Technical Tip: Create an Access-list on a Route-Map that would deny specific network on a BGP peering

[acvaldez]

Description

This article describes how to configure an access-list on a route-map that would deny specific routes on BGP.

Scope

Solution

This example will show how to create a route-map-in on FortiGate2 that has an access-list rule that would deny 10.10.30.0/24 from FortiGate1.

Configuration.

FortiGate1 BGP GUI configuration:

FortiGate2 BGP GUI configuration:

FortiGate2 CLI configuration to deny 10.10.30.0/24 network and allow any other network.

Access-list:

show

config router access-list
    edit "BLOCK_10.10.30.0/24"
        config rule
            edit 1
                set prefix 10.10.30.0 255.255.255.0
                set exact-match enable
            next
        end
    next
    edit "ALLOW_OTHER"
        config rule
            edit 1
                set prefix any
                set exact-match enable
            next
        end
    next
end

Route-map:

config router router-map

    show

    config route route-map

        edit "BLOCK_10.10.30.0/24"

            config rule

                edit 1

                    set action deny

                    set match-ip-address "BLOCK_10.10.30.0/24"

                next

                edit 2

                    set match-ip-address "ALLOW_OTHER"

                next

            end

Note that if the second access-list named 'ALLOW_OTHER' with the prefix set to 'any' is not added in route-map config, none of the subnets will be allowed even though they are not explicitly mentioned with the action set to 'deny'. There should be an entry with the action set to 'permit' to be able to receive/advertise the desired subnet(s). In the above configuration, 'set action permit' is not visible, because it is the default configuration of route-map entry.

Assign that route-map to the route-map-in of the FortiGate2 neighbor configuration:

config router bgp

        config neighbor

            edit 192.168.175.230

    end

show

    config router bgp

        set as 20

        set router-id 192.168.175.231

            config neighbor

                edit "192.168.175.230"

                    set remote-as 10

                    set route-map-in "BLOCK_10.10.30.0/24"

                next

            end

            config redistribute "connected"

            end

            config redistribute "rip"

            end

            config redistribute "ospf"

            end

            config redistribute "static"

            end

            config redistribute "isis"

            end

            config redistribute "connected"

            end

            config redistribute "rip"

            end

            config redistribute "ospf"

            end

            config redistribute "static"

            end

            config redistribute "isis"

            end

end

Clear the BGP peering using this command for the route-map access-list to take effect:

exec router clear bgp as 10        <------------ because the AS on FortiGate1 is set to 10.

Or:

exec router clear bgp all

Troubleshooting.
To confirm that it is already being blocked, run these commands:

diag ip router bgp all en
diag ip router bgp level info
diag debug en

Note: to stop the debugging of the BGP, run these commands:

diag debug disable
diag ip router bgp all disable
diag ip router bgp level none

Here is the sample result:

BGP: [NETWORK] Accept Thread: Incoming conn from host 192.168.175.230 (FD=24)
BGP: 192.168.175.230-Outgoing [FSM] State: Idle Event: 14
BGP: 192.168.175.230-Outgoing [FSM] State: Idle Event: 3
BGP: 192.168.175.230-Outgoing [NETWORK] FD=24, Sock Status: 0-Success
BGP: 192.168.175.230-Outgoing [FSM] State: Connect Event: 17
BGP: 192.168.175.230-Outgoing [ENCODE] Msg-Hdr: Type 1
BGP: 192.168.175.230-Outgoing [ENCODE] Open: Ver 4 MyAS 20 Holdtime 180
BGP: 192.168.175.230-Outgoing [ENCODE] Open: Msg-Size 61
BGP: 192.168.175.230-Outgoing [DECODE] Msg-Hdr: type 1, length 61
BGP: 192.168.175.230-Outgoing [DECODE] Open: Optional param len 32
BGP: 192.168.175.230-Outgoing [DECODE] Open Opt: Option Type 2, Option Len 6
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: Cap Code 1, Cap Len 4
BGP: 192.168.175.230-Outgoing [DECODE] Open Opt: Option Type 2, Option Len 6
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: Cap Code 1, Cap Len 4
BGP: 192.168.175.230-Outgoing [DECODE] Open Opt: Option Type 2, Option Len 2
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: Cap Code 128, Cap Len 0
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: RR Cap(old) for all address-families
BGP: 192.168.175.230-Outgoing [DECODE] Open Opt: Option Type 2, Option Len 2
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: Cap Code 2, Cap Len 0
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: RR Cap(new) for all address-families
BGP: 192.168.175.230-Outgoing [DECODE] Open Opt: Option Type 2, Option Len 6
BGP: 192.168.175.230-Outgoing [DECODE] Open Cap: Cap Code 65, Cap Len 4
BGP: 192.168.175.230-Outgoing [FSM] State: OpenSent Event: 19
BGP: 192.168.175.230-Outgoing [ENCODE] Msg-Hdr: Type 4
BGP: 192.168.175.230-Outgoing [ENCODE] Keepalive: 28 KAlive msg(s) sent
BGP: bgp_keepalive_proc: notif_rcv 4-4
BGP: 192.168.175.230-Outgoing [DECODE] Msg-Hdr: type 4, length 19
BGP: 192.168.175.230-Outgoing [DECODE] KAlive: Received!
BGP: 192.168.175.230-Outgoing [FSM] State: OpenConfirm Event: 26
id=20300 logdesc="BGP neighbor status changed" msg="BGP: %BGP-5-ADJCHANGE: neighbor 192.168.175.230 Up "
BGP: 192.168.175.230-Outgoing [DECODE] Msg-Hdr: type 2, length 55
BGP: 192.168.175.230-Outgoing [DECODE] Update: Starting UPDATE decoding... Bytes To Read (36), msg_size (36)
BGP: 192.168.175.230-Outgoing [DECODE] Update: NLRI Len(12)
BGP: 192.168.175.230-Outgoing [FSM] State: Established Event: 27
BGP: 192.168.175.230-Outgoing [RIB] Update: Received Prefix 10.10.10.0/24
BGP: 192.168.175.230-Outgoing [RIB] Update: Prefix 10.10.30.0/24 denied due to route-map
BGP: 192.168.175.230-Outgoing [RIB] Update: Received Prefix 10.10.20.0/24
BGP: 192.168.175.230-Outgoing [FSM] State: Established Event: 34
BGP: [RIB] Scanning BGP Network Routes...

To make sure that the route 10.10.30.0/24 is not installed on the routing table of FortiGate2, run this command:

get router info routing-table all

Here is the result of this lab:

get router info routing-table all

Routing table for VRF=0
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
       O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default

B       10.10.10.0/24 [20/0] via 192.168.175.230, port1, 00:01:19
B       10.10.20.0/24 [20/0] via 192.168.175.230, port1, 00:01:19
C       192.168.175.0/24 is directly connected, port1