© 2005 Cisco Systems, Inc. All rights reserved. BGP v Route Selection Using Attributes Addressing BGP Communities

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Outline Overview Selecting the Proper Return Path BGP Communities Overview Using Communities Configuring BGP Communities Configuring Route Tagging with BGP Communities Configuring Community Propagation Defining BGP Community-Lists BGP Named Community Lists BGP Cost Community BGP Link Bandwidth BGP Support for Sequenced Entries in Extended Community Lists Matching BGP Communities with Route-Maps Monitoring Communities Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Selecting the Proper Return Path Q:How do you select the proper return path from AS 387 without using AS-path prepending in AS 213? A:Use local preference in AS 387. Q:Will the administrator of AS 387 configure it? A:Unlikely.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Communities Overview BGP communities are a means of tagging routes to ensure consistent filtering or route selection policy. Any BGP router can tag routes in incoming and outgoing routing updates or when doing redistribution. Any BGP router can filter routes in incoming or outgoing updates or select preferred routes based on communities. By default, communities are stripped in outgoing BGP updates.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Communities Overview (Cont.) The community attribute is a transitive optional attribute. Its value is a 32-bit number (range 0 to 4,294,967,200). Each network in a BGP routing table can be tagged with a set of communities. The standards define several filtering-oriented communities: –no-advertise: Do not advertise routes to any peer. –no-export: Do not advertise routes to real EBGP peers. –local-as: Do not advertise routes to any EBGP peers. –internet: Advertise this route to the Internet community. Routers that do not support communities pass them along unchanged.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Communities Overview (Cont.) Defining your own communities A 32-bit community value is split into two parts: –High-order 16 bits contain the AS number of the AS that defines the community meaning. –Low-order 16 bits have local significance. Values of all zeroes and all ones in high-order 16 bits are reserved. Cisco IOS parser allows you to specify a 32-bit community value as: –[AS-number]:[low-order-16-bits]

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Using Communities Define administrative policy goals. Design filters and route selection policy to achieve administrative goals. Define communities that signal individual goals. Configure route tagging on entry points or let BGP neighbors tag the routes. Configure community distribution. Configure route filters and route selection parameters based on communities.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Using Communities (Cont.) Define administrative policy goals. –Solve asymmetrical customer routing problems. Design filters and path selection policy to achieve administrative goals. –Set local preference of customer routes to 50 for customers using the backup ISP. Define communities that signal individual goals. –Community 387:17 is used to indicate that the local preference of the route should be lowered to 50.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring BGP Communities Configure BGP communities as follows: Configure route tagging with BGP communities. Configure BGP community propagation. Define BGP community access-lists (community-lists) to match BGP communities. Configure route-maps that match on community-lists and filter routes or set other BGP attributes. Apply route-maps to incoming or outgoing updates.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Route Tagging with BGP Communities route-map name match condition set community value [ value … ] [additive] route-map name match condition set community value [ value … ] [additive] router(config)# Route tagging with communities is always done with a route- map. You can specify any number of communities. Communities specified in the set keyword overwrite existing communities unless you specify the additive option.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Route Tagging with BGP Communities (Cont.) neighbor ip-address route-map map in | out router(config-router)# This command applies a route-map to inbound or outbound BGP updates. The route-map can set BGP communities or other BGP attributes. redistribute protocol route-map map router(config-router)# Applies a route-map to redistributed routes

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Route Tagging with BGP Communities (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Community Propagation neighbor ip-address send-community router(config-router)# By default, communities are stripped in outgoing BGP updates. You must manually configure community propagation to BGP neighbors. BGP peer groups are ideal for configuring BGP community propagation toward a large number of neighbors.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Community Propagation (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v ip community-list 1-99 permit|deny value [ value … ] router(config)# This command defines a simple community-list. Community-lists are similar to access-liststhey are evaluated sequentially, line by line. All values listed in one line have to match for the line to match and permit or deny a route. You can use the keyword internet to match any community. Defining BGP Community-Lists

© 2005 Cisco Systems, Inc. All rights reserved. BGP v ip community-list permit|deny regexp router(config)# This command defines an extended community-list. Extended community-lists are like simple community-lists, but they match based on regular expressions. Communities attached to a route are ordered, converted to string, and matched with regexp. Use.* to match any community value. Defining BGP Community-Lists (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Defining BGP Community-Lists (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Named Community-Lists Allows the network operator to assign meaningful names to community-lists and increases the number of community- lists that can be configured Can be configured with regular expressions and with numbered community-lists No limitation on the number of community attributes that can be configured for a named community-list Increases the number of community-lists that can be configured by a network operator because there is no limitation on the number of named community-list that can be configured

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Cost Community Allows you to customize the BGP best-path selection process for a local AS or confederation Applied to internal routes by configuring the set extcommunity cost command in a route map Influences the BGP best-path selection process at the POI Can be used as a tie breaker during the best-path selection process

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Link Bandwidth Feature Used to enable multipath load balancing for external links with unequal bandwidth capacity Enabled under an IPv4 or VPNv4 address family sessions by entering the bgp dmzlink-bw command Routes learned from directly connected external neighbor propagated through the IBGP network with the bandwidth of the source external link

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Link Bandwidth Configuration

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Support for Sequenced Entries in Extended Community Lists Allows automatic sequencing of individual entries in BGP extended community-lists Provides the ability to remove or resequence extended community-list entries without deleting the entire existing extended community list Configures sequence numbers for extended community-list entries Resequences existing sequence numbers for extended community-list entries Configures an extended community-list to use default values

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Matching BGP Communities with Route-Maps route-map name permit | deny match community clist-number [exact] set attributes route-map name permit | deny match community clist-number [exact] set attributes router(config)# Community-lists are used in match conditions in route-maps to match on communities attached to BGP routes. A route-map with a community-list matches a route if at least some communities attached to the route match the community- list. With the exact option, all communities attached to the route have to match the community-list. You can use route-maps to filter routes or set other BGP attributes based on communities attached to routes.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Matching BGP Communities with Route-Maps (Cont.) Route selection –You can use route-maps to set weights, local preference, or metric based on BGP communities attached to the BGP route. –Normal route selection rules apply afterward. –Routes not accepted by route-map are dropped. Default filters –Routes tagged with community no-export are sent to IBGP peers and intra-confederation EBGP peers. –Routes tagged with local-as are sent to IBGP peers. –Routes tagged with no-advertise are not sent in any outgoing BGP updates.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Matching BGP Communities with Route-Maps (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Communities Communities are displayed in show ip bgp prefix printout. Communities are not displayed in debugging outputs. Routes in the BGP table tagged with a set of communities or routes matching a community-list can be displayed.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Communities (Cont.) Communities are displayed only in show ip bgp prefix printout.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Communities (Cont.) show ip bgp community router> Displays all routes in a BGP table that have at least one community attached show ip bgp community as:nn [as:nn...] router> Displays all routes in a BGP table that have all the specified communities attached

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Communities (Cont.) show ip bgp community as:nn [as:nn …] exact router> Displays all routes in BGP table that have exactly the specified communities attached show ip bgp community-list clist router> Displays all routes in BGP table that match community-list clist

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Summary You can use the BGP community attribute to create an AS-wide routing policy or to provide services to neighboring autonomous systems. Community attributes are usually used between neighboring autonomous systems. Routers that do not support the community attribute will pass the attribute to other neighbors because it is a transitive attribute. A community is an attribute that is used to tag BGP routes that you can use to manipulate path selection and enforce administrative policies. To set the community attribute, you must use a route-map. In route-map configuration mode, you should use the set community command. You must configure propagation of BGP communities on the routers on a per-neighbor basis; otherwise, the BGP communities are removed from the outgoing BGP updates.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Summary (Cont.) You can use community-lists to match against the community attribute as a method of route selection. Communities are designed to give the network operator the ability to apply policies to large numbers of routes by using match and set clauses in the configuration of route maps. The BGP Named Community Lists feature allows the network operator to assign meaningful names to community-lists and increases the number of community-lists that can be configured by a network operator. The configuration of the BGP Cost Community feature allows you to customize the BGP best path selection process for a local AS or confederation by assigning cost values to specific routes. The BGP Link Bandwidth feature is used to enable multipath load balancing for external links with unequal bandwidth capacity.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Summary (Cont.) BGP Support for Sequenced Entries in Extended Community Lists allows automatic sequencing of individual entries in BGP extended community-lists and also provides the ability to remove or resequence extended community list entries without deleting the entire existing extended community-list. A route-map is used to match networks that carry a subset of communities that are permitted by the community-list. You can view communities only if you use the show ip bgp prefix command.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v