Default BGP communities sorting or selection criteria used by Juniper

By default, BGP communities are sorted in order: Normal-community < Extended-community < Large-community...

All community types are internally sorted using a comparison function.

The comparison follows a step-by-step approach to ensure consistent ordering. The lowest value is preferred over the highest:

• Community type is compared first

            Communities are grouped based on their type (normal, extended, IPv6 extended, or large) to keep similar types together.

• Extended community subtype is compared next

            For extended communities, the subtype is used to maintain consistent ordering within those groups.

• First value is then compared.
  • For IPv6 extended communities, the IPv6 address is compared.
  • For all other community types, the AS number or address value is compared.
• Second value is compared next.
  
  • The primary value of the community is used to further differentiate entries.
• Third value is compared for large communities.

Communities that include IP addresses may also appear in a different order:

Big-endian* [which stores the most significant byte (the "big end") first ]

For instance - Big-endian sorting with respect to rt-import extended communities:

user@ASBR2_RE# run show route advertising-protocol bgp 10.2.2.2 extensive 10.1.11.1 | match communit 

   Communities: target:64514:64515 src-as:64514:0 src-as:64515:0 rt-import:172.16.20.0:8 rt-import:172.31.11.11:8 --> Starting from most significant bit, lowest value is 16 over 31 [as 1st octet is the same, i.e. 172, contention moves to 2nd octet], so rt-import:172.16.20.0:8 takes precedence over rt-import:172.31.11.11:8

Little-endian* [which stores the least significant byte (the "little end") first]:

For instance - Little-endian sorting with respect to rt-import extended communities:

user@ASBR2_RE# run show route advertising-protocol bgp 10.2.2.2 extensive 10.1.11.1 | match communit 

   Communities: target:64514:64515 src-as:64514:0 src-as:64515:0 rt-import:172.31.20.0:8 rt-import:172.31.11.11:8 --> Starting from least significant bit, lowest value is 0 over 11, so rt-import:172.31.20.0:8 takes precedence over rt-import:172.31.11.11:8

*Endianness refers to the order in which bytes are arranged in system memory. Big/little-endian sorting selection varies across platforms/vendors, but is consistent within the same model