Information Technology Reference
In-Depth Information
Filter Processing Order
When multiple filters are configured per neighbor, each filter is processed in a specific
order, as shown in Figure 4-1. For inbound updates, the filter list is processed first, followed
by the route map. The distribute list or prefix list is processed last. On the outbound side,
the distribute list or prefix list is processed first, and then the prefix list received via
Outbound Route Filtering (ORF), and then the filter list. The route map is processed last.
Figure 4-1
Filter Processing Order
Filter List Route Map Distribute List/Prefix List
Start
Inbound
Distribute List/Prefix List Prefix List via ORF Filter List Route Map
Start
Outbound
An update has to pass through all the filters. One filter does not take precedence over
another. If any filter does not match, the update is not permitted. For example, if an inbound
update is permitted by the filter list and the route map but is denied by the prefix list,
the update is denied. The same rule applies on the outbound side.
When a policy for a neighbor is configured in the
neighbor
command but the policy is not
defined, the following are the default behaviors:
•
For distribute lists and prefix lists, permit any.
•
For filter lists and route maps, deny any.
Conditional Advertisement
BGP by default advertises the permitted best paths in its BGP routing information base
(RIB) to external peers. In certain cases, this might be undesirable. Advertisement of some
routes might depend on the existence and nonexistence of some other routes. In other
words, the advertisement is conditional.
In a multihomed network, some prefixes are to be advertised to one of the providers only if
information from the other provider is missing, such as a failure in the peering session or
partial reachability. The conditional BGP announcements are in addition to the normal
announcements that a BGP router sends to its peers.
A conditional advertisement does not create routes; it only withholds them until the
condition is met. These routes must already be present in the BGP RIB.
NOTE
