© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-1 BGP Overview Understanding BGP Path Attributes

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-2 Outline Overview BGP Path Attributes Well-Known BGP Attributes Optional BGP Attributes AS-Path Attribute Next-Hop Attribute Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-3 BGP Path Attributes BGP metrics are called path attributes. BGP attributes are categorized as “well-known” and “optional.” Well-known attributes must be recognized by all compliant implementations. Optional attributes are recognized only by some implementations (could be private); expected not to be recognized by all.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-4 Well-Known BGP Attributes Well-known attributes are divided into mandatory and discretionary. Mandatory well-known attributes must be present in all update messages. Discretionary well-known attributes are optional; they could be present in update messages. All well-known attributes are propagated to other neighbors.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-5 Mandatory Well-Known BGP Attributes Origin –The origin of a BGP route iRoute originated in an IGP eRoute originated in EGP ?Route was redistributed into BGP AS-path –Sequence of AS numbers through which the network is accessible Next-hop –IP address of the next-hop router

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-6 Discretionary Well-Known BGP Attributes Local preference –Used for consistent routing policy within AS Atomic aggregate –Informs the neighbor AS that the originating router aggregated routes

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-7 Optional BGP Attributes Optional BGP attributes are transitive or nontransitive. Transitive optional attributes –Propagated to other neighbors if not recognized; partial bit set to indicate that the attribute was not recognized Nontransitive optional attributes –Discarded if not recognized Recognized optional attributes are propagated to other neighbors based on their meaning (not constrained by transitive bit).

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-8 Optional BGP Attributes (Cont.) Nontransitive attributes Multi-exit discriminator –Used to discriminate between multiple entry points to a single AS Transitive attributes Aggregator –Specifies IP address and AS number of the router that performed route aggregation Community –Used for route tagging

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-9 AS-Path Attribute The AS-path attribute is empty when a local route is inserted in the BGP table. The AS number of the sender is prepended to the AS- path attribute when the routing update crosses AS boundary. The receiver of BGP routing information can use the AS-path attribute to determine through which AS the information has passed. An AS that receives routing information with its own AS number in the AS path silently ignores the information.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-10 AS-Path Attribute Example

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-11 AS-Path Attribute Example

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-12 AS-Path Attribute Example

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-13 Next-Hop Attribute Indicates the next-hop IP address used for packet forwarding Usually set to the IP address of the sending External Border Gateway Protocol (EBGP) router Can be set to a third-party IP address to optimize routing

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-14 Next-hop attribute is usually set to the IP address of the sending router. Next-Hop Processing Next-Hop Attribute Example

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-15 Next-hop attribute is usually set to the IP address of the sending router. Next-Hop Attribute Example Next-Hop Processing

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-16 Next-Hop Attribute Example If the receiving BGP router is in the same subnet as the current next-hop address, the next-hop address remains unchanged to optimize packet forwarding. Next-Hop Processing on Shared Media

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-17 Next-Hop Attribute Example Next-Hop Processing on Shared Media If the receiving BGP router is in the same subnet as the current next-hop address, the next-hop address remains unchanged to optimize packet forwarding.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-18 Next-Hop Attribute Example BGP next-hop processing can break connectivity with improper network designs over partially meshed WAN networks. Next-Hop Processing on NBMA Network

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-19 Next-Hop Attribute Example BGP next-hop processing can break connectivity with improper network designs over partially meshed WAN networks. Next-Hop Processing on NBMA Network

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-20 Summary BGP metrics attached to a BGP route are called “path attributes.” Some path attributes are well-known and should be recognized by every BGP implementation. Some of the well-known attributes are mandatory and have to be present in every BGP update. These are the AS-path, next-hop, and origin attributes. Other well-known attributes are discretionary. Attributes that are not required to be recognized by every BGP implementation are called “optional.” These attributes could be transitive (propagated if not recognized) or nontransitive (dropped).

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-21 Summary (Cont.) The AS-path attribute lists the autonomous systems that the routing update has already crossed. This attribute is used for BGP loop detection and BGP route selection. The next-hop attribute specifies the IP address that is to be used for packet forwarding. The next hop is usually set to the IP address of the BGP router sending the update.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-22