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-221109	CISC-RT-000550	SV-221109r622190_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 NX-OS Switch RTR Security Technical Implementation Guide	2021-03-29

Details

Check Text ( C-22824r409816_chk )
Review the switch configuration to verify the switch 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. 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 AS_PATH permit ^yy$ ip as-path access-list 10 AS_PATH .* 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 router-id 10.1.1.1 neighbor x.1.12.2 remote-as xx password 3 7b07d1b3023056a9 address-family ipv4 unicast filter-list AS_PATH in If the switch 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-22824r409816_chk )

Review the switch configuration to verify the switch 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.

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

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
router-id 10.1.1.1
neighbor x.1.12.2 remote-as xx
password 3 7b07d1b3023056a9
address-family ipv4 unicast
filter-list AS_PATH in

If the switch 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-22813r409817_fix)
Configure the switch to deny updates received from eBGP peers that do not list their AS number as the first AS in the AS_PATH attribute. Step 1: Configure the as-path ACL as shown in the example below: SW1(config)# ip as-path access-list AS_PATH permit ^22$ SW1(config)# ip as-path access-list AS_PATH deny .* Step 2: Apply the as-path filter inbound as shown in the example below: SW1(config)# router bgp xx SW1(config-router)# neighbor x.1.12.2 SW1(config-router-neighbor)# address-family ipv4 unicast SW1(config-router-neighbor-af)# filter-list AS_PATH in SW1(config-router-neighbor-af)# end

Fix Text (F-22813r409817_fix)

Configure the switch to deny updates received from eBGP peers that do not list their AS number as the first AS in the AS_PATH attribute.

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

SW1(config)# ip as-path access-list AS_PATH permit ^22$
SW1(config)# ip as-path access-list AS_PATH deny .*

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

SW1(config)# router bgp xx
SW1(config-router)# neighbor x.1.12.2
SW1(config-router-neighbor)# address-family ipv4 unicast
SW1(config-router-neighbor-af)# filter-list AS_PATH in
SW1(config-router-neighbor-af)# end