16-8 Riverstone Networks RS Switch Router User Guide Release 8.0
Basic BGP Tasks
BGP Configuration Guide
•
To prevent looping of routing announcements within the confederation, the AS-path attribute uses
two new path segment types: as-confed-set and as-confed-sequence are similar to the as-set and
as-sequence attributes, except they are only used
within
a confederation.
The confederation structure is hidden whenever an EBGP session takes place between a router in a sub-AS and a router
outside the confederation. In
Figure 16-1
, when R1 advertises a route to R2, R1 strips any as-confed-sequence and
as-confed-set path segments from the as-path attribute and adds AS 64801 to the as-path attribute. When R1 learns a
route from R2 and advertises the route via EIBGP to a router in another member AS, R1 adds the as-confed-sequence
path segment to the as-path attribute and includes its sub-AS number (102) in the new path segment.
16.2.9
Creating Community Lists
A community is a group of destinations that share a common property. A community is defined by the administrator
of the autonomous system and can be used as a way of filtering BGP routes.
To create and define community lists on an RS, enter the following commands in Configure mode:
The
<community-string>
is in the form "
<AS-identifier>
:
<community-identifier>.
" The
bgp show routes
command shows communities associated with a BGP route.
Community lists can be specified in route maps. Communities in a BGP route can be compared to the community list.
The communities in the route can either be a superset of the specified communities or they can exactly match the
specified communities. Route map actions can include removing the community attribute from a route, setting the
value of the community attribute, adding communities to the route, or deleting communities from the route. With route
maps, you can also specify keywords for certain well-known communities:
Create a community list.
ip-router policy create community-list
<identifier>
<community-string>
Add communities to a community list.
ip-router policy add community-list
<identifier>
<community-string>
Table 16-1 Keywords for well-known communities
Keyword
Community
Description
no_export
65535:65281
Do not advertise to EBGP peers.
no_advertise
65535:65282
Do not advertise to any peer.
no_export_subconfed
65535:65283
Do not advertise to EBGP peers, including peers inside a BGP
confederation.
