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The Cisco BGP switch must be configured to reject outbound route advertisements for any prefixes that do not belong to any customers or the local autonomous system (AS).

Overview

Finding IDVersionRule IDIA ControlsSeverity
V-221026CISC-RT-000520SV-221026r622190_ruleCCI-001368medium
Description
Advertisement of routes by an autonomous system for networks that do not belong to any of its customers pulls traffic away from the authorized network. This causes a denial of service (DoS) on the network that allocated the block of addresses and may cause a DoS on the network that is inadvertently advertising it as the originator. It is also possible that a misconfigured or compromised switch within the GIG IP core could redistribute Interior Gateway Protocol (IGP) routes into BGP, thereby leaking internal routes.
STIGDate
Cisco IOS XE Switch RTR Security Technical Implementation Guide2025-05-20

Details

Check Text (C-221026r622190_chk)

Step 1: Verify that a prefix list has been configured containing prefixes belonging to customers as well as the local AS as shown in the example below: ip prefix-list CE_PREFIX_ADVERTISEMENTS seq 5 permit x.13.1.0/24 le 32 ip prefix-list CE_PREFIX_ADVERTISEMENTS seq 10 permit x.13.2.0/24 le 32 ip prefix-list CE_PREFIX_ADVERTISEMENTS seq 15 permit x.13.3.0/24 le 32 ip prefix-list CE_PREFIX_ADVERTISEMENTS seq 20 permit x.13.4.0/24 le 32 … … … ip prefix-list CE_PREFIX_ADVERTISEMENTS seq 80 deny 0.0.0.0/0 ge 8 Step 2: Verify that the prefix lists has been applied to all CE peers as shown in the example below: router bgp 64512 no synchronization bgp log-neighbor-changes neighbor x.12.4.14 remote-as 64514 neighbor x.12.4.14 prefix-list CE_PREFIX_ADVERTISEMENTS out neighbor x.12.4.16 remote-as 64516 neighbor x.12.4.16 prefix-list CE_PREFIX_ADVERTISEMENTS out If the switch is not configured to reject outbound route advertisements that do not belong to any customers or the local AS, this is a finding.

Fix Text (F-22730r408873_fix)

Step 1: Configure a prefix list for containing all customer and local AS prefixes as shown in the example below: SW1(config)#ip prefix-list CE_PREFIX_ADVERTISEMENTS permit x.13.1.0/24 le 32 SW1(config)#ip prefix-list CE_PREFIX_ADVERTISEMENTS permit x.13.2.0/24 le 32 SW1(config)#ip prefix-list CE_PREFIX_ADVERTISEMENTS permit x.13.3.0/24 le 32 SW1(config)#ip prefix-list CE_PREFIX_ADVERTISEMENTS permit x.13.4.0/24 le 32 … … … SW1(config)#ip prefix-list CE_PREFIX_ADVERTISEMENTS deny 0.0.0.0/0 ge 8 Step 2: Apply the prefix list filter outbound to each CE neighbor as shown in the example. SW1(config)#router bgp 64512 SW1(config-switch)#neighbor x.12.4.14 prefix-list CE_PREFIX_ADVERTISEMENTS out SW1(config-switch)#neighbor x.12.4.16 prefix-list CE_PREFIX_ADVERTISEMENTS out