Building Secure Layer-2 Data Center Fabric with Cisco Nexus Switches

One of my readers is designing a layer-2-only data center fabric (no SVI interfaces on switches) with stringent security requirements using Cisco Nexus switches, and he wondered whether a host connected to such a fabric could attack a switch, and whether it would be possible to reach the management network in that way.

Do you think it’s possible to reach the MANAGEMENT PLANE from the DATA PLANE? Is it valid to think that there is a potential attack vector that someone can compromise to source traffic from the front of the device (ASIC) through the PCI bus across the CPU to the across the PCI bus to the Platform Controller Hub through the I/O card to spew out the Management Port onto that out-of-band network?

My initial answer was “of course there’s always a conduit from the switching ASIC to the CPU, how would you handle STP/CDP/LLDP otherwise”. I also asked Lukas Krattiger for more details; here’s what he sent me:

A destination MAC address that is not programmed in the ASIC as pointing to a local interface will not be considered local and hence will not be directed to the control/management plane. In fact, unknown traffic could be flooded to all ports in the VLAN but still won’t hit the CPU.

In a layer-2 switch, there is no ARP/ND process enabled and hence IP traffic to local DMAC will not be processed. There are, however, specific DMACs that need to be processed to support STP, IGMP snooping, DHCP snooping, 802.1x… These layer-2 protocols are specifically programmed in the switching ASIC and responded to, and yes, the response comes from the control plane which runs things like STP or IGMP snooping. However, whenever a control-plane protocol has to be sent or redirected to the control plane, Control-Plane Policing (CoPP) is always present.

Looking a bit beyond layer-2-only networks: of course every reachable IP interface of a router or switch is an attack vector. The only question is: how are you going to prevent the attacks? In Nexus OS, the first step is that functions that are not enabled with feature are not started and hence no protocol ports are listening, and no send-to-CPU entries are programmed in the ASIC.

Second, if there is traffic that need to be sent to the CPU, CoPP will ensure appropriate treatment. Third is the hardening of the operating system and closing unnecessary ports. Fourth is using ACLs to protect Telnet, SSH, SNMP, and other protocols that need to be answered locally (for example BGP is TCP/179 and for sure we have to listen to this). Fifth, limit the amount of protocols to a minimum.

Finally, with regard to the out-of-band management network, the answer is NO. Our out-of-band management has VRF separation, this means we use a dedicated routing table and only allow necessary control-plane protocol to run (CDP, LLDP, DHCP…). You will see that you don’t have implicit access to routing protocols that run in default VRF over the out-of-band port.

Further, we are reducing the functions to what the out-of-band portion actually can and need to support. For example HW telemetry that is streamed from the ASIC is not available via out-of-band as it would have to cross the CPU.

Beyond this, at the core of the defensive network is the principle of proven trustworthy hardware and software working in conjunction to protect network devices from attack. We tried to capture the importance of a trustworthy system/network in the following blog with references to the wider efforts to do advancement of security in the internet (ICASI -

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