The Cisco BGP router must be configured to reject route advertisements from BGP peers that do not list their autonomous system (AS) number as the first AS in the AS

The Cisco BGP router must be configured to reject route advertisements from BGP peers that do not list their autonomous system (AS) number as the first AS in the AS_PATH attribute.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-96771	CISC-RT-000540	SV-105909r1_rule		Low

Description
Verifying the path a route has traversed will ensure the IP core is not used as a transit network for unauthorized or possibly even Internet traffic. All autonomous system boundary routers (ASBRs) must ensure updates received from eBGP peers list their AS number as the first AS in the AS_PATH attribute.

STIG	Date
Cisco IOS XR Router RTR Security Technical Implementation Guide	2020-06-30

Details

Check Text ( C-95607r1_chk )
Review the router configuration to verify the router is configured to deny updates received from eBGP peers that do not list their AS number as the first AS in the AS_PATH attribute. By default Cisco IOS enforces the first AS in the AS_PATH attribute for all route advertisements. Review the router configuration to verify that the command bgp enforce-first-as disable is not configured as shown in the example below. router bgp nn bgp enforce-first-as disable If the router is not configured to reject updates from peers that do not list their AS number as the first AS in the AS_PATH attribute, this is a finding.

Check Text ( C-95607r1_chk )

Review the router configuration to verify the router is configured to deny updates received from eBGP peers that do not list their AS number as the first AS in the AS_PATH attribute.

By default Cisco IOS enforces the first AS in the AS_PATH attribute for all route advertisements. Review the router configuration to verify that the command bgp enforce-first-as disable is not configured as shown in the example below.

router bgp nn
bgp enforce-first-as disable

If the router is not configured to reject updates from peers that do not list their AS number as the first AS in the AS_PATH attribute, this is a finding.

Fix Text (F-102449r1_fix)
Configure the router to deny updates received from eBGP peers that do not list their AS number as the first AS in the AS_PATH attribute. RP/0/0/CPU0:R2(config)#router bgp 2 RP/0/0/CPU0:R2(config-bgp)#no bgp enforce-first-as disable