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This chapter contains the following sections:
First Hop Security (FHS) is an access security feature that provides security to the network at the access (where the host attaches to the first switch in the network). The Dot1x, port-security and DHCP Snooping are examples of access security features. Together, these security features authorize and authenticate the host and thereby protect the network by ensuring that only legitimate hosts are allowed to use the network.
In a VXLAN fabric, the host can be attached to an interface on one VTEP, while the DHCP server can be attached to an interface on a different VTEP.
As shown in the figure, the host H1 is attached to VTEP1, while the DHCP server is attached to VTEP3.
The host and the DHCP server exchange a set of messages as part of this host IP assignment procedure. These are popularly known as Discover-Offer-Request-Ack (DORA) exchange messages.
The DORA exchange, for a particular host (H1), must now be sent over the VXLAN fabric to reach remote DHCP servers (VTEP3).
VTEP3 checks that the “Offer” and “Ack” messages (that are part of a DORA sequence) and coming from the DHCP server, are received on a Trusted Interface on VTEP3.
Upon completion of the DORA exchange, the VTEP1 creates a “DHCP snooping DB” entry. This DB contains the MAC-address of the host, the IP-address assigned to the host by the DHCP server, VLAN, and other details like the “lease time”. The major driving part of this feature is that the snooping DB entry created on VTEP1 for host (H1) as a "Local snooping DB entry" is also propagated to remote VTEPs using BGP-EVPN and will be seen as "Remote snooping DB entry" for host (H1). Thus this DHCP snooping DB will be seen as a "Distributed DB" across the VTEPs and the snooping entries will be in sync with all VTEPs.
For use-cases where the IP address assignment to the host is predefined, the snooping DB entry can be configured using the ip source binding ip address vlan vlan-id interface interface command. Snooping entries added through this command are referred as static entries and even these are also distributed across all VTEPs.
The Distributed DHCP Snooping DB is used as follows:
To validate ARPs/GARPs sent from the host using DAI - This ensures that any spoofing of the ARP/GARP using different host credentials, and consequent malicious-ARP-storm in the network, is prevented.
In a VXLAN environment, we must account for host-move. Since the DHCP Snooping DB is replicated across the fabric, DAI can now work across the fabric after the host-move also. Thus, the control plane is protected in a VXLAN environment.
 Note |
If there is no matching entry in the DB, the ARP/GARP will be dropped. |
To validate the data-plane traffic from the host using IPSG. This validates the data-traffic and prevents malicious hosts from sending data traffic to the network.
The DHCP snooping entry is replicated across the fabric. Only local DHCP clients for that VTEP are programmed in the IPSG. The local DHCP clients are identified with anchor flag set to true in the DHCP snooping table. If a host moves to a different VTEP and settles down, IPSG has to reprogram the client behind the new VTEP to validate the data-traffic. On the old VTEP, IPSG has to remove this DHCP client. The anchor flag will change accordingly. The host move is triggered by the receipt of an ARP request from the host which is received on the new VTEP that the host moved to.
DHCP Snooping on VXLAN feature has the following configuration guidelines and limitations:
Beginning with Cisco NX-OS Release 10.4(1)F, DHCP snooping and associated features such as Dynamic ARP Inspection (DAI) and IP Source Guard (IPSG) support is extended to VXLAN fabric on Cisco Nexus 9300-EX/FX/FX2/FX3/GX/GX2 platform switches and Cisco Nexus 9500 switches with 9700-EX/FX/GX line cards.
Beginning with Cisco NX-OS Release 10.4(2)F, First Hop Security feature is supported on Cisco Nexus 9332D-H2R, and 93400LD-H1 switches.
Beginning with Cisco NX-OS Release 10.4(3)F, First Hop Security feature is supported on Cisco Nexus 9364C-H1 switches.
Ensure that the DHCP snooping, DAI and IPSG together are enabled on all VTEPs.
Note |
DAI and IPSG depend on DHCP snooping. DHCP snooping creates the snooping DB and this DB is used by DAI and IPSG. |
Only IPv4 multicast underlay is supported. However, IPv4 ingress replication underlay, IPv6 ingress replication underlay and IPv6 multicast underlay are not supported.
Only IPv4 DHCP hosts is supported.
The host-move is indicated by ARP/GARP/RARP receipt. In case of RARP (which contains MAC info alone), VTEPs start ARP Refreshes for the IPs learned against MAC. Hence, essentially, ARP-GARP is the trigger for host-move and not any other data packet.
For vPC VTEPs, only physical MCT is supported.
This feature cannot coexist with FabricPath to VXLAN migration feature and counter ACL (CNT ACL) feature.
In the ingress SUP region, the TCAM must be carved out to 768 entries instead of the default 512 entries to set up the ingress ACLs using the hardware access-list tcam region ing-sup command. Reload of a switch is required for the TCAM carving changes to reflect.
In case of multisite and with vPC BGW, if DHCP snooping is enabled on the vPC BGW, ensure that DHCP clients and DHCP servers are on same sites.
Note |
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The DHCP server cannot be deployed behind the EoR.
DHCP has the following prerequisites:
You should be familiar with DHCP before you configure DHCP snooping or the DHCP relay agent.
Make sure that the DHCP Snooping, DAI and IPSG features are enabled together on a leaf VTEP.
You can enable or disable DHCP snooping on a single-box feature or enable this feature for a VLAN for the entire fabric. By default, DHCP snooping is disabled on all VLANs.
Make sure that the DHCP feature is enabled.
Make sure that the nv overlay evpn command is configured.
Make sure that the DHCP Snooping, DAI and IPSG features are enabled. For more information see Prerequisites for DHCP Snooping section.
Make sure that DHCP snooping and DAI are enabled on all the VXLAN nodes. For more information on configuration, see Configuring DHCP Snooping section of Cisco Nexus 9000 Series NX-OS Security Configuration Guide.
Make sure that DHCP snooping trust and ARP inspection trust are enabled on interfaces connected to the DHCP server nodes. For more information on configuration, see Configuring DHCP Snooping section of Cisco Nexus 9000 Series NX-OS Security Configuration Guide.
Make sure that IP Source Guard is enabled on the interfaces connected to the DHCP client nodes. For more information on configuration, see Configuring DHCP Snooping section of Cisco Nexus 9000 Series NX-OS Security Configuration Guide.
| Command or Action | Purpose | |||
|---|---|---|---|---|
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Step 1 |
configure terminal Example: |
Enters global configuration mode. |
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Step 2 |
[no] ip dhcp snooping vlan vlan-list evpn Example: |
Enables DHCP snooping on the VLANs specified by vlan-list . Beginning with Cisco NX-OS Release 10.4(1)F, the evpn option is provided to support host move to other interfaces on the same VTEP or other VTEPs
The no form of this command disables DHCP snooping on the VLANs specified. |
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Step 3 |
(Optional) show running-config dhcp Example: |
(Optional)
Displays the DHCP configuration. |
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Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
The mobility and duplicate detection logic for DHCP clients in FHS enabled VTEPs is same as BGP EVPN mobility and duplicate detection logic. However duplicate detection may happen in any of the VTEPs in non-FHS deployments. In the FHS deployments, the host duplicate will be detected always on a VTEP where DHCP binding entry is remote.
For more information on mobility and duplicate detection, see Duplicate Detection for IP and MAC Addresses section.
Once the MAC or MAC-IP is permanently frozen, there is no auto recovery mechanism to re-initiate mobility or duplicate check sequences. To clear MAC and MAC-IP permanent freeze state, use the following commands:
For MAC:
clear l2route evpn mac [mac-address] [topo] permanently-frozen-list For MAC-IP:
clear fabric forwarding dup-host [{ ip|ipv6 address }] [vrf {vrf-name | vrf-known-name | all}] To display DHCP snooping bindings information, enter the following commands:
|
Command |
Purpose |
|---|---|
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show ip dhcp snooping binding evpn |
Displays all entries from the DHCP snooping binding database. |
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show l2route fhs [topology topology id | all] |
Displays all entries from the L2RIB database. |
switch(config)# show ip dhcp snooping binding evpn
MacAddress IpAddress Lease(Sec) Type BD Interface anchor Freeze
----------------- -------------- ----------- ---------- ---- ----------------- ---------- ---------
00:10:00:10:00:10 10.10.10.10 infinite static 2001 Ethernet1/48 YES NONE
00:15:06:00:00:01 100.1.150.156 86282 dhcp-snoop 2001 Ethernet1/31 YES NONE
00:17:06:00:00:01 100.1.150.155 86265 dhcp-snoop 2001 nve1(peer-id: 1) NO NONEswitch(config)# show l2route fhs all
Flags - (Stt):Static (Dyn):Dynamic (R):Remote
Topo ID Mac Address Host IP Prod Flags Seq No Next-Hops
---- -- ---- ----------- ------------------ ---------- ----------- ------------ -------------
2001 0015.0600.0001 100.1.150.156 DHCP_DYNAMIC Dyn, 0 Eth1/31
2001 0017.0600.0001 100.1.150.155 BGP Dyn,R, 0 1.13.13.13 (Label: 0)
switch(config)#feature dhcp
service dhcp
ip dhcp snooping
ip dhcp snooping vlan 2001-2002 evpn
ip arp inspection vlan 2001-2002
interface Ethernet1/31
ip verify source dhcp-snooping-vlanfeature dhcp
service dhcp
ip dhcp snooping
ip dhcp snooping vlan 2001-2002 evpn
ip arp inspection vlan 2001-2002
interface Ethernet1/47
ip dhcp snooping trust
ip arp inspection trust
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