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Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 35 Configuring IP Unicast Routing
Configuring BGP
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:
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. The attribute is a way to groups destinations into communities and to apply
routing decisions based on the communities. This method simplifies configuration of a BGP speaker to
control distribution of routing information.
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 community. The community is identified by the
COMMUNITIES attribute, an optional, transitive, global attribute in the numerical range from
1 to 4294967200. These are some predefined, well-known communities:
• internet—Advertise this route to the Internet community. All routers belong to it.
• no-export—Do not advertise this route to EBGP peers.
• no-advertise—Do not advertise this route to any peer (internal or external).
• local-as—Do not advertise this route to peers outside the local autonomous system.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip prefix-list list-name [seq seq-value] deny |
permit network/len [ge ge-value] [le le-value]
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
Step 3
ip prefix-list list-name seq seq-value deny |
permit network/len [ge ge-value] [le le-value]
(Optional) Add an entry to a prefix list, and assign a sequence
number to the entry.
Step 4
end Return to privileged EXEC mode.
Step 5
show ip prefix list [detail | summary] name
[network/len] [seq seq-num] [longer]
[first-match]
Verify the configuration by displaying information about a prefix list
or prefix list entries.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.
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