Users Guide

Use Case 2: Configuration of source-interface CLI, link selection, and
server-override and VSS suboptions
In this example, the DHCP client is connected to eth 1/1/1 on the default VRF and eth 1/1/3 is connected to 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.
DHCP Relay Agent
Global config:
ip dhcp-relay server-override
ip dhcp-relay link-selection
ip dhcp-relay vss
VRF config for source-interface:
OS10# show running-configuration vrf
!
ip vrf default
update-source-ip loopback0
ip dhcp-relay source-interface loopback0
!
ip vrf hello
update-source-ip loopback1
ip dhcp-relay source-interface loopback0
System management
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