Order of inheritance precedence

When multiple configuration sources apply to a BGP neighbor or group, the effective value is determined in the following order (from highest to lowest precedence):

1. Direct neighbor configuration: Values set directly on the neighbor override all inherited values.

2. Session group or address family group: If not set directly, values from a session group or an address family group override those from a neighbor group.

3. Neighbor group: If not set in the neighbor or higher-level groups, values from a neighbor group (including any values it inherits) are applied.

4. System default: If a value is not configured at any level, the Cisco IOS XR system default is used.

Inheritance rules summary

• Neighbors can inherit configuration from both session groups and neighbor groups, as well as address family groups.

• Neighbor groups can inherit from session groups, address family groups, and other neighbor groups.

• Session groups can inherit from other session groups.

• Address family groups can inherit from other address family groups.

Address family inheritance

• Inheritance rules apply to both address family-independent and address family-dependent configurations.

• If a property can be set in both address family-dependent and independent contexts, the most specific value takes precedence, following the same order.

Examples

Example 1: Direct neighbor configuration takes highest precedence

If you set advertisement-interval both on a neighbor group and directly on a neighbor, the neighbor setting is applied.

Router(config)# router bgp 140
Router(config-bgp)# neighbor-group AS_1
Router(config-bgp-nbrgrp)# advertisement-interval 15
Router(config-bgp-nbrgrp)# exit
Router(config-bgp)# neighbor 10.1.1.1
Router(config-bgp-nbr)# remote-as 1
Router(config-bgp-nbr)# use neighbor-group AS_1
Router(config-bgp-nbr)# advertisement-interval 20

This sample output from the show bgp neighbors command shows that the advertisement interval used is 20 seconds.

  Router# show bgp neighbors 10.1.1.1
  
  BGP neighbor is 10.1.1.1, remote AS 1, local AS 140, external link
   Remote router ID 0.0.0.0
    BGP state = Idle
    Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
    Received 0 messages, 0 notifications, 0 in queue
    Sent 0 messages, 0 notifications, 0 in queue
    Minimum time between advertisement runs is 20 seconds
  
   For Address Family: IPv4 Unicast
    BGP neighbor version 0
    Update group: 0.1
    eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
    Route refresh request: received 0, sent 0
    0 accepted prefixes
    Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
    Threshold for warning message 75%
  
    Connections established 0; dropped 0
    Last reset 00:00:14, due to BGP neighbor initialized
    External BGP neighbor not directly connected.
  

Example 2: Session group or address family group overrides neighbor group

If a value is set both in a session-group (or address family group) and in a neighbor group, but not directly on the neighbor, the session group or address family group value is used.

  Router(config)# router bgp 140
  Router(config-bgp)# session-group AS_2
  Router(config-bgp-sngrp)# advertisement-interval 15
  Router(config-bgp-sngrp)# exit
  Router(config-bgp)# neighbor-group AS_1
  Router(config-bgp-nbrgrp)# advertisement-interval 20
  Router(config-bgp-nbrgrp)# exit
  Router(config-bgp)# neighbor 192.168.0.1
  Router(config-bgp-nbr)# remote-as 1
  Router(config-bgp-nbr)# use session-group AS_2
  Router(config-bgp-nbr)# use neighbor-group AS_1
 

This sample output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds.

  Router# show bgp neighbors 192.168.0.1
  
  BGP neighbor is 192.168.0.1, remote AS 1, local AS 140, external link
   Remote router ID 0.0.0.0
    BGP state = Idle
    Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
    Received 0 messages, 0 notifications, 0 in queue
    Sent 0 messages, 0 notifications, 0 in queue
    Minimum time between advertisement runs is 15 seconds
   For Address Family: IPv4 Unicast
    BGP neighbor version 0
    Update group: 0.1
    eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
    Route refresh request: received 0, sent 0
    0 accepted prefixes
    Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
    Threshold for warning message 75%
    Connections established 0; dropped 0
    Last reset 00:03:23, due to BGP neighbor initialized
    External BGP neighbor not directly connected.
  

Example 3: Neighbor group inherited value is used when higher precedence is absent

If the neighbor uses only a neighbor group (not a session group or address family group), and no value is set directly on the neighbor, the neighbor group value is used, either directly or through its own inheritance.

  Router(config)# router bgp 150
  Router(config-bgp)# session-group AS_2
  Router(config-bgp-sngrp)# advertisement-interval 20
  Router(config-bgp-sngrp)# exit
  Router(config-bgp)# neighbor-group AS_1
  Router(config-bgp-nbrgrp)# advertisement-interval 15
  Router(config-bgp-nbrgrp)# exit
  Router(config-bgp)# neighbor 192.168.1.1
  Router(config-bgp-nbr)# remote-as 1
  Router(config-bgp-nbr)# use neighbor-group AS_1
    

This sample output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds.

   Router# show bgp neighbors 192.168.1.1
  
  BGP neighbor is 192.168.2.2, remote AS 1, local AS 140, external link
   Remote router ID 0.0.0.0
    BGP state = Idle
    Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
    Received 0 messages, 0 notifications, 0 in queue
    Sent 0 messages, 0 notifications, 0 in queue
    Minimum time between advertisement runs is 15 seconds
  
   For Address Family: IPv4 Unicast
    BGP neighbor version 0
    Update group: 0.1
    eBGP neighbor with no outbound policy; defaults to 'drop'
    Route refresh request: received 0, sent 0
    Inbound path policy configured
    Policy for incoming advertisements is POLICY_1
    0 accepted prefixes
    Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
    Threshold for warning message 75%
  
    Connections established 0; dropped 0
    Last reset 00:01:14, due to BGP neighbor initialized
    External BGP neighbor not directly connected.
  

Example 4: Default value is applied if not configured or inherited

If the advertisement-interval is not configured directly or inherited, the system default (30 seconds) applies.

Additional notes

• The same inheritance and precedence rules apply when groups inherit values from other groups.

• For any inheritable property, always refer to the effective configuration displayed by the show bgp neighbors command to verify which value is ultimately used.

Result

These rules ensure that BGP neighbor and group configurations in Cisco IOS XR software are inherited and applied in a predictable order, allowing flexible and scalable BGP policy management.