Configuring Bgp Community Filtering - Cisco 3845 - Security Bundle Router Software Manual

Chapter 29 Configuring IP Unicast Routing
•
By default, sequence numbers are generated automatically and incremented in units of five. If you
disable the automatic generation of sequence numbers, you must specify the sequence number for each
entry. You can specify sequence values in any increment. If you specify increments of one, you cannot
insert additional entries into the list; if you choose very large increments, you might run out of values.
You do not need to specify a sequence number when removing a configuration entry. Show commands
include the sequence numbers in their output.
Before using a prefix list in a command, you must set up the prefix list. Beginning in privileged EXEC
mode, follow these steps to create a prefix list or to add an entry to a prefix list:
Command
Step 1
configure terminal
Step 2
ip prefix-list list-name [seq seq-value] deny |
permit network/len [ge ge-value] [le le-value]
Step 3 ip prefix-list list-name seq seq-value deny |
permit network/len [ge ge-value] [le le-value]
Step 4
end
Step 5
show ip prefix list [detail | summary] name
[network/len] [seq seq-num] [longer]
[first-match]
Step 6 copy running-config startup-config
To delete a prefix list and all of its entries, use the no ip prefix-list list-name global configuration
command. To delete an entry from a prefix list, use the no ip prefix-list seq seq-value global
configuration command. To disable automatic generation of sequence numbers, use the no ip prefix-list
sequence number command; to reenable automatic generation, use the ip prefix-list sequence number
command. To clear the hit-count table of prefix list entries, use the clear ip prefix-list privileged EXEC
command.

Configuring BGP Community Filtering

One way that BGP controls the distribution of routing information based on the value of the
COMMUNITIES attribute. A community is a group of destinations that share some common attribute.
Each destination can belong to multiple communities. AS administrators can define to which
communities a destination belongs. By default, all destinations belong to the general Internet
OL-23400-01
When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry
identifies the entry with the lowest sequence number.
Purpose
Enter global configuration mode.
Create a prefix list with an optional sequence number to deny or
permit access for matching conditions. You must enter at least one
permit or deny clause.
network/len is the network number and length (in bits) of the
•
network mask.
• (Optional) ge and le values specify the range of the prefix length
to be matched.The specified ge-value and le-value must satisfy
this condition: len < ge-value < le-value < 32
(Optional) Add an entry to a prefix list, and assign a sequence
number to the entry.
Return to privileged EXEC mode.
Verify the configuration by displaying information about a prefix list
or prefix list entries.
(Optional) Save your entries in the configuration file.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Configuring BGP
29-53