Troubleshooting guide

Additional BGP Configuration

deny <network ipv4 address/length> Denies access to entries matching the specified network IPv4

address and the corresponding network prefix length (for example, 10.10.10.0/24).

le <value> Specifies the upper end of the range. Range is 0 to 32.

ge <value> Specifies the lower end of the range. Range is 0 to 32.

Routes to subnets within the larger network can be permitted or denied by specifying the permitted range

of prefix lengths. For example, the filter could allow all routes to subnets in the 10.1.0.0 /16 network with

a prefix length up to and including 24:

(config)#ip prefix-list TEST seq 5 permit 10.1.0.0/16 ge 24 le 24

A filter that exactly matches a prefix length can be created by entering the length for both the ge and le

values. For example, the filter could allow any routes to a /24 subnet in the 10.1.0.0 /16 range, but not

accept a route to the entire 10.1.0.0 /16 network:

(config)#ip prefix-list TEST seq 5 permit 10.1.0.0/16 ge 24 le 24

After a prefix list has been defined, use the prefix-list <name> command to assign the prefix list to a BGP

neighbor and specify whether the list will be used to filter inbound or outbound routes.

If the IPv4 network address is entered without specifying a range for prefix lengths, the

router assumes that the route must be an exact match. For example, if the command ip

prefix-list TEST seq 5 permit 10.1.0.0/16 is entered, the BGP interface will only accept

routes to the entire 10.1.0.0 /16 subnet. It will not accept routes to a network, such as

10.1.1.0/ 24, which was subdivided from the /16 network.

The ge keyword indicates that the length must be greater than or equal to that specified in

order to match. The le keyword indicates that the length must be less than or equal to that

specified in order to match. If ge is only specified, the router assumes 32 as the upper limit.

If le is only specified, the router assumes the IPv4 network address’s length as the lower

limit.

Table 2. Common Prefix List Examples

Action Example Prefix List Command

Deny all routes ip prefix-list ALL seq 10 deny 0.0.0.0/0 le 32

Allow default route ip prefix-list DEFAULT seq 10 permit 0.0.0.0/0

Deny default route, but

allow everything else

ip prefix-list DEFAULT seq 10 deny 0.0.0.0/0

ip prefix-list DEFAULT seq 20 permit 0.0.0.0/le 32

Only allow RFC 1918

private addresses

ip prefix-list PRIVATE seq 10 permit 10.0.0.0/8 le 32

ip prefix-list PRIVATE seq 20 permit 172.16.0.0/12 le 32

ip prefix-list PRIVATE seq 30 permit 192.168.0.0/16 le 32

ip prefix-list PRIVATE seq 40 deny 0.0.0.0/0 le 32

Only allow 192.168.x.0/24

summarized route

ip prefix-list REMOTES seq 10 permit 192.168.0.0/16 ge 24 le 24