Filter
Top Products
Hierarchy of the Different Filters
Page 8 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
Hierarchy of the Different Filters
For distribute filters (ACLs), path filters, and prefix filters, the order of application is not
important. If an update is denied by any given filter, it is discarded immediately, and is not run
through any of the other filters. If an update is permitted by one filter, it is passed through to
the next filter to be considered. At the end, you end up with the set of updates that all the
filters agree should not be discarded.
However, route maps are applied last, after the other types of filter. This is because route
maps can modify updates, not just accept or discard them.
BGP: Example
This How To Note illustrates each type of filter with an example from a very simple BGP
network. This section describes the basic network configuration. The following sections add
filtering options to it:
"Example: Distribute filters" on page 12
"Example: AS path filters" on page 18
"Example: Prefix filters" on page 23
The network consists of two BGP peers in different Autonomous Systems (AS):
Switch running
AlliedWare Plus
ASN 34567
Switch running
AlliedWare
ASN 34568
port1.0.2, vlan 64:
64.64.64.64/4
port 50, vlan 1:
45.45.45.46/8
port1.0.1, vlan1:
45.45.45.45/24
port 52, vlan 2:
52.52.52.5/8