© 2001, Cisco Systems, Inc. A_BGP_Confed BGP Confederations

© 2001, Cisco Systems, Inc. BGP Confederations-2 Objectives 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 © 2001, Cisco Systems, Inc. BGP Confederations-3

© 2001, Cisco Systems, Inc. BGP Confederations-4 Objectives 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

© 2001, Cisco Systems, Inc. BGP Confederations-5 IBGP 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

© 2001, Cisco Systems, Inc. BGP Confederations-6 AS 61AS 62 AS 63AS 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 EBGP session Intra-confederation EBGP session IBGP session

© 2001, Cisco Systems, Inc. BGP Confederations-7 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

© 2001, Cisco Systems, Inc. BGP Confederations-8 AS Path Changes within BGP Confederation X(61) 12 X 12X(61) 12 X(61) 12 X(61) 12 X(62 61) 12 X(63 61) 12X 42 12

© 2001, Cisco Systems, Inc. BGP Confederations-9 Details 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

© 2001, Cisco Systems, Inc. BGP Confederations-10 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

© 2001, Cisco Systems, Inc. BGP Confederations-11 Summary 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

© 2001, Cisco Systems, Inc. BGP Confederations-12 Review 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 Confederations © 2001, Cisco Systems, Inc. BGP Confederations-13

© 2001, Cisco Systems, Inc. BGP Confederations-14 Objectives 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

© 2001, Cisco Systems, Inc. BGP Confederations-15 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

© 2001, Cisco Systems, Inc. BGP Confederations-16 BGP 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

© 2001, Cisco Systems, Inc. BGP Confederations-17 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

© 2001, Cisco Systems, Inc. BGP Confederations-18 Configuring BGP Confederation no router bgp as-number router 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

© 2001, Cisco Systems, Inc. BGP Confederations-19 BGP Confederation Configuration Example router bgp ! internal AS ! ! Confederation parameter bgp confederation identifier 123 bgp confederation peers ! ! IBGP neighbor neighbor remote-as ! ! EBGP with intra-confed AS neighbor remote-as neighbor remote-as ! ! real EBGP neighbor remote-as 222

© 2001, Cisco Systems, Inc. BGP Confederations-20 Monitoring 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

© 2001, Cisco Systems, Inc. BGP Confederations-21 Monitoring Intra-Confederation EBGP Neighbors Wilma#show ip bgp neighbor BGP neighbor is , remote AS 65002, external link Index 2, Offset 0, Mask 0x4 BGP version 4, remote router ID 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 BGP neighbor is , remote AS 65002, external link Index 2, Offset 0, Mask 0x4 BGP version 4, remote router ID 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.

© 2001, Cisco Systems, Inc. BGP Confederations-22 Monitoring Confederation Routes Fred#show ip bgp BGP routing table entry for /8, version 5 Paths: (2 available, best #2, advertised over IBGP, EBGP) (65001) (metric ) from ( ) Origin IGP, metric 0, localpref 60, valid, confed-internal (65001) (metric ) from ( ) Origin IGP, metric 0, localpref 60, valid, confed-external, best Intra-confederation part of AS-Path External part of AS- Path Route received from intra- confederation EBGP session Route received from intra- confederation IBGP session Next-hop points to real EBGP peer in both cases

© 2001, Cisco Systems, Inc. BGP Confederations-23 Summary 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

© 2001, Cisco Systems, Inc. BGP Confederations-24 Review 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?

© 2001, Cisco Systems, Inc. BGP Confederations-25 Summary 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

© 2001, Cisco Systems, Inc. BGP Confederations-26