© 2005 Cisco Systems, Inc. All rights reserved. BGP v Route Selection Using Attributes Setting BGP Local Preferences

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Outline Overview Consistent Route Selection Within the AS BGP Local Preference Configuring Default Local Preference Configuring Local Preference with Route-Maps Monitoring Local Preference Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Consistent Route Selection Within the AS Q1:Which routing protocol must be run in AS 213? A1:You must run IBGP in AS 213.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Consistent Route Selection Within the AS (Cont.) Q2: How will you influence the route selection on routers in AS 213 so that they select the fastest route? A2:Use weights on EBGP and IBGP sessions.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Consistent Route Selection Within the AS (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Consistent Route Selection Within the AS (Cont.) Have the traffic run over the fastest line available.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Local Preference You can use local preference to ensure AS-wide route selection policy. Any BGP router can set local preference when processing incoming route updates, when doing redistribution, or when sending outgoing route updates. Local preference is used to select routes with equal weight. Local preference is stripped in outgoing EBGP updates except in EBGP updates with confederation peers.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Local Preference (Cont.) Local preference is the second strongest BGP route selection parameter. Remember the BGP route selection rules: –Highest weight preferred (local to router) –Highest local preference preferred (global within AS) –Other BGP route selection rules Weights configured on a router override local preference settings. To ensure consistent AS-wide route selection: –Do not change local preference within the AS. –Do not use BGP weights.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Local Preference (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Default Local Preference bgp default local-preference preference router(config-router)# This command changes the default local preference value. The specified value is applied to all routes that do not have local preference set (EBGP routes). The default value of this parameter is 100, allowing you to specify more desirable or less desirable routers.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Default Local Preference (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Local Preference with Route-Maps route-map name permit sequence match condition set local-preference value route-map name permit sequence match condition set local-preference value router(config)# Changes BGP local preference only for routes matched by the route-map entry neighbor address route-map name in | out router(config-router)# Applies route-map to incoming updates from specified neighbor or outgoing updates to specified neighbor Per-neighbor local preference configured by using a route-map with no match condition

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring Local Preference with Route-Maps (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Local Preference Nondefault local preference is displayed in the show ip bgp printout. Local preference is displayed in the show ip bgp prefix printout. Local preference is displayed in BGP update debugging (only for inbound updates, starting with Cisco IOS Software Release 12.0).

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Local Preference (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Local Preference (Cont.) Nondefault local preference is displayed in the show ip bgp printout.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Local Preference (Cont.) All values for local preference are displayed in the show ip bgp prefix printout.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Monitoring Local Preference (Cont.) Local preference received in a BGP update is displayed in debugging outputs in Cisco IOS Software Release 12.0 and later.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Summary Local preference is similar to the weight attribute in that you can use both to influence BGP path selection, but it differs from the BGP weight attribute in that weight is local to the specific router on which it is configured. You can use local preference to ensure AS-wide route selection policy because it can be seen on neighboring routers without the need to reset it. You should avoid mixing weight and local preference because weight has priority when you are selecting the best path. Local preference can be configured using either the bgp default local-preference preference command or with route-map statements. You can display local preference with the show ip bgp or show ip bgp prefix commands. The former displays only nondefault local preference settings.

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