© 2001, Cisco Systems, Inc. A_BGP_Confed BGP Confederations

© 2001, Cisco Systems, Inc.

A_BGP_ConfedA_BGP_Confed

BGP ConfederationsBGP Confederations

© 2001, Cisco Systems, Inc. BGP Confederations-2

ObjectivesObjectives

Upon completion of this lesson, you will be able to perform the following tasks:

• Understand the motivation behind BGP Confederations

• Describe how BGP confederations modify BGP AS-path propagation rules

• Plan IBGP backbone migration toward BGP confederation

• Configure and monitor BGP confederation

Introduction to BGP Confederations

Introduction to BGP Confederations

www.cisco.com© 2001, Cisco Systems, Inc. BGP Confederations-3



Upon completion of this section, you will be able to perform the following tasks:

• Understand the need for BGP confederations

• Understand and describe AS path propagation in BGP confederation

• Understand the difference between IBGP, intra-confederation EBGP and real EBGP sessions


IBGP Transit AS - ProblemsIBGP Transit AS - Problems

IBGP requires full-mesh between all BGP-speaking routers

• large number of TCP sessions

• unnecessary duplicate routing traffic

Solutions• route reflectors modify IBGP split horizon

rules

• BGP confederations modify IBGP AS Path processing


AS 61 AS 62

AS 63 AS 64

BGP Confederation - Split Transit AS in smaller AS

AS 12

AS 14

Splitting the AS into smaller AS would reduce the number of IBGP sessions, but we cannot get extra AS numbers

AS 42

Confederations enable us to hide internal AS numbers and announce only one (external) AS number to the EBGP neighbors

real EBGPsession

Intra-confederationEBGP session

IBGP session


AS Path Changes within BGP Confederation


IBGP session

Intra-confederation EBGP session

EBGP session with external peer

• AS path is not changed

• Intra-confederation AS number is prepended to AS path

• Intra-confederation AS numbers are removed from AS path

• External AS number is prepended to the AS path



AS 42

AS 12

AS 14

AS 61 AS 62

AS 63 AS 64

X (61) 12

X 12 X (61) 12

X (61) 12

X (61) 12

X (62 61) 12

X (63 61) 12 X 42 12


Details of AS Path ProcessingDetails of AS Path Processing

• Intra-confederation AS path is encoded as a separate segment of the AS path• Displayed in parenthesis when using IOS show

commands

• All routers within the BGP confederation have to support BGP confederations• A router not supporting BGP confederations

will reject AS path with unknown segment type


Other Properties of Intra-Confederation EBGP Session

Other Properties of Intra-Confederation EBGP Session

• Behaves like EBGP session during session establishment

• EBGP neighbor has to be directly connected or you have to configure ebgp-multihop on the neighbor

• Behaves like IBGP session when propagating routing updates

• Local preference, MED and next-hop attributes are retained

• The whole confederation can run one IGP, giving optimal routing based on next-hop attribute in BGP routing table


SummarySummary

After completing this section, you should be able to perform the following tasks:

• Understand the need for BGP confederations

• Understand and describe AS path propagation in BGP confederation

• Understand the difference between IBGP, intra-confederation EBGP and real EBGP sessions


Review QuestionsReview Questions

• Describe how BGP confederations reduce the IBGP full-mesh

• List the AS Path processing rules in BGP confederation

• What happens if a router not supporting BGP confederations is placed inside the confederation?

• What is the difference between EBGP session and intra-confederation EBGP session?

Deploying BGP ConfederationsDeploying BGP Confederations

www.cisco.com© 2001, Cisco Systems, Inc. BGP Confederations-13



Upon completion of this section, you will be able to perform the following tasks:

• Plan migration of existing IBGP backbone toward BGP confederation

• Configure BGP confederation

• Monitor BGP backbone where BGP confederation is configured


BGP Confederation Design Rules

BGP Confederation Design Rules

• IBGP full-mesh within each member AS is required• Route reflectors might be used within each

AS to relax the IBGP full-mesh requirements

• No topology limitation on EBGP sessions between AS within confederation • The intra-confederation EBGP sessions shall

follow the physical topology of the network


BGP Confederation PlanningBGP Confederation Planning

Divide transit AS into smaller areas

• Follow physical topology of the network

Define AS number for each area.

• Use AS numbers reserved for private use (higher than 64512)

Verify IOS release level

• All routers have to support BGP confederations

Convert each area into autonomous system

• Total rewrite of BGP configuration is required


Configuring BGP Confederation

Configuring BGP Confederation

• Start BGP process with member AS number

• Specify external AS number• Must be defined in all routers within

confederation

• List all member AS numbers in the confederation• Must be defined in all routers with an EBGP

session


Configuring BGP ConfederationConfiguring BGP Confederation

no router bgp as-numberrouter bgp member-AS-number

router(config)#

• Remove old BGP process and configure BGP process with member AS number

bgp confederation identifier external-as-number

router(config-router)#

• Configure external confederation-wide AS number

bgp confederation peers list-of-intra-confederation-AS

router(config-router)#

• Define all the other autonomous systems in the confederation


Autonomous system 123

Internal AS65003

Internal AS65002

Internal AS 65001

1.0.0.11.0.0.2

1.0.0.3 1.0.0.4

2.7.1.1EBGP inAS 222

BGP Confederation Configuration Example

BGP Confederation Configuration Example

router bgp 65001 ! internal AS!! Confederation parameterbgp confederation identifier 123bgp confederation peers 65002 65003! ! IBGP neighborneighbor 1.0.0.3 remote-as 65001 !! EBGP with intra-confed ASneighbor 1.0.0.2 remote-as 65002neighbor 1.0.0.1 remote-as 65003!! real EBGPneighbor 2.7.1.1 remote-as 222


Monitoring BGP ConfederationMonitoring BGP Confederation

show ip bgp neighbor

router#

• Displays whether a neighbor is within the confederation

show ip bgp prefix [mask]

router#

• Displays internal and external segments of the AS Path

• Displays whether the path is external, internal or intra-confederation external


Monitoring Intra-Confederation EBGP Neighbors

Monitoring Intra-Confederation EBGP Neighbors

Wilma#show ip bgp neighbor 1.0.0.4BGP neighbor is 1.0.0.4, remote AS 65002, external link Index 2, Offset 0, Mask 0x4 BGP version 4, remote router ID 12.1.2.3 Neighbor under common administration BGP state = Established, table version = 5, up for 00:09:15 Last read 00:00:16, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 13 messages, 0 notifications, 0 in queue Sent 13 messages, 0 notifications, 0 in queue Prefix advertised 1, suppressed 0, withdrawn 0 Connections established 1; dropped 0 Last reset never 1 accepted prefixes consume 32 bytes 0 history paths consume 0 bytes External BGP neighbor may be up to 255 hops away.

Wilma#show ip bgp neighbor 1.0.0.4BGP neighbor is 1.0.0.4, remote AS 65002, external link Index 2, Offset 0, Mask 0x4 BGP version 4, remote router ID 12.1.2.3 Neighbor under common administration BGP state = Established, table version = 5, up for 00:09:15 Last read 00:00:16, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 13 messages, 0 notifications, 0 in queue Sent 13 messages, 0 notifications, 0 in queue Prefix advertised 1, suppressed 0, withdrawn 0 Connections established 1; dropped 0 Last reset never 1 accepted prefixes consume 32 bytes 0 history paths consume 0 bytes External BGP neighbor may be up to 255 hops away.


Monitoring Confederation RoutesMonitoring Confederation Routes

Fred#show ip bgp 14.0.0.0BGP routing table entry for 14.0.0.0/8, version 5Paths: (2 available, best #2, advertised over IBGP, EBGP) (65001) 387 1.3.0.3 (metric 54357248) from 1.0.0.1 (11.0.0.1) Origin IGP, metric 0, localpref 60, valid, confed-internal (65001) 387 1.3.0.3 (metric 54357248) from 1.0.0.2 (10.1.1.1) Origin IGP, metric 0, localpref 60, valid, confed-external,

best

Intra-confederation part of AS-PathIntra-confederation part of AS-Path

External part of AS-PathExternal part of AS-Path

Route received from intra-confederation EBGP sessionRoute received from intra-confederation EBGP session

Route received from intra-confederation IBGP sessionRoute received from intra-confederation IBGP session

Next-hop points to real EBGP peer in both casesNext-hop points to real EBGP peer in both cases


SummarySummary

After completing this section, you should be able to perform the following tasks:

• Plan migration of existing IBGP backbone toward BGP confederation

• Configure BGP confederation

• Monitor BGP backbone where BGP confederation is configured


Review QuestionsReview Questions

• How can you reduce the IBGP full-mesh within a confederation AS?

• List the BGP Confederation planning steps

• How would you split your AS into smaller autonomous systems?

• Why do you have to verify IOS release level before deploying confederations?

• Which BGP parameters have to be specified in every router within confederation?

• How will the show ip bgp command display intra-confederation segment of the AS path?


SummarySummary

After completing this lesson, you should be able to perform the following tasks:

• Understand the motivation behind BGP Confederations

• Describe how BGP confederations modify BGP AS-path propagation rules

• Plan IBGP backbone migration toward BGP confederation

• Configure and monitor BGP confederation


Documents

© 2001, Cisco Systems, Inc. A_BGP_Confed BGP Confederations