Filtering routes in the Border Gateway Protocol (BGP) allows network administrators to control the routes advertised or received from BGP peers. Here is a step-by-step guide with commands to filter routes in different ways in the BGP protocol

Step 1: Access the BGP Configuration Mode:

Access the configuration mode for BGP on your router. The specific command to enter this mode may vary depending on the router platform. For Cisco devices, the command is typically:

# configure terminal
# router bgp [AS-number]

Replace [AS-number] with the Autonomous System (AS) number of your network.

Step 2: Filtering Based on Prefix:

To filter routes based on the IP prefix, you can use prefix-list or access-list. Here’s an example using prefix-list:

# ip prefix-list [list-name] seq [sequence-number] permit [prefix]

Replace [list-name] with a descriptive name for the prefix-list, [sequence-number] with a numerical value to determine the order of the entry, and [prefix] with the IP prefix you want to permit.

To apply the prefix-list in the BGP configuration:

# address-family ipv4
# neighbor [IP-address] prefix-list [list-name] out

Step 3: Filtering Based on AS Path:

To filter routes based on the AS path attribute, you can use the as-path access-list. Here’s an example:

# ip as-path access-list [list-number] permit [AS-path-regex]

Replace [list-number] with a numerical value to identify the as-path access-list, and [AS-path-regex] with the regular expression to match the desired AS path.

To apply the as-path access-list in the BGP configuration:

# address-family ipv4
# neighbor [IP-address] filter-list [list-number] out

Replace [IP-address] with the IP address of the BGP neighbor you want to apply the as-path access-list to.

Step 4: Filtering Based on Community:

To filter routes based on BGP communities, you can use community-list or community-based route maps. Here’s an example using community-list:

# ip community-list [list-number] permit [community-value]

Replace [list-number] with a numerical value to identify the community-list, and [community-value] with the specific BGP community value to permit.

To apply the community-list in the BGP configuration:

# address-family ipv4
# neighbor [IP-address] route-map [route-map-name] out

Replace [IP-address] with the IP address of the BGP neighbor, and [route-map-name] with the name of the route map that contains the community-list.

Step 5: Filtering Based on Route Origin:

To filter routes based on the route origin attribute, you can use the origin access-list. Here’s an example:

# ip as-path access-list [list-number] permit [origin-regex]

Replace [list-number] with a numerical value to identify the origin access-list, and [origin-regex] with the regular expression to match the desired route origin.

To apply the origin access-list in the BGP configuration:

# address-family ipv4
# neighbor [IP-address] filter-list [list-number] out

Replace [IP-address] with the IP address of the BGP neighbor you want to apply the origin access-list to.

Step 6: Commit Configuration:

Save and apply the BGP configuration changes:

# end
# copy running-config startup-config