Border Gateway Protocol (BGP4) Border Gateway Protocol (BGP) Routing/Forwarding basics Building blocks Exercises BGP protocol basics Exercises BGP path

Border Gateway Protocol (BGP4)

Border Gateway Protocol (BGP)

• Routing/Forwarding basics• Building blocks• Exercises• BGP protocol basics• Exercises• BGP path attributes• Best path computation• Exercises

Border Gateway Protocol (BGP)...

• Typical BGP topologies• Routing Policy• Exercises• Redundancy/Load sharing• Best current practices

Routing/ForwardingBasics

IP route lookup:Longest match routing

R2

R3

R1 R4

All 10/8 except10.1/16

10.1/16

Packet: DestinationIP address: 10.1.1.1

10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..

R2’s IP routing table

IP route lookup: Longest match routing

R2

R3

R1 R4


10.1/16

10/8 -> R310.1/16 -> R420/8 -> R5

…..


10.1.1.1 & FF.0.0.0 is equal to10.0.0.0 & FF.0.0.0

Match!



R2

R3

R1 R4


10.1/16

10/8 -> R310.1/16 -> R420/8 -> R5

…..


10.1.1.1 & FF.FF.0.0 is equal to10.1.0.0 & FF.FF.0.0

Match as well!



R2

R3

R1 R4


10.1/16

10/8 -> R310.1/16 -> R420/8 -> R5…..


10.1.1.1 & FF.0.0.0 is equal to20.0.0.0 & FF.0.0.0

Does not match!



R2

R3

R1 R4


10.1/16

10/8 -> R310.1/16 -> R420/8 -> R5

…..



Longest match, 16 bit netmask


• default is 0.0.0.0/0

• can handle it using the normal longest match algorithm

• matches everything. Always the shortest match.

Forwarding

• Uses the routing table built by routing protocols

• Performs the lookup to find next-hop and outgoing interface

• Switches the packet with new encapsulation as per the outgoing interface

Building Blocks

• Autonomous System (AS)

• Types of Routes

• IGP/EGP

• DMZ

• Policy

• Egress

• Ingress

Autonomous System (AS)

• Collection of networks with same policy• Single routing protocol• Usually under single administrative control• IGP to provide internal connectivity

AS 100

Autonomous System(AS)...

• Identified by ‘AS number’• Public & Private AS numbers• Examples:

– Service provider– Multi-homed customers– Anyone needing policy discrimination

Routing flow and packet flow

For networks in AS1 and AS2 to communicate: AS1 must announce routes to AS2

AS2 must accept routes from AS1

AS2 must announce routes to AS1

AS1 must accept routes from AS2

accept

announce

announceacceptAS 1 AS2

packet flow

packet flow

Routing flow

egress

ingress

Egress Traffic

• Packets exiting the network

• Based on – Route availability (what others send you)– Route acceptance (what you accept from others)– Policy and tuning (what you do with routes from

others)– Peering and transit agreements

Ingress Traffic• Packets entering your network

• Ingress traffic depends on:– What information you send and to who– Based on your addressing and ASes– Based on others’ policy (what they accept from

you and what they do with it)

Types of Routes• Static Routes

– configured manually

• Connected Routes– created automatically when an interface is ‘up’

• Interior Routes– Routes within an AS

• Exterior Routes– Routes exterior to AS

What Is an IGP?

• Interior Gateway Protocol• Within an Autonomous System• Carries information about internal prefixes• Examples—OSPF, ISIS, EIGRP…

What Is an EGP?

• Exterior Gateway Protocol• Used to convey routing information between

ASes• De-coupled from the IGP• Current EGP is BGP4

Why Do We Need an EGP?

• Scaling to large network– Hierarchy– Limit scope of failure

• Define administrative boundary• Policy

– Control reachability to prefixes

• Interior– Automatic

discovery– Generally trust

your IGP routers– Routes go to all

IGP routers

• Exterior

Specifically configured peers

Connecting with outside networks

Set administrative boundaries

Interior vs. Exterior Routing Protocols

Hierarchy of Routing Protocols

Local NAPFDDI

Other ISP’s

BGP4 BGP4/Static

BGP4 / OSPF

Customers

BGP4

AS 100 AS 101

AS 102

DMZ Network

AA

BB

CC

DD

EE

• Shared network between ASes

Demilitarized Zone (DMZ)

Addressing - ISP

• Need to reserve address space for its network.

• Need to allocate address blocks to its customers.

• Need to take “growth” into consideration

• Upstream link address is allocated by upstream provider

BGP Basics

• Terminology

• Protocol Basics

• Messages

• General Operation

• Peering relationships (EBGP/IBGP)

• Originating routes

Terminology

• Neighbor– Configured BGP peer

• NLRI/Prefix– NLRI - network layer reachability information– Reachability information for a IP address & mask

• Router-ID– Highest IP address configured on the router

• Route/Path– NLRI advertised by a neighbor

Protocol Basics

• Routing protocol used between ASes

–if you aren’t connected to multiple ASes, you don’t need BGP :)

• Runs over TCP• Path vector protocol• Incremental update

AS 100 AS 101

AS 102

EE

BB DD

AA CC

Peering

BGP Basics ...

• Each AS originates a set of NLRI

• NLRI is exchanged between BGP peers• Can have multiple paths for a given prefix• Picks the best path and installs in the IP

forwarding table• Policies applied (through attributes)

influences BGP path selection

AS 100 AS 101

AS 102

AA CC

BGP speakers are called peers

BGP Peers

eBGP TCP/IP

Peer Connection

Peers in different AS’sare called External Peers

Note: eBGP Peers normally should be directly connected.

EE

BB DD220.220.8.0/24 220.220.16.0/24

220.220.32.0/24

AS 100 AS 101

AA CC

BGP speakers are called peers

BGP Peers

iBGP TCP/IP

Peer Connection

Peers in the same ASare called Internal Peers

AS 102

EE

BB DD

Note: iBGP Peers don’t have to be directly connected.

220.220.8.0/24 220.220.16.0/24

220.220.32.0/24

AS 100 AS 101

AA CC

BGP Peers

AS 102

DD220.220.8.0/24 220.220.16.0/24

220.220.32.0/24

EE

BB

BGP Peers exchange Update messages containing Network Layer Reachability Information (NLRI)

BGP Update

Messages

Configuring BGP Peers

interface Serial 0ip address 222.222.10.2 255.255.255.252

router bgp 100 network 220.220.8.0 mask 255.255.255.0 neighbor 222.222.10.1 remote-as 101neighbor 222.222.10.1 remote-as 101



eBGP TCP Connection

• BGP Peering sessions are established using the BGP “neighbor” configuration command

222.222.10.0/30

BB CC DDAA

AS 100 AS 101

.2220.220.8.0/24 220.220.16.0/24.2 .1 .2 .1.1

– External (eBGP) is configured when AS numbers are different

– Internal (iBGP) is configured when AS numbers are same

AS 100 AS 101


222.222.10.0/30

.2



BB



CC

iBGP TCP Connection

• BGP Peering sessions are established using the BGP “neighbor” configuration command

DD220.220.8.0/24 220.220.16.0/24AA .2 .1 .2 .1.1

– External (eBGP) is configured when AS numbers are different


• Each iBGP speaker must peer with every other iBGP speaker in the AS

iBGP TCP/IP

Peer Connection

AS 100

AABB

CC


• Loopback interface are normally used aspeer connection end-points

AS 100215.10.7.1

215.10.7.2

215.10.7.3

AABB

CC

iBGP TCP/IP

Peer Connection

iBGP TCP/IP

Peer Connection


AS 100

AA

215.10.7.1215.10.7.2

215.10.7.3

CC

BB

interface loopback 0 ip address 215.10.7.1 255.255.255.255

router bgp 100 network 220.220.1.0 neighbor 215.10.7.2 remote-as 100 neighbor 215.10.7.2 update-source loopback0neighbor 215.10.7.2 update-source loopback0 neighbor 215.10.7.3 remote-as 100 neighbor 215.10.7.3 update-source loopback0neighbor 215.10.7.3 update-source loopback0

AA


AS 100

AA

215.10.7.1215.10.7.2

215.10.7.3

CC

AA



BB

iBGP TCP/IP

Peer Connection


AS 100

AA

215.10.7.1215.10.7.2

215.10.7.3

AABB



CC

iBGP TCP/IP

Peer Connection

BGP Updates — NLRI

• Network Layer Reachability Information

• Used to advertise feasible routes

• Composed of:– Network Prefix– Mask Length

BGP Updates — Attributes• Used to convey information associated with

NLRI– AS path– Next hop– Local preference– Multi-Exit Discriminator (MED)– Community– Origin – Aggregator

• Sequence of ASes a route has traversed

• Loop detection

• Apply policy

AS 100

AS 300

AS 200

AS 500

AS 400

170.10.0.0/16 180.10.0.0/16

150.10.0.0/16

Network Path

180.10.0.0/16 300 200 100

170.10.0.0/16 300 200

150.10.0.0/16 300 400

Network Path180.10.0.0/16 300 200 100170.10.0.0/16 300 200

AS-Path Attribute

160.10.0.0/16

150.10.0.0/16

192.10.1.0/30

.2

AS 100

AS 200

Network Next-Hop Path160.10.0.0/16 192.20.2.1 100

CC

Next Hop Attribute

.1

BGP Update

Messages

BB

AA

.1

.2

192.

20.2

.0/3

0

AS 300

EE

DD

• Next hop to reach a network

• Usually a local network is the next hop in eBGP session

140.10.0.0/16

• Next hop to reach a network

• Usually a local network is the next

hop in eBGP session

160.10.0.0/16

150.10.0.0/16

192.10.1.0/30

.2

AS 100

AS 200CC

Next Hop Attribute

.1

BB

AA

.1

.2

192.

20.2

.0/3

0

BGP Update

Messages

EE

DD

• Next Hop updated betweeneBGP Peers

AS 300140.10.0.0/16

Network Next-Hop Path150.10.0.0/16 192.10.1.1 200160.10.0.0/16 192.10.1.1192.10.1.1 200 100

• Next hop not changedbetween iBGP peers

160.10.0.0/16

150.10.0.0/16

192.10.1.0/30

.2

AS 100

AS 200

Network Next-Hop Path150.10.0.0/16 192.10.1.1 200160.10.0.0/16 192.10.1.1192.10.1.1 200 100

CC

Next Hop Attribute

.1

BB

AA

.1

.2

192.

20.2

.0/3

0

BGP Update

Messages

DD

EE

AS 300140.10.0.0/16

Next Hop Attribute (more)

• IGP should carry route to next hops

• Recursive route look-up

• Unlinks BGP from actual physical topology

• Allows IGP to make intelligent forwarding decision

BGP Updates — Withdrawn Routes

• Used to “withdraw” network reachability

• Each Withdrawn Route is composed of:– Network Prefix– Mask Length

BGP Updates — Withdrawn Routes

AS 321AS 123

192.168.10.0/24

192.192.25.0/24

.1 .2

x

Connectivity lost

BGP Update

Message

Withdraw Routes192.192.25.0/24Withdraw Routes192.192.25.0/24

Network Next-Hop Path150.10.0.0/16 192.168.10.2 321 200192.192.25.0/24 192.168.10.2 321

BGP Routing Information BaseBGP RIB

D 10.1.2.0/24D 160.10.1.0/24D 160.10.3.0/24R 153.22.0.0/16S 192.1.1.0/24

Network Next-Hop Path

router bgp 100 network 160.10.0.0 255.255.0.0 no auto-summary

Route Table

*>i160.10.1.0/24 192.20.2.2 i*>i160.10.3.0/24 192.20.2.2 i

BGP ‘network’ commands are normally used to populate the BGP RIB with routes from the Route Table


router bgp 100 network 160.10.0.0 255.255.0.0 aggregate-address 160.10.0.0 255.255.0.0 summary-only no auto-summary

Route Table


D 10.1.2.0/24D 160.10.1.0/24D 160.10.3.0/24R 153.22.0.0/16S 192.1.1.0/24

*> 160.10.0.0/16 0.0.0.0 i*> 160.10.0.0/16 0.0.0.0 i* i 192.20.2.2 i* i 192.20.2.2 is> 160.10.1.0/24 192.20.2.2 is> 160.10.3.0/24 192.20.2.2 i

BGP ‘aggregate-address’ commands may be used to install summary routes in the BGP RIB

BGP ‘redistribute’ commands can also be used to populate the BGP RIB with routes from the Route Table



router bgp 100 network 160.10.0.0 255.255.0.0 redistribute static route-map foo no auto-summary

access-list 1 permit 192.1.0.0 0.0.255.255

route-map foo permit 10 match ip address 1

Route Table

D 10.1.2.0/24D 160.10.1.0/24D 160.10.3.0/24R 153.22.0.0/16S 192.1.1.0/24

*> 160.10.0.0/16 0.0.0.0 i* i 192.20.2.2 is> 160.10.1.0/24 192.20.2.2 is> 160.10.3.0/24 192.20.2.2 i*> 192.1.1.0/24 192.20.2.2 ?*> 192.1.1.0/24 192.20.2.2 ?

BGP Routing Information Base

BGP RIBIN Process

Update

Network Next-Hop Path173.21.0.0/16 192.20.2.1 100

* 173.21.0.0/16 192.20.2.1 100

• BGP “in” process• receives path information from peers

• results of BGP path selection placed in the BGP table

• “best path” flagged (denoted by “>”)

Update

Network Next-Hop Path*>i160.10.1.0/24 192.20.2.2 i*>i160.10.3.0/24 192.20.2.2 i

OUT Process

>


OUT Process

Network Next-Hop Path160.10.1.0/24 192.20.2.2 200160.10.3.0/24 192.20.2.2 200173.21.0.0/16 192.20.2.2 200 100192.20.2.1 192.20.2.1

BGP RIB

> 173.21.0.0/16 192.20.2.1 100

Network Next-Hop Path*>i160.10.1.0/24 192.20.2.2 i*>i160.10.3.0/24 192.20.2.2 i*

IN Process

Update Update

• BGP “out” process• builds update using info from RIB

• may modify update based on config

• Sends update to peers

Next-Hop changed


BGP RIB

D 10.1.2.0/24D 160.10.1.0/24D 160.10.3.0/24R 153.22.0.0/16S 192.1.1.0/24

Network Next-Hop Path*>i160.10.1.0/24 192.20.2.2 i*>i160.10.3.0/24 192.20.2.2 i*> 173.21.0.0/16 192.20.2.1 100

• Best paths installed in routing table if:

B 173.21.0.0/16

Route Table

• prefix and prefix length are unique• lowest “protocol distance”

The ‘Bible’ & other resources

• Route-views.oregon-ix.net

• Internet Routing Architectures– Bassam Halabi– pg. 168 BGP Decision Process Summary

Types of BGP Messages

• OPEN– To negotiate and establish peering

• UPDATE– To exchange routing information

• KEEPALIVE– To maintain peering session

• NOTIFICATION– To report errors (results in session reset)

Internal BGP Peering (IBGP)

• BGP peer within the same AS• Not required to be directly connected• Maintain full IBGP mesh or use Route Reflection

AS 100

AA

EE

BB

DD

External BGP Peering (EBGP)

AS 100 AS 101CC

BB

AA

• Between BGP speakers in different AS• Directly connected or peering address is reachable

An Example…

Learns about 35.0.0.0/8 from F & D

AS3561

B

E

C

D

F

A

AS200

AS101

AS21

AS675

35.0.0.0/8

Basic BGP commands

Configuration commandsrouter bgp <AS-number>

neighbor <ip address> remote-as <as-number>

Show commandsshow ip bgp summary

show ip bgp neighbors

Originating routes...

• Using network command or redistributionnetwork <ipaddress>

redistribute <protocol name>

• Requires the route to be present in the routing table

Originating routes/Inserting prefixes into BGP

• network command

• network 198.10.4.0 mask 255.255.254.0

• ip route 198.10.0.0 255.255.254.0 serial 0

• matching route must exist in the routing table before network is announced!

• Origin: IGP

Update message

• Withdrawn routes• Path Attributes• Advertised routes

Stable IBGP peering

• Unlinks IBGP peering from physical topology.

• Carry loopback address in IGProuter ospf <ID>

passive-interface loopback0

• Unlink peering from physical topologyrouter bgp <AS1>

neighbor <x.x.x.x> remote-as <AS1>

neighbor <x.x.x.x> update-source loopback0

BGP4 continued...

BGP Path Attributes: Why ?

• Encoded as Type, Length & Value (TLV)

• Transitive/Non-Transitive attributes

• Some are mandatory

• Used in path selection

• To apply policy for steering traffic

BGP Path Attributes...

• Origin• AS-path• Next-hop• Multi-Exit Discriminator (MED)• Local preference• BGP Community• Others...

AS-PATH

• Updated by the sending router with its AS number

• Contains the list of AS numbers the update traverses.

• Used to detect routing loops

– Each time the router receives an update, if it finds its AS number, it discards the update

• Sequence of ASes a route has traversed

• Loop detection

AS-Path

AS 100

AS 300

AS 200

AS 500

AS 400

170.10.0.0/16 180.10.0.0/16

150.10.0.0/16

180.10.0.0/16 300 200 100

170.10.0.0/16 300 200

150.10.0.0/16 300 400

180.10.0.0/16 dropped

Next-Hop

• Next hop router to reach a network• Advertising router/Third party in EBGP• Unmodified in IBGP

160.10.0.0/16

150.10.0.0/16

150.10.1.1 150.10.1.2

AS 100

AS 300AS 200

150.10.0.0/16 150.10.1.1160.10.0.0/16 150.10.1.1

AA BB

20Cisco Systems Confidential0799_04F7_c2

Third Party Next Hop

192.68.1.0/24

150.1.1.3150.1.1.3

150.1.1.1

150.1.1.2

192.68.1.0/24 150.1.1.3

AS 201

AS 200

CC

AA BB

• More efficient, but bad idea!

peering

Next Hop...

• IGP should carry route to next hops• Recursive route look-up• Unlinks BGP from actual physical topology• Allows IGP to make intelligent forwarding

decision

Local Preference

• Not for EBGP, mandatory for IBGP

• Default value is 100 on Ciscos

• Local to an AS

• Used to prefer one exit over another

• Path with highest local preference wins

Local Preference

AS 400

AS 200

160.10.0.0/16

AS 100

AS 300

160.10.0.0/16 500> 160.10.0.0/16 800

500 800 EE

BB

CC

AA

DD

Multi-Exit Discriminator

• Non-transitive

• Represented as a numeric value (0-0xffffffff)

• Used to convey the relative preference of entry points

• Comparable if paths are from the same AS

• Path with lower MED wins

• IGP metric can be conveyed as MED

Multi-Exit Discriminator (MED)

AS 201

AS 200

192.68.1.0/24

CC

AA BB

192.68.1.0/24 1000192.68.1.0/24 2000

preferred

Origin

• Conveys the origin of the prefix

• Three values: – IGP - Generated using “network” statement

• ex: network 35.0.0.0

– EGP - Redistributed from EGP

– Incomplete - Redistribute IGP• ex: redistribute ospf

• IGP < EGP < INCOMPLETE

Communities

• Transitive, Non-mandatory• Represented as a numeric value (0-0xffffffff)• Used to group destinations• Each destination could be member of multiple

communities• Flexibility to scope a set of prefixes within or

across AS for applying policy

Customer AS 201

Service Provider AS 200

192.68.1.0/24

CC

AA BB

Community:201:110 Community:201:120

DD

Community Local Preference201:110 110201:120 120

Community...

Synchronization

• C not running BGP (non-pervasive BGP)• A won’t advertise 35/8 to D until the IGP is in sync• Turn synchronization off!

– Run pervasive BGP

router bgp 1880no sync

1880

209

690

B

AC

35/8

D OSPF

BGP Route Selection (bestpath)Only one path as the bestpath !

• Route has to be synchronized Prefix in forwarding table

• Next-hop has to be accessibleNext-hop in forwarding table

• Largest weightLocal to the router

• Largest local preferenceSpread within AS

• Locally sourcedVia redistribute or network statement

BGP Route Selection ...

• Shortest AS-path lengthnumber of ASes in the AS-path attribute

• Lowest origin IGP < EGP < INCOMPLETE

• Lowest MEDbetween paths from same AS

• External over internal closest exit from a router

• Closest next-hop Lower IGP metric, closer exit from as AS

• Lowest router-id • Lowest IP address of neighbor

BGP Route Selection...

AS 400

AS 200

AS 100

AS 300

BBAA

DD

AS 400’s Policy to reach AS100

AS 200 preferred path

AS 300 backup

Increase AS path attribute length by at least 1

Stub AS

• Typically no need for BGP

• Point default towards the ISP

• ISP advertises the stub network to Internet

• Policy confined within ISP policy

Stub AS

AS 100

AS 101BB

AA

Provider

Customer

Multi-homed AS

• Only border routers speak BGP

• IBGP only between border routers

• Exterior routes must be redistributed in a controlled fashion into IGP or use defaults

Multi-homed AS

AS 100

AS 200

AS 300

DD

CCBB

AAprovider

provider

customer

Service Provider Network

• IBGP used to carry exterior routes

• IGP keeps track of topology

• Full IBGP mesh is required

Common Service Provider Network

AS 100 AS 200

AS 400

AS 300

FF

EE

DD

GG

HH

CCBB

AA

provider

Routing Policy

• Why?– To steer traffic through preferred paths– Inbound/Outbound prefix filtering – To enforce Customer-ISP agreements

• How ?– AS based route filtering - filter list– Prefix based route filtering - distribute list– BGP attribute modification - route maps

Distribute list - using IP access lists

access-list 1 deny 10.0.0.0access-list 1 permit anyaccess-list 2 permit 20.0.0.0 … more access-lists as prefixes are added ...

router bgp 100neighbor 171.69.233.33 remote-as 33neighbor 171.69.233.33 distribute-list 1 inneighbor 171.69.233.33 distribute-list 2 out

Filter list rules Regular Expressions

• RE is a pattern to match against an input string

• Used to match against AS-path attribute

• ex: ^3561.*100.*1$

• Flexible enough to generate complex filter list rules

Filter list - using as-path access list

ip as-path access-list 1 permit 3561

ip as-path access-list 2 deny 35

ip as-path access-list 2 permit .*

router bgp 100

neighbor 171.69.233.33 remote-as 33

neighbor 171.69.233.33 filter-list 1 in

neighbor 171.69.233.33 filter-list 2 out

Route Maps

router bgp 300neighbor 2.2.2.2 remote-as 100neighbor 2.2.2.2 route-map SETCOMMUNITY out!route-map SETCOMMUNITY permit 10match ip address 1match community 1set community 300:100!access-list 1 permit 35.0.0.0ip community-list 1 permit 100:200

Route-map match & set clauses

Match Clauses Set Clauses• AS-path• Community• IP address

• AS-path prepend• Community• Local-Preference• MED• Origin• Weight• Others...

H HethH H

eth

H Heth

H Heth

C31

C22C21

C32

ISP3

ISP2

Inbound route-mapto set community

Route-map Configuration Example

neighbor <x.x.x.x> route-map AS100_IN in!route-map AS100_IN permit 10

set community 100:200

neighbor <y.y.y.y> route-map AS200_IN in!route-map AS200_IN permit 10

match community 1set local-preference 200

!ip community-list 1 permit 100:200

Load Sharing & Redundancyusing BGP

Load-sharing - single path

AS100 AS200

Router A:interface loopback 0ip address 20.200.0.1 255.255.255.255!router bgp 100 neighbor 10.200.0.2 remote-as 200neighbor 10.200.0.2 update-source loopback0neighbor 10.200.0.2 ebgp-multi-hop 2!ip route 10.200.0.2 255.255.255.255 <DMZ-link1, link2>

ALoopback 010.200.0.2

Loopback 020.200.0.1

100 200A

Note:A still only advertises one “best” path to ibgp peers

Router A:router bgp 100 neighbor 10.200.0.1 remote-as 200neighbor 10.300.0.1 remote-as 200maximum-paths 2

Load Sharing - Multiple paths from the same AS

Redundancy - Multi-homing

• Reliable connection to Internet• 3 common cases of multi-homing:

- default from all providers- customer + default routes from all- full routes from all

Default from all providers

• Low memory/CPU solution

• Provider sends BGP default– provider is selected based on IGP metric

• Inbound traffic decided by providers’ policy– Can influence using outbound policy, example:

AS-path prepend

Default from all providers

AS 400

Provider

AS 200

Provider

AS 300

EE

BB

CC

AA

DD

Customer + default from all providers

• Medium memory and CPU solution

• Granular routing for customer routes and

default for the rest

• Inbound traffic decided by providers’ policy– Can influence using outbound policy

Customer routes from all providers

AS 400

Provider

AS 200

Customer

AS 100160.10.0.0/16

Provider

AS 300

EE

BB

CC

AA

DD

C chooses shortest AS path

Full routes from all providers

• More memory/CPU

• Full granular routing

• Usually transit ASes take full routes

• Usually pervasive BGP

Full routes from all providers

AS 400

AS 200

AS 100

AS 300

EE

BB

CC

AA

DD

C chooses shortest AS path

AS 500

Best PracticesIGP in Backbone

• IGP connects your backbone together, not your client’s routes

• IGP must converge quickly

• IGP should carry netmask information - OSPF, IS-IS, EIGRP

Best Practices...Connecting to a customer

• Static routes– You control directly– No route flaps

• Shared routing protocol or leaking– You must filter your customers info– Route flaps

• BGP for multi-homed customers

Best Practices...Connecting to other ISPs

• Use BGP4• Advertise only what you serve• Take back as little as you can• Take the shortest exit

Best Practices...The Internet Exchange

• Long distance connectivity is expensive

• Connect to several providers at a single

point

Q & A

Documents

Border Gateway Protocol (BGP4) Border Gateway Protocol (BGP) Routing/Forwarding basics Building blocks Exercises BGP protocol basics Exercises BGP path