ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1837 –
separated by one colon. The 2-byte network number can range
from 0 to 65535.
One or more community numbers can be entered, separated by
a space. Up to 3 community numbers are supported.
100-500 – Expanded community list number that identifies one or
more groups of communities.
expanded community-list-name – Name of expanded access list.
(Maximum length: 32 characters, no spaces or other special
characters)
regular-expression – Regular expression indicating the community
list number or name. Syntax complies with the IEEE POSIX Basic
Regular Expressions (BRE) standard.
COMMAND MODE
Global Configuration
DEFAULT SETTING
No extended community lists are defined.
COMMAND USAGE
◆ Standard community lists are used to configure well-known
communities or community numbers. Expanded community lists are
used to filter communities using a regular expression.
◆ When multiple values are entered in the same community list, they
form a logical AND condition. When multiple values are configured in
separate community lists, the form a logical OR condition, where the
first list that matches a condition is processed.
◆ If the criteria specified for a community list is matched, then the
deny/permit condition is applied to the routing message.
◆ If a permit value is applied to a community list, the filter will implicitly
deny other community values.
◆ The route target (RT) attribute is used to identify sites that may receive
routes tagged with a specific route target. Using this attribute allows
that route to be placed in per-site forwarding tables used for routing
traffic received from the corresponding sites.
◆ The site of origin (SOO) attribute is used to identify the site from which
the provider edge (PE) router learned the route. All routes learned from
a particular site are assigned the same site of origin attribute, no
matter if a site is connected to a single PE router or multiple PE routers.
Filtering based on this extended community attribute can prevent
routing loops from occurring when a site is multi-homed.
◆ Use this command in conjunction with the neighbor filter-list to filter
route updates sent to or received from a neighbor, or with the match
extcommunity route map command to implement a more
comprehensive filter for policy-based routing.