The Cisco BGP switch must be configured to reject inbound route advertisements from a customer edge (CE) switch for prefixes that are not allocated to that customer.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221025	CISC-RT-000510	SV-221025r622190_rule		Medium

Description
As a best practice, a service provider should only accept customer prefixes that have been assigned to that customer and any peering autonomous systems. A multi-homed customer with BGP speaking switches connected to the Internet or other external networks could be breached and used to launch a prefix de-aggregation attack. Without ingress route filtering of customers, the effectiveness of such an attack could impact the entire IP core and its customers.

Description

As a best practice, a service provider should only accept customer prefixes that have been assigned to that customer and any peering autonomous systems. A multi-homed customer with BGP speaking switches connected to the Internet or other external networks could be breached and used to launch a prefix de-aggregation attack. Without ingress route filtering of customers, the effectiveness of such an attack could impact the entire IP core and its customers.

STIG	Date
Cisco IOS-XE Switch RTR Security Technical Implementation Guide	2021-03-26

Details

Check Text ( C-22740r408869_chk )
Review the switch configuration to verify that there are ACLs defined to only accept routes for prefixes that belong to specific customers. Step 1: Verify prefix list has been configured for each customer containing prefixes belonging to each customer as shown in the example below: ip prefix-list PREFIX_FILTER_CUST1 seq 5 permit x.13.1.0/24 le 32 ip prefix-list PREFIX_FILTER_CUST1 seq 10 deny 0.0.0.0/0 ge 8 ip prefix-list PREFIX_FILTER_CUST2 seq 5 permit x.13.2.0/24 le 32 ip prefix-list PREFIX_FILTER_CUST2 seq 10 deny 0.0.0.0/0 ge 8 Step 2: Verify that the prefix lists has been applied to all to the applicable CE peers as shown in the example below: router bgp xx no synchronization bgp log-neighbor-changes neighbor x.12.4.14 remote-as 64514 neighbor x.12.4.14 prefix-list FILTER_PREFIXES_CUST1 in neighbor x.12.4.16 remote-as 64516 neighbor x.12.4.16 prefix-list FILTER_PREFIXES_CUST2 in Note: Routes to PE-CE links within a VPN are needed for troubleshooting end-to-end connectivity across the MPLS/IP backbone. Hence, these prefixes are an exception to this requirement. If the switch is not configured to reject inbound route advertisements from each CE switch for prefixes that are not allocated to that customer, this is a finding.

Check Text ( C-22740r408869_chk )

Review the switch configuration to verify that there are ACLs defined to only accept routes for prefixes that belong to specific customers.

Step 1: Verify prefix list has been configured for each customer containing prefixes belonging to each customer as shown in the example below:

ip prefix-list PREFIX_FILTER_CUST1 seq 5 permit x.13.1.0/24 le 32
ip prefix-list PREFIX_FILTER_CUST1 seq 10 deny 0.0.0.0/0 ge 8
ip prefix-list PREFIX_FILTER_CUST2 seq 5 permit x.13.2.0/24 le 32
ip prefix-list PREFIX_FILTER_CUST2 seq 10 deny 0.0.0.0/0 ge 8

Step 2: Verify that the prefix lists has been applied to all to the applicable CE peers as shown in the example below:

router bgp xx
no synchronization
bgp log-neighbor-changes
neighbor x.12.4.14 remote-as 64514
neighbor x.12.4.14 prefix-list FILTER_PREFIXES_CUST1 in
neighbor x.12.4.16 remote-as 64516
neighbor x.12.4.16 prefix-list FILTER_PREFIXES_CUST2 in

Note: Routes to PE-CE links within a VPN are needed for troubleshooting end-to-end connectivity across the MPLS/IP backbone. Hence, these prefixes are an exception to this requirement.

If the switch is not configured to reject inbound route advertisements from each CE switch for prefixes that are not allocated to that customer, this is a finding.

Fix Text (F-22729r408870_fix)
Configure the switch to reject inbound route advertisements from each CE switch for prefixes that are not allocated to that customer. Step 1: Configure a prefix list for each customer containing prefixes belonging to each. SW1(config)#ip prefix-list PREFIX_FILTER_CUST1 permit x.13.1.0/24 le 32 SW1(config)#ip prefix-list PREFIX_FILTER_CUST1 deny 0.0.0.0/0 ge 8 SW1(config)#ip prefix-list PREFIX_FILTER_CUST2 permit x.13.2.0/24 le 32 SW1(config)#ip prefix-list PREFIX_FILTER_CUST2 deny 0.0.0.0/0 ge 8 Step 2: Apply the prefix list filter inbound to each CE neighbor as shown in the example. SW1(config)#router bgp xx SW1(config-switch)#neighbor x.12.4.14 prefix-list FILTER_PREFIXES_CUST1 in SW1(config-switch)#neighbor x.12.4.16 prefix-list FILTER_PREFIXES_CUST2 in

Fix Text (F-22729r408870_fix)

Configure the switch to reject inbound route advertisements from each CE switch for prefixes that are not allocated to that customer.

Step 1: Configure a prefix list for each customer containing prefixes belonging to each.

SW1(config)#ip prefix-list PREFIX_FILTER_CUST1 permit x.13.1.0/24 le 32
SW1(config)#ip prefix-list PREFIX_FILTER_CUST1 deny 0.0.0.0/0 ge 8
SW1(config)#ip prefix-list PREFIX_FILTER_CUST2 permit x.13.2.0/24 le 32
SW1(config)#ip prefix-list PREFIX_FILTER_CUST2 deny 0.0.0.0/0 ge 8

Step 2: Apply the prefix list filter inbound to each CE neighbor as shown in the example.

SW1(config)#router bgp xx
SW1(config-switch)#neighbor x.12.4.14 prefix-list FILTER_PREFIXES_CUST1 in
SW1(config-switch)#neighbor x.12.4.16 prefix-list FILTER_PREFIXES_CUST2 in