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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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Filtering BGP using IP prefix lists

An IP prefix lists contains a list of networks. When applying an IP prefix list to a BGP neighbor, you can able to send or receive the routes whose destination is in the IP prefix list.

Filtering BGP routes based on IP prefix lists involves the following steps:

  • Create a prefix list. The steps 1 to 3 in the following configuration commands shows how to create a prefix list.

  • Applying the created prefix list to a BGP neighbor. The steps 4 and 5 show how the prefix list is applied to a neighbor.

To configure an IP prefix list, use the following commands.

  1. Create a prefix list and assign it a name.
    CONFIGURATION mode
    ip prefix-list prefix-name
  2. Create multiple prefix list filters with a deny or permit action.
    CONFIG-PREFIX LIST mode
    seq sequence-number {deny | permit} {any | ip-prefix [ge | le] }
    • ge: minimum prefix length to be matched.
    • le: maximum prefix length to me matched.
    For information about configuring prefix lists, refer to Access Control Lists (ACLs).
  3. Return to CONFIGURATION mode.
    CONFIG-PREFIX LIST mode
    exit
  4. Enter ROUTER BGP mode.
    CONFIGURATION mode
    router bgp as-number
  5. Filter routes based on the criteria in the configured prefix list.
    CONFIG-ROUTER-BGP mode
    neighbor {ip-address | ipv6-address | peer-group-name} distribute-list prefix-list-name {in | out}
    Configure the following parameters:
    • ip-address or ipv6-address or peer-group-name: enter the neighbor’s IPv4 or IPv6 address or the peer group’s name.
    • prefix-list-name: enter the name of a configured prefix list.
    • in: apply the prefix list to inbound routes.
    • out: apply the prefix list to outbound routes.
As a reminder, the following are rules concerning prefix lists:
  • If the prefix list contains no filters, all routes are permitted.
  • If none of the routes match any of the filters in the prefix list, the route is denied. This action is called an implicit deny. (If you want to forward all routes that do not match the prefix list criteria, you must configure a prefix list filter to permit all routes. For example, you could have the following filter as the last filter in your prefix list permit 0.0.0.0/0 le 32).
  • After a route matches a filter, the filter’s action is applied. No additional filters are applied to the route.

Example of configuring an IP prefix list and applying it to a BGP neighbor

DellEMC# configure terminal
DellEMC(conf)# ip prefix-list route10 
DellEMC(conf)# seq 10 permit 10.10.10.0/24 ge 32
DellEMC(conf)#exit
DellEMC(conf)# router bgp
DellEMC(conf-router_bgp)#neighbor 10.10.10.2 distribute-list route10 out

The commands show the configuration of an IP prefix list named route10, which permits routes to network 10.10.10.0/24. The neighbor command configures to use IP prefix list route10 to determine, which routes to be distributed to the neighbor 10.10.10.2. So the routes from 10.10.10.1/24 network is distributed to neighbor 10.10.10.2 since the IP prefix list route10 explicitly permits the routes to be distributed to the neighbor.

To view the BGP configuration, use the show config command in ROUTER BGP mode. To view a prefix list configuration, use the show ip prefix-list detail or show ip prefix-list summary commands in EXEC Privilege mode.

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