The Cisco BGP switch must be configured to reject route advertisements from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221029	CISC-RT-000550	SV-221029r622190_rule		Low

Description
Verifying the path a route has traversed will ensure that the local AS is not used as a transit network for unauthorized traffic. To ensure that the local AS does not carry any prefixes that do not belong to any customers, all PE switches must be configured to reject routes with an originating AS other than that belonging to the customer.

STIG	Date
Cisco IOS-XE Switch RTR Security Technical Implementation Guide	2022-09-13

Details

Check Text ( C-22744r408881_chk )
Review the switch configuration to verify the switch is configured to deny updates received from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer. Step 1: Review switch configuration and verify that there is an as-path access-list statement defined to only accept routes from a CE switch whose AS did not originate the route. The configuration should look similar to the following: ip as-path access-list 10 permit ^yy$ ip as-path access-list 10 deny .* Note: The characters “^” and “$” representing the beginning and the end of the expression respectively are optional and are implicitly defined if omitted. Step 2: Verify that the as-path access-list is referenced by the filter-list inbound for the appropriate BGP neighbors as shown in the example below: router bgp xx neighbor x.1.4.12 remote-as yy neighbor x.1.4.12 filter-list 10 in If the switch is not configured to reject updates from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer, this is a finding.

Check Text ( C-22744r408881_chk )

Review the switch configuration to verify the switch is configured to deny updates received from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer.

Step 1: Review switch configuration and verify that there is an as-path access-list statement defined to only accept routes from a CE switch whose AS did not originate the route. The configuration should look similar to the following:

ip as-path access-list 10 permit ^yy$
ip as-path access-list 10 deny .*

Note: The characters “^” and “$” representing the beginning and the end of the expression respectively are optional and are implicitly defined if omitted.

Step 2: Verify that the as-path access-list is referenced by the filter-list inbound for the appropriate BGP neighbors as shown in the example below:

router bgp xx
neighbor x.1.4.12 remote-as yy
neighbor x.1.4.12 filter-list 10 in

If the switch is not configured to reject updates from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer, this is a finding.

Fix Text (F-22733r408882_fix)
Configure the switch to reject updates from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer. Step 1: Configure the as-path ACL as shown in the example below: SW1(config)#ip as-path access-list 10 permit ^yy$ SW1(config)#ip as-path access-list 10 deny .* Step 2: Apply the as-path filter inbound as shown in the example below: SW1(config)#router bgp xx SW1(config-switch)#neighbor x.1.4.12 filter-list 10 in

Fix Text (F-22733r408882_fix)

Configure the switch to reject updates from CE switches with an originating AS in the AS_PATH attribute that does not belong to that customer.

Step 1: Configure the as-path ACL as shown in the example below:

SW1(config)#ip as-path access-list 10 permit ^yy$
SW1(config)#ip as-path access-list 10 deny .*

Step 2: Apply the as-path filter inbound as shown in the example below:

SW1(config)#router bgp xx
SW1(config-switch)#neighbor x.1.4.12 filter-list 10 in