The Cisco BGP switch must be configured to check whether a single-hop eBGP peer is directly connected.

As described in RFC 3682, GTSM is designed to protect a switch's IP-based control plane from DoS attacks. Many attacks focused on CPU load and line-card overload can be prevented by implementing GTSM on all Exterior Border Gateway Protocol-speaking switches.

GTSM is based on the fact that the vast majority of control plane peering is established between adjacent switches; that is, the Exterior Border Gateway Protocol peers are either between connecting interfaces or between loopback interfaces. Since TTL spoofing is considered nearly impossible, a mechanism based on an expected TTL value provides a simple and reasonably robust defense from infrastructure attacks based on forged control plane traffic.

Review the BGP configuration to verify that checking whether a single-hop eBGP peer is directly connected. The example below disables this mechanism.

router bgp xx router-id 10.1.1.1 neighbor x.1.12.2 remote-as xx disable-connected-check address-family ipv4 unicast

Note: BGP triggers a connection check automatically for all eBGP peers that are known to be a single hop away, unless this check is disabled with the disable-connected-check command. BGP does not bring up sessions if the check fails.

If the switch is configured to disable checking whether a single-hop eBGP peer is directly connected, this is a finding.

Remove the command that disables checking whether a single-hop eBGP peer is directly connected for all external BGP neighbors as shown in the example below:

SW1(config)# router bgp xx SW1(config-router)# neighbor x.1.12.2 SW1(config-router-neighbor)# no disable-connected-check SW1(config-router-neighbor)# end