Remove Private BGP AS Numbers from the AS Path

Some end customers use private BGP AS numbers when running BGP with their Internet Service Providers (ISPs). Those AS numbers should not appear in the BGP AS path when the ISP advertises those prefixes to its peers and upstream providers.

In this lab exercise, you’ll use the remove private AS feature available in many BGP implementations to remove the customer’s private BGP AS number from the BGP AS path sent in EBGP updates.

Existing BGP Configuration

The routers in your lab use the following BGP AS numbers. Each router advertises an IPv4 prefix.

Node/ASN	Router ID	Advertised prefixes
AS64500
rtr	10.0.0.1	10.0.0.1/32
AS64507
x2	10.0.0.11	10.0.0.11/32
AS65000
x1	10.0.0.10	192.168.42.0/24

Your router has these EBGP neighbors. netlab configures them automatically; if you’re using some other lab infrastructure, you’ll have to configure EBGP neighbors and advertised prefixes manually.

Node	Neighbor	Neighbor AS	Neighbor IPv4
rtr	x1	65000	10.1.0.2
	x2	64507	10.1.0.6

Start the Lab

Assuming you already set up your lab infrastructure:

• Change directory to session/4-removeprivate
• Execute netlab up (device requirements, other options)
• Log into your router (RTR) with the netlab connect rtr command and verify that the IP addresses and the EBGP sessions are properly configured.

The Problem

Log into X2 and check its BGP table. You’ll notice that the AS path for the prefix 192.168.42.0/24 contains a private AS number¹ 65000 that should not be visible outside your autonomous system. For example, you would get the following printout when running Cumulus Linux on X2:²

$ netlab connect x2 -q --show ip bgp
BGP table version is 3, local router ID is 10.0.0.11, vrf id 0
Default local pref 100, local AS 64507
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.0.0.1/32      10.1.0.5                               0 64500 i
*> 10.0.0.11/32     0.0.0.0                  0         32768 i
*> 192.168.42.0/24  10.1.0.5                               0 64500 65000 i

Displayed  3 routes and 3 total paths

You must change your router’s configuration to make AS 65000 disappear from the AS path.

Configuration Tasks

Most BGP implementations have a nerd knob that removes private AS numbers from the AS path. It’s usually configured with a command similar to neighbor remove-private-as.

• Configure removal of private AS numbers on the EBGP session between rtr and x2.

Verification

You can use the netlab validate command if you’ve installed netlab release 1.8.3 or later and use Cumulus Linux, FRR, or Arista EOS on the external routers. The validation tests check:

• The state of the EBGP session between RTR and X1/X2
• Whether RTR sends the prefix received from X1 to X2
• Whether RTR removes private AS numbers (AS 65000) from the AS path before sending the updates to X2.

For example, this is the result you’d get if you forgot to remove the private AS numbers from the updates sent to X2:

If the netlab validate command fails or you’re using another network operating system on the ISP routers, do manual validation.

Check the BGP table on X2. None of the AS paths should contain private AS numbers. This is the printout you should get on Cumulus Linux:

$ netlab connect x2 -q --show ip bgp
BGP table version is 4, local router ID is 10.0.0.11, vrf id 0
Default local pref 100, local AS 64507
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.0.0.1/32      10.1.0.5                               0 64500 i
*> 10.0.0.11/32     0.0.0.0                  0         32768 i
*> 192.168.42.0/24  10.1.0.5                               0 64500 i

Displayed  3 routes and 3 total paths

Reference Information

This lab uses a subset of the 4-router lab topology. The following information might help you if you plan to build custom lab infrastructure:

Device Requirements

• Use any device supported by the netlab BGP configuration module for the customer- and provider routers.
• Git repository contains initial Cumulus Linux device configurations for X1 and X2.

Lab Wiring

Origin Device	Origin Port	Destination Device	Destination Port
rtr	Ethernet1	x1	swp1
rtr	Ethernet2	x2	swp1

Lab Addressing

Node/Interface	IPv4 Address	IPv6 Address	Description
rtr	10.0.0.1/32		Loopback
Ethernet1	10.1.0.1/30		rtr -> x1
Ethernet2	10.1.0.5/30		rtr -> x2
x1	192.168.42.1/24		Loopback
swp1	10.1.0.2/30		x1 -> rtr
x2	10.0.0.11/32		Loopback
swp1	10.1.0.6/30		x2 -> rtr