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Chapter 9
BGP Scalability and Advanced Features
The ge-value syntax is used to specify the range of the prefix length that is to
be matched for prefixes that are more than the subnet mask identified in the
network/len
syntax. If the range runs from the /len value to 32, then only the
ge
syntax needs to be specified. The le-value syntax is used to specify the
range of the prefix length to be matched, for prefixes that are of higher value
specified in the specific network/len syntax. The le syntax identifies the val-
ues from the len to le-value specified, indicating a range of networks. Both
ge
and le are optional syntaxes and are used only when you need to specify
a range of the prefix that is more specific than that identified in the network/
len
syntax. Just remember this rule: len < ge-value < le-value <= 32.
An exact match is assumed when neither ge nor le is specified, as shown
below:
Cisco3640(config)#ip prefix-list list1 seq 15 permit
172.16.0.0/24
Cisco3640(config)#
The available syntaxes for the ip prefix-list command are shown
below:
ip prefix-list list-name [seq seq-value] {deny | permit}
network/len [ge ge-value] [le le-value]
Now that we have created the prefix list, we need to apply it to BGP using
the neighbor command. Let's look at the syntaxes and then apply the small
prefix list we created above:
neighbor {ip-address | peer-group-name} prefix-list
prefix-listname {in | out}
Now let's apply the access list created above:
RouterA(config)#router bgp 31400
RouterA(config-router)#neighbor 172.16.11.254 remote-as
31400
RouterA(config-router)#neighbor 172.16.12.254 remote-as
31400
RouterA(config-router)#neighbor 172.16.12.254 prefix-list
list1 in
RouterA(config-router)#exit
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