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PowerConnect B-Series FCX Configuration Guide 1035
53-1002266-01
Filtering
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If you enable the software to display IP subnet masks in CIDR format, the mask is saved in the file
in “/<mask-bits>” format. To enable the software to display the CIDR masks, enter the ip
show-subnet-length command at the global CONFIG level of the CLI. You can use the CIDR format to
configure the filter regardless of whether the software is configured to display the masks in CIDR
format.
The <mask> parameter specifies the network mask. If you want the filter to match on all
destination addresses, enter any. The wildcard works the same as described above.
Filtering AS-paths
You can filter updates received from BGP4 neighbors based on the contents of the AS-path list
accompanying the updates. For example, if you want to deny routes that have the AS 4.3.2.1 in the
AS-path from entering the BGP4 route table, you can define a filter to deny such routes.
The Layer 3 Switch provides the following methods for filtering on AS-path information:
• AS-path filters
• AS-path ACLs
NOTE
The Layer 3 Switch cannot actively support AS-path filters and AS-path ACLs at the same time. Use
one method or the other but do not mix methods.
NOTE
Once you define a filter or ACL, the default action for updates that do not match a filter is “deny”. To
change the default action to “permit”, configure the last filter or ACL as “permit any any”.
AS-path filters or AS-path ACLs can be referred to by a BGP neighbor's filter list number as well as
by match statements in a route map.
Defining an AS-path filter
To define AS-path filter 4 to permit AS 2500, enter the following command.
PowerConnect(config-bgp-router)#as-path-filter 4 permit 2500
Syntax: as-path-filter <num> permit | deny <as-path>
The <num> parameter identifies the filter position in the AS-path filter list and can be from 1
through 100. Thus, the AS-path filter list can contain up to 100 filters. The Layer 3 Switch applies
the filters in numerical order, beginning with the lowest-numbered filter. When a filter match is true,
the Layer 3 Switch stops and does not continue applying filters from the list.
NOTE
If the filter is referred to by a route map match statement, the filter is applied in the order in which
the filter is listed in the match statement.
The permit | deny parameter indicates the action the router takes if the filter match is true.
• If you specify permit, the router permits the route into the BGP4 table if the filter match is true.
• If you specify deny, the router denies the route from entering the BGP4 table if the filter match
is true.