Use Case 2: Configuration of source-interface CLI, link selection, and server-override and VSS suboptions
NOTE: You cannot use Dell SmartFabric OS10 switch as a DHCP Server for the Use Case 2 scenario since it is not VRF aware and the DHCP server that you use must be compliant with the RFC standards mentioned under the DHCP Server configuration example section. Configure at least one interface to match one of the configured network pools. An interface matches a network pool when its IP address is included in the subnet defined for that network pool. For example, an interface with IP address 1.1.1.1/24 matches a pool configured with network 1.1.1.0/24. For the DHCP server to process DHCP discovery packets, you must configure the IP address pool for all the interfaces on which DHCP packets are received. Also, these interfaces must be connected to the DHCP server. This configuration is required even if the IP addresses are not leased to any DHCP client in the subnet directly connected to the DHCP server.
In this example, the DHCP client is connected to
eth 1/1/1 on the default VRF and
eth 1/1/3 on the VRF hello.
Loopback 0 is used as the relay source-interface for the default VRF clients. The server-override option is enabled globally.
Configure the DHCP relay agent globally to insert the server ID override suboption and link selection suboption into the relay agent information option of the DHCP packet. The VSS option is enabled globally and the
vss-info value is configured on the interfaces to send VRF information on the client-connected interfaces to the DHCP server.
DHCP server is also configured with VRF pool
default and
hello for the same
1.1.1.x network range to assign the IP addresses to the clients requesting from the respective VRFs.
Consider the following scenarios:
The DHCP client sends a broadcast DHCP request on the network.
The DHCP relay agent inserts the VRF
vss-info value, server ID override suboption, and link selection suboption to its relay agent information option in the DHCP packet. The link selection suboptions contains the incoming interface IP address, which is the IP address on the relay agent that is accessible from the client
1.1.1.1. In this case, as the interface IP is present, link selection uses the
1.1.1.x subnet. If the interface IP is not present, the virtual gateway address is used in VXLAN scenarios. The server-override option carries the virtual anycast gateway IP.
Based on the
vss-info value in the DHCP packet, the DHCP server offers an address from the respective VRF pool; for example, pool
hello_clients as it matches with the
type0 value
serverVRF is sent in the
vss-info.
As the source interface is explicitly configured on a Loopback 0, the relay agent uses that address as the source IP address (giaddr) for messages relayed to the DHCP server
3.1.1.1.
If the DHCP server is reachable on a different VRF, configure route leaking on VRF hello to reach the DHCP server.
In the above case, as there is no virtual anycast IP, the server-override option value is not added. As a result, further DHCP packets from the client directly go to the server.
NOTE: SmartFabric OS10 switch configured as a DHCP Server is not VRF aware. For this use case scenario, you must use a DHCP server that is VRF aware.
DHCP Relay Agent
You must configure the following settings at the DHCP relay agent level:
Global configuration
ip dhcp-relay server-override
ip dhcp-relay vss
ip dhcp-relay link-selection
VRF configuration for source-interface:
OS10# show running-configuration vrf
!
ip vrf default
update-source-ip loopback0
ip dhcp-relay source-interface loopback0
!
ip vrf serverVRF
!
ip vrf hello
update-source-ip loopback1
!
Interface configuration
!
interface loopback0
no shutdown
ip address 3.1.1.1/32
!
interface loopback1
no shutdown
ip vrf forwarding hello
ip address 5.1.1.1/32
!
interface ethernet1/1/1
no shutdown
ip address 1.1.1.1/24
ip helper-address 20.1.1.2
!
interface ethernet1/1/2
no shutdown
ip address 20.1.1.1/24
!
interface ethernet1/1/3
no shutdown
ip vrf forwarding hello
ip address 1.1.1.1/24
ip helper-address 30.1.1.2 vrf hello
ip dhcp-relay vss-info type 0 serverVRF
!
interface ethernet1/1/4
no shutdown
ip vrf forwarding hello
ip address 30.1.1.1/24
DHCP Server
You must use a VRF aware DHCP server that is compliant with the following RFC standards:
