCP reachability to peer address

• If peer address is unreachable peering goesIf peer address is unreachable, peering goes down– e.g. if IP address of Serial link is used for peering and

Serial link is down, peer goes down

• Using Loopback addresses for peerings allows rerouting around link failures and adds redundancy


redundancy– e.g. as long as any link is up, Loopback can be

reached

• Defined as update-source for TCP session

BGP Loopback Redundancy ExampleBGP Loopback Redundancy ExampleR1 and R2 Peer Using Their

Directly Connected Link

Directly Connected Link GoesDown and BGP Peering Is Lost

R1 and R2 Peer Using TheirLoopback Interfaces

Physical Link Goes DownBut Peering Is Rerouted




Update Source ConfigurationUpdate Source Configuration

• Modify peering source address– neighbor [address] update-source [interface][interface]






Route Reflector ClustersRoute Reflector Clusters

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BGP Route ReflectorsBGP Route Reflectors

• Eliminates need for full mesh– Only need peering(s) to the RR(s)

• Like OSPF DR , minimizes prefix replication– Send one update to the RR

– RR sends the update to its “clients”

Loop prevention through Cluster ID


• Loop prevention through Cluster-ID– If I am a RR and I receive a route with my own

Cluster-ID, discard it

Route Reflector ExampleRoute Reflector Example

RouteReflector




Design GuidelinesDesign Guidelines

• Redundant RRs have iBGP between them

• Client have no iBGP connections except to ther RRs

• Cluster ID set to the same value on RRs in the same cluster.


Route ReflectorsRoute Reflectors

• Enabling Route Reflection– neighbor [address] route-reflector-clientreflector-client




Route ReflectorRoute Reflector






ConfederationsConfederations

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BGP ConfederationBGP Confederation

• Reduces full mesh iBGP need by splitting AS into smaller Sub-ASes

Inside Sub AS full mesh or RR need remains– Inside Sub-AS full mesh or RR need remains– Between Sub-AS acts like EBGP

• Devices outside the confederation do not know about the internal structure– Sub-AS numbers are stripped from

advertisements to true EBGP peers


advertisements to true EBGP peers

• Typically uses ASNs in private range (64512 – 65535)

BGP Confederation ExampleBGP Confederation Example

RouteReflector




Confederation ImplementationConfederation Implementation


ConfederationsConfederations

• Enabling Confederation– Enable global BGP process

•router bgp [Sub ASN]•router bgp [Sub-ASN]

– Define global ASN•bgp confederation-id [ASN]

– Define other Sub-ASes•bgp confederation peers [Sub-ASN1] [Sub-ASN2] [Sub-ASNn]




ConfederationConfederation


ConfederationConfederation




AS Path for InterAS Path for Inter--ConfederationConfederation






Peer GroupsPeer Groups

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BGP Peer GroupsBGP Peer Groups

• Typically many peers share the same update policy– e.g. a route reflector’s clients

• BGP Peer Groups reduce configuration and processing overhead by applying a template to the peers

• Peer group is assigned parameters such as…– remote-as– route-reflector-client


– route-map• Neighbor is specified as a member of the group

– Peers in a group must be either all iBGP or all EBGP

Peer group ConfigurationPeer group Configuration

• Configuring BGP peer group– neighbor [Peer-Group-Name] peer-group

neighbor [Peer Group Name] [attrbiutes]– neighbor [Peer-Group-Name] [attrbiutes]

– neighbor [address] peer-group [Peer-Group-Name]




BGP Peer Group ExampleBGP Peer Group Examplerouter bgp 1neighbor IBGP_PEER_GROUP peer-groupneighbor IBGP_PEER_GROUP remote-as 1neighbor IBGP_PEER_GROUP update-source Loopback0i hb G G O fl lineighbor IBGP_PEER_GROUP route-reflector-client

neighbor IBGP_PEER_GROUP next-hop-selfneighbor 1.2.3.4 peer-group IBGP_PEER_GROUPneighbor 5.6.7.8 peer-group IBGP_PEER_GROUPneighbor 9.10.11.12 peer-group IBGP_PEER_GROUPneighbor 13.14.15.16 peer-group IBGP_PEER_GROUP


Dynamic Update PeerDynamic Update Peer--GroupsGroups

• Separate outbound routing policies




PeerPeer--groupsgroups






Authentication and TTL SecurityAuthentication and TTL Security

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BGP AuthenticationBGP Authentication

• Like IGP authentication, BGP peer authentication protects control plane against attacks and misconfigurationsattacks and misconfigurations– Without authentication, BGP susceptible to TCP RST

attacks• Interesting read: “Slipping in the Window: TCP Reset attacks”

• Uses MD5 as defined in RFC 2385– “Protection of BGP Sessions via the TCP MD5


Signature Option”

• Simply configured as neighbor [address] password [password]



Authentication ConfigurationAuthentication Configuration

• Enabling BGP authentication– neighbor [address] password [password]


TTL SecurityTTL Security

• Configured per eBGP peer

• Chan ges the TTL for eBGP to 255

• You choose how may hops max the peer is away. Any incoming eBGP updates with TTL below the acceptable level are silently discarded.






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Text of - Amazon S3 · • CCIE Routing and Switching – 2001 • CCIE S itCCIE Security – 2003 •...