Dening and applying IP prex lists
An IP prex list species a list of networks. When you apply an IP prex list to a neighbor, the device sends or receives only a route
whose destination is in the IP prex list. The software interprets the prex lists in order, beginning with the lowest sequence number.
To congure an IP prex list and apply it to a neighbor, enter commands such as the following.
device(config)# ip prefix-list Routesfor20 permit 10.20.0.0/24
device(config)# router bgp
device(config-bgp-router)# neighbor 10.10.10.1 prefix-list Routesfor20 out
These commands congure an IP prex list named Routesfor20, which permits routes to network 10.20.0.0/24. The neighbor
command congures the device to use IP prex list Routesfor20 to determine which routes to send to neighbor 10.10.10.1. The device
sends routes that go to 10.20.x.x to neighbor 10.10.10.1 because the IP prex list explicitly permits these routes to be sent to the
neighbor.
Syntax: [no] ip prex-list name [ seq seq-value ] [ description string ] deny | permit network-addr / mask-bits [ ge ge-value ] [ le le-
value ]
The name parameter species the prex list name. Use this name when applying the prex list to a neighbor.
The description string parameter is a text string describing the prex list.
The seq seq-value parameter is optional and species the sequence number of the IP prex list. If you do not specify a sequence
number, the software numbers the entries in increments of 5, beginning with prex list entry 5. The software interprets the prex list
entries in numerical order, beginning with the lowest sequence number.
The deny and permit parameters specify the action the software takes if a neighbor route is in this prex list.
The network-addr and mask-bits parameters specify the network number and the number of bits in the network mask.
You can specify a range of prex length for prexes that are more specic than network-addr and mask-bits .
The prex-list matches only on this network unless you use the ge ge-value or le le-value parameters.
• If you specify only ge ge-value, the mask-length range is from ge-value to 81.
• If you specify only le le-value, the mask-length range is from length to le-value .
The ge-value or le-value you specify must meet the following condition:
length < ge-value <= le-value <= 81
If you do not specify ge ge-value or le le-value , the prex list matches only on the exact network prex you specied with the network-
addr and mask-bits parameters.
In the following example, only default routes are allowed:
device(config)# ip prefix-list match-default-routes permit 0.0.0.0/0
In the following example, only default routes are denied:
device(config)# ip prefix-list match-default-routes deny 0.0.0.0/0
In the following example, all routes are allowed, including all subnet masks and all prexes:
device(config)# ip prefix-list match-all-routes permit 0.0.0.0/0 le 32
NOTE
Be careful to determine exactly which routes you want to allow using a prex
list.
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FastIron Ethernet Switch Layer 3 Routing
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