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Cumulus Linux

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DHCP Relays

DHCP is a client server protocol that automatically provides IP hosts with IP addresses and other related configuration information. A DHCP relay (agent) is a host that forwards DHCP packets between clients and servers that are not on the same physical subnet.

This topic describes how to configure DHCP relays for IPv4 and IPv6 using the following topology:

Basic Configuration

To set up DHCP relay, you need to provide the IP address of the DHCP server and the interfaces participating in DHCP relay (facing the server and facing the client). In an MLAG configuration, you must also specify the peerlink interface in case the local uplink interfaces fail.

In the example commands below:

  • The DHCP server IPv4 address is 172.16.1.102
  • The DHCP server IPv6 address is 2001:db8:100::2
  • vlan10 is the SVI for VLAN 10 and the uplinks are swp51 and swp52
  • peerlink.4094 is the MLAG interface
cumulus@leaf01:~$ nv set service dhcp-relay default interface swp51
cumulus@leaf01:~$ nv set service dhcp-relay default interface swp52
cumulus@leaf01:~$ nv set service dhcp-relay default interface vlan10
cumulus@leaf01:~$ nv set service dhcp-relay default interface peerlink.4094
cumulus@leaf01:~$ nv set service dhcp-relay default server 172.16.1.102
cumulus@leaf01:~$ nv config apply

cumulus@leaf01:~$ nv set service dhcp-relay6 default interface upstream swp51 server-address 2001:db8:100::2
cumulus@leaf01:~$ nv set service dhcp-relay6 default interface upstream swp52 server-address 2001:db8:100::2
cumulus@leaf01:~$ nv set service dhcp-relay6 default interface downstream vlan10
cumulus@leaf01:~$ nv set service dhcp-relay6 default interface downstream peerlink.4094
cumulus@leaf01:~$ nv config apply

  1. Edit the /etc/default/isc-dhcp-relay-default file to add the IP address of the DHCP server and the interfaces participating in DHCP relay.

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
SERVERS="172.16.1.102"
INTF_CMD="-i vlan10 -i swp51 -i swp52 -i peerlink.4094"
OPTIONS=""

  2. Enable, then restart the dhcrelay service so that the configuration persists between reboots:

    cumulus@leaf01:~$ sudo systemctl enable dhcrelay@default.service
cumulus@leaf01:~$ sudo systemctl restart dhcrelay@default.service

  1. Edit the /etc/default/isc-dhcp-relay6-default file to add the IP address of the DHCP server and the interfaces participating in DHCP relay.

    cumulus@leaf01:$ sudo nano /etc/default/isc-dhcp-relay6-default
SERVERS=" -u 2001:db8:100::2%swp51 -u 2001:db8:100::2%swp52"
INTF_CMD="-l vlan10 -l peerlink.4094"

  2. Enable, then restart the dhcrelay6 service so that the configuration persists between reboots:

    cumulus@switch:~$ sudo systemctl enable dhcrelay6@default.service
cumulus@switch:~$ sudo systemctl restart dhcrelay6@default.service

  • You configure a DHCP relay on a per-VLAN basis, specifying the SVI, not the parent bridge. In the example above, you specify vlan10 as the SVI for VLAN 10 but you do not specify the bridge named bridge.
  • When you configure DHCP relay with VRR, the DHCP relay client must run on the SVI; not on the -v0 interface.
  • For every instance of a DHCP relay in a non-default VRF, you need to create a separate default file in the /etc/default directory. See DHCP with VRF.

Optional Configuration

This section describes optional DHCP relay configurations. The steps provided in this section assume that you have already configured basic DHCP relay, as described above.

DHCP Agent Information Option (Option 82)

Cumulus Linux supports DHCP Agent Information Option 82, which allows a DHCP relay to insert circuit or relay specific information into a request that the switch forwards to a DHCP server. You can use the following options:

  • Circuit ID includes information about the circuit on which the request comes in, such as the SVI or physical port. By default, this is the printable name of the interface that receives the client request.
  • Remote ID includes information that identifies the relay agent, such as the MAC address. By default, this is the system MAC address of the device on which DHCP relay is running.

To configure DHCP Agent Information Option 82:

The following example enables Option 82 and enables circuit ID:

cumulus@leaf01:~$ nv set service dhcp-relay <vrf-id> agent enable on
cumulus@leaf01:~$ nv set service dhcp-relay <vrf-id> agent use-pif-circuit-id enable on
cumulus@leaf01:~$ nv config apply

The following example enables Option 82 and sets the remote ID to MAC address 44:38:39:BE:EF:AA:

cumulus@leaf01:~$ nv set service dhcp-relay <vrf-id> agent enable on
cumulus@leaf01:~$ nv set service dhcp-relay default agent remote-id 44:38:39:BE:EF:AA
cumulus@leaf01:~$ nv config apply

  1. Edit the /etc/default/isc-dhcp-relay-default file and add one of the following options:

    To inject the ingress SVI interface against which DHCP processes the relayed DHCP discover packet, add -a to the OPTIONS line:

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-a"

    To inject the physical switch port on which the relayed DHCP discover packet arrives instead of the SVI, add -a --use-pif-circuit-id to the OPTIONS line:

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-a --use-pif-circuit-id"

    To customize the Remote ID sub-option, add -a -r to the OPTIONS line followed by a custom string (up to 255 characters):

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-a -r CUSTOMVALUE"

  2. Restart the dhcrelay service to apply the new configuration:

    cumulus@leaf01:~$ sudo systemctl restart dhcrelay@default.service

Control the Gateway IP Address with RFC 3527

When you need DHCP relay in an environment that relies on an anycast gateway (such as EVPN), a unique IP address is necessary on each device for return traffic. By default, in a BGP unnumbered environment with DHCP relay, the source IP address is the loopback IP address and the gateway IP address is the SVI IP address. However with anycast traffic, the SVI IP address is not unique to each rack; it is typically shared between racks. Most EVPN ToR deployments only use a single unique IP address, which is the loopback IP address.

RFC 3527 enables the DHCP server to react to these environments by introducing a new parameter to the DHCP header called the link selection sub-option, which the DHCP relay agent builds. The link selection sub-option takes on the normal role of the gateway address in relaying to the DHCP server which subnet correlates to the DHCP request. When using this sub-option, the gateway address continues to be present but only relays the return IP address that the DHCP server uses; the gateway address becomes the unique loopback IP address.

When enabling RFC 3527 support, you can specify an interface, such as the loopback interface or a switch port interface to use as the gateway address. The relay picks the first IP address on that interface. If the interface has multiple IP addresses, you can specify a specific IP address for the interface.

RFC 3527 supports IPv4 DHCP relays only.

To enable RFC 3527 support and control the gateway address:

Run the nv set service dhcp-relay default gateway-interface command with the interface or IP address you want to use. The following example uses the first IP address on the loopback interface as the gateway IP address:

cumulus@leaf01:~$ nv set service dhcp-relay default gateway-interface lo

The first IP address on the loopback interface is typically the 127.0.0.1 address. This example uses IP address 10.10.10.1 on the loopback interface as the gateway address:

cumulus@leaf01:~$ nv set service dhcp-relay default gateway-interface lo address 10.10.10.1

This example uses the first IP address on swp2 as the gateway address:

cumulus@leaf01:~$ nv set service dhcp-relay default gateway-interface swp2

This example uses IP address 10.0.0.4 on swp2 as the gateway address:

cumulus@leaf01:~$ nv set service dhcp-relay default gateway-interface swp2 address 10.0.0.4

  1. Edit the /etc/default/isc-dhcp-relay-default file and provide the -U option with the interface or IP address you want to use as the gateway address.

    This example uses the first IP address on the loopback interface as the gateway address:

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-U lo"

    The first IP address on the loopback interface is typically the 127.0.0.1 address. This example uses IP address 10.10.10.1 on the loopback interface as the gateway address:

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-U 10.10.10.1%lo"

    This example uses the first IP address on swp2 as the gateway address:

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-U swp2"

    This example uses IP address 10.0.0.4 on swp2 as the gateway address:

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
...
# Additional options that are passed to the DHCP relay daemon?
OPTIONS="-U 10.0.0.4%swp2"

  2. Restart the dhcrelay service to apply the configuration change:

    cumulus@leaf01:~$ sudo systemctl restart dhcrelay@default.service

DHCP Relay for IPv4 in an EVPN Symmetric Environment with MLAG

In a multi-tenant EVPN symmetric routing environment with MLAG, you must enable RFC 3527 support. You can specify an interface, such as the loopback or VRF interface for the gateway address. The interface must be reachable in the tenant VRF that you configure for DHCP relay and must have a unique IPv4 address. For EVPN symmetric routing with an anycast gateway that reuses the same SVI IP address on multiple leaf switches, you must assign a unique IP address for the VRF interface and include the layer 3 VNI for this VRF in the DHCP relay configuration.

The following example:

  • Configures VRF RED with IPv4 address 20.20.20.1/32.
  • Configures the SVIs vlan10 and vlan20, and the layer 3 VNI VLAN interface for VRF RED vlan4024_l3 to be part of the INTF_CMD list to service DHCP packets.
  • Sets the DHCP server to 10.1.10.104.
  • Configures VRF RED to advertise connected routes as type-5 so that the VRF RED loopback IPv4 address is reachable.
cumulus@leaf01:~$ nv set vrf RED loopback ip address 20.20.20.1/32
cumulus@leaf01:~$ nv set service dhcp-relay RED interface vlan10
cumulus@leaf01:~$ nv set service dhcp-relay RED interface vlan20
cumulus@leaf01:~$ nv set service dhcp-relay RED interface vlan4024_l3
cumulus@leaf01:~$ nv set service dhcp-relay RED server 10.1.10.104
cumulus@leaf01:~$ nv set vrf RED router bgp address-family ipv4-unicast redistribute connected enable on
cumulus@leaf01:~$ nv set vrf RED router bgp address-family ipv4-unicast route-export to-evpn enable on
cumulus@leaf01:~$ nv config apply

  1. Edit the /etc/network/interfaces file to configure VRF RED with IPv4 address 20.20.20.1/32

    cumulus@leaf01:mgmt:~$ sudo nano /etc/network/interfaces
...
auto RED
iface RED
        address 20.20.20.1/32
        vrf-table auto

  2. Configure VRF RED to advertise the connected routes as type-5 so that the loopback IPv4 address is reachable:

    cumulus@leaf01:mgmt:~$ sudo vtysh 
...
leaf01# configure terminal
leaf01(config)# router bgp 65101 vrf RED
leaf01(config-router)# address-family l2vpn evpn
leaf01(config-router-af)# advertise ipv4 unicast 
leaf01(config-router-af)# end
leaf01# write memory

    The /etc/frr/frr.conf file now contains the following entries:

    ...
router bgp 65101 vrf RED
 bgp router-id 10.10.10.1
..
 !
 address-family ipv4 unicast
  redistribute connected
  maximum-paths 64
  maximum-paths ibgp 64
 exit-address-family
 !
 address-family l2vpn evpn
  advertise ipv4 unicast
 exit-address-family
exit

  3. Edit the /etc/default/isc-dhcp-relay-RED file.

    cumulus@leaf01:mgmt:~$ sudo nano /etc/default/isc-dhcp-relay-RED
SERVERS="10.1.10.104"
INTF_CMD=" -i vlan10 -i vlan20 -i vlan4024_l3" 
OPTIONS="-U RED"

  4. Start and enable the DHCP service so that it starts automatically the next time the switch boots:

    sudo systemctl start dhcrelay@RED.service
sudo systemctl enable dhcrelay@RED.service

DHCP Relay for IPv4 in an EVPN Symmetric Environment without MLAG

In a multi-tenant EVPN symmetric routing environment without MLAG, the VLAN interface (SVI) IPv4 address is typically unique on each leaf switch, which does not require RFC 3527 configuration.

The following example:

  • Configures the SVIs vlan10 and vlan20, and the layer 3 VNI VLAN interface for VRF RED vlan4024_l3 to be part of INTF_CMD list to service DHCP packets.
  • Sets the DHCP server IP address to 10.1.10.104.
cumulus@leaf01:~$ nv set service dhcp-relay RED interface vlan10
cumulus@leaf01:~$ nv set service dhcp-relay RED interface vlan20
cumulus@leaf01:~$ nv set service dhcp-relay RED interface vlan4024_l3
cumulus@leaf01:~$ nv set service dhcp-relay RED server 10.1.10.104
cumulus@leaf01:~$ nv config apply

  1. Edit the /etc/default/isc-dhcp-relay-RED file.

    cumulus@leaf01:mgmt:~$ sudo nano /etc/default/isc-dhcp-relay-RED
SERVERS="10.1.10.104"
INTF_CMD=" -i vlan10 -i vlan20 -i vlan4024_l3" 
OPTIONS=""

  2. Start the DHCP service and enable it to start automatically when the switch boots:

    sudo systemctl start dhcrelay@RED.service
sudo systemctl enable dhcrelay@RED.service

DHCP Relay for IPv6 in an EVPN Symmetric Environment

For IPv6 DHCP relay in a symmetric routing environment, you must assign a unique IPv6 address to the non-default VRF interfaces that participate in DHCP relay. Cumulus Linux uses this IPv6 address as the source address when sending packets to the DHCP server and the DHCP server replies to this address.

RFC 3527 does not apply to IPv6. IPv6 has the functionality described in RFC 3527 as part of its normal operations.

The following example:

  • Configures VRF RED with the unique IPv6 address 2001:db8:666::1/128.
  • Configures VLAN 10 and 20 in VRF RED to service DHCP requests from downstream hosts.
  • Sets the DHCP server to 2001:db8:199::2.
  • Configures the layer 3 VNI interface for VRF RED vlan4024_l3 to process DHCP packets from the upstream server.
  • Configures VRF RED to advertise the connected routes so that the loopback IPv6 address is reachable.
cumulus@leaf01:~$ nv set vrf RED loopback ip address 2001:db8:666::1/128
cumulus@leaf01:~$ nv set service dhcp-relay6 RED interface downstream vlan10
cumulus@leaf01:~$ nv set service dhcp-relay6 RED interface downstream vlan20
cumulus@leaf01:~$ nv set service dhcp-relay6 RED interface upstream RED server-address 2001:db8:199::2
cumulus@leaf01:~$ nv set service dhcp-relay6 RED interface upstream vlan4024_l3
cumulus@leaf01:~$ nv set vrf RED router bgp address-family ipv6-unicast route-export to-evpn enable on
cumulus@leaf01:~$ nv config apply

  1. Edit the /etc/network/interfaces file to configure VRF RED with IPv6 address 2001:db8:666::1/128:

    cumulus@leaf01:mgmt:~$ sudo nano /etc/network/interfaces
...
auto RED
iface RED
        address 2001:db8:666::1/128
        vrf-table auto

  2. Configure VRF RED to advertise the connected routes so that the loopback IPv6 address is reachable:

    cumulus@leaf01:mgmt:~$ sudo vtysh 
...
leaf01# configure terminal
leaf01(config)# router bgp 65101 vrf RED
leaf01(config-router)# address-family l2vpn evpn
leaf01(config-router-af)# advertise ipv6 unicast 
leaf01(config-router-af)# end
leaf01# write memory

    The /etc/frr/frr.conf file now contains the following entries:

    ...
router bgp 65101 vrf RED
 bgp router-id 10.10.10.1
..
 !
 address-family ipv6 unicast
  redistribute connected
  maximum-paths 64
  maximum-paths ibgp 64
 exit-address-family
 !
 address-family l2vpn evpn
  advertise ipv6 unicast
 exit-address-family
exit

  3. Edit the /etc/default/isc-dhcp-relay6-RED file.

    • Set the -l option to the VLANs that receive DHCP requests from hosts.
    • Set the <ip-address-dhcp-server>%<interface-facing-dhcp-server> option to associate the DHCP Server with VRF RED.
    • Set the -u option to indicate where the switch receives replies from the DHCP server (SVI vlan4024_l3).
    cumulus@leaf01:mgmt:~$ sudo nano /etc/default/isc-dhcp-relay6-RED
INTF_CMD="-l vlan10 -l vlan20"
SERVERS="-u 2001:db8:199::2%RED -u vlan4024_l3"

  4. Start and enable the DHCP service so that it starts automatically the next time the switch boots:

    sudo systemctl start dhcrelay6@RED.service
sudo systemctl enable dhcrelay6@RED.service

Gateway IP Address as Source IP for Relayed DHCP Packets (Advanced)

You can configure the dhcrelay service to forward IPv4 (only) DHCP packets to a DHCP server and ensure that the source IP address of the relayed packet is the same as the gateway IP address.

This option impacts all relayed IPv4 packets globally.

To use the gateway IP address as the source IP address:

cumulus@leaf01:~$ nv set service dhcp-relay default source-ip gateway
cumulus@leaf01:~$ nv config apply

  1. Edit the /etc/default/isc-dhcp-relay-default file to add --giaddr-src to the OPTIONS line.

    cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay-default
SERVERS="172.16.1.102"
INTF_CMD="-i vlan10 -i swp51 -i swp52 -U swp2"
OPTIONS="--giaddr-src"

  2. Restart the dhcrelay service to apply the configuration change:

    cumulus@leaf01:~$ sudo systemctl restart dhcrelay@default.service

Configure Multiple DHCP Relays

Cumulus Linux supports multiple DHCP relay daemons on a switch to enable relaying of packets from different bridges to different upstream interfaces.

To configure multiple DHCP relay daemons on a switch:

  1. In the /etc/default directory, create a configuration file for each DHCP relay daemon. Use the naming scheme isc-dhcp-relay-<dhcp-name> for IPv4 or isc-dhcp-relay6-<dhcp-name> for IPv6. This is an example configuration file for IPv4:

    # Defaults for isc-dhcp-relay initscript
# sourced by /etc/init.d/isc-dhcp-relay
# installed at /etc/default/isc-dhcp-relay by the maintainer scripts

#
# This is a POSIX shell fragment
#

# What servers should the DHCP relay forward requests to?
SERVERS="102.0.0.2"
# On what interfaces should the DHCP relay (dhrelay) serve DHCP requests?
# Always include the interface towards the DHCP server.
# This variable requires a -i for each interface configured above.
# This will be used in the actual dhcrelay command
# For example, "-i eth0 -i eth1"
INTF_CMD="-i swp2s2 -i swp2s3"

# Additional options that are passed to the DHCP relay daemon?
OPTIONS=""

  2. Run the following command to start a dhcrelay instance, where <dhcp-name> is the instance name or number.

    cumulus@leaf01:~$ sudo systemctl start dhcrelay@<dhcp-name>

Troubleshooting

This section provides troubleshooting tips.

Show DHCP Relay Status

To show the DHCP relay status:

Run the nv show service dhcp-relay command for IPv4 or the nv show service dhcp-relay6 command for IPv6:

cumulus@leaf01:~$ nv show service dhcp-relay
           source-ip  Summary
---------  ---------  -----------------------
+ default  auto       gateway-interface: lo
  default             interface:        swp51
  default             interface:        swp52
  default             interface:        vlan10
  default             server:    172.16.1.102

Run the Linux systemctl status dhcrelay@default.service command for IPv4 or the systemctl status dhcrelay6@default.service command for IPv6:

cumulus@leaf01:~$ sudo systemctl status dhcrelay@default.service
● dhcrelay@default.service - DHCPv4 Relay Agent Daemon default in vrf default
   Loaded: loaded (/lib/systemd/system/dhcrelay@.service; enabled; vendor preset: enabled)
  Drop-In: /run/systemd/generator/dhcrelay@.service.d
           └─vrf.conf
   Active: active (running) since Tue 2023-04-18 18:23:55 UTC; 9min ago
     Docs: man:dhcrelay(8)
 Main PID: 30904 (dhcrelay)
    Tasks: 1 (limit: 2056)
   Memory: 2.3M
   CGroup: /system.slice/system-dhcrelay.slice/dhcrelay@default.service
           └─vrf
             └─30904 /usr/sbin/dhcrelay --nl -d -i swp51 -i swp52 -i vlan10 -i peerlink.4094 172.16.1.102

Check systemd

If you are experiencing issues with DHCP relay, check if there is a problem with systemd:

  • For IPv4, run the /usr/sbin/dhcrelay -4 -i <interface-facing-host> <ip-address-dhcp-server> -i <interface-facing-dhcp-server> command. For example:

    cumulus@leaf01:~$ /usr/sbin/dhcrelay -4 -i vlan10 172.16.1.102 -i swp51

  • For IPv6, run the /usr/sbin/dhcrelay -6 -l <interface-facing-host> -u <ip-address-dhcp-server>%<interface-facing-dhcp-server> command. For example:

    cumulus@leaf01:~$ /usr/sbin/dhcrelay -6 -l vlan10 -u 2001:db8:100::2%swp51

To see how DHCP relay is working on your switch, run the journalctl command:

cumulus@leaf01:~$ sudo journalctl -l -n 20 | grep dhcrelay
Dec 05 20:58:55 leaf01 dhcrelay[6152]: sending upstream swp52
Dec 05 20:58:55 leaf01 dhcrelay[6152]: sending upstream swp51
Dec 05 20:58:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.
Dec 05 20:58:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.
Dec 05 21:03:55 leaf01 dhcrelay[6152]: Relaying Renew from fe80::4638:39ff:fe00:3 port 546 going up.
Dec 05 21:03:55 leaf01 dhcrelay[6152]: sending upstream swp52
Dec 05 21:03:55 leaf01 dhcrelay[6152]: sending upstream swp51
Dec 05 21:03:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.
Dec 05 21:03:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.

To specify a time period with the journalctl command, use the --since flag:

cumulus@leaf01:~$ sudo journalctl -l --since "2 minutes ago" | grep dhcrelay
Dec 05 21:08:55 leaf01 dhcrelay[6152]: Relaying Renew from fe80::4638:39ff:fe00:3 port 546 going up.
Dec 05 21:08:55 leaf01 dhcrelay[6152]: sending upstream swp52
Dec 05 21:08:55 leaf01 dhcrelay[6152]: sending upstream swp51

Configuration Errors

If you configure DHCP relays by editing the /etc/default/isc-dhcp-relay-default file manually, you can introduce configuration errors that cause the switch to crash.

For example, if you see an error similar to the following, check that there is no space between the DHCP server address and the interface you use as the uplink.

Core was generated by /usr/sbin/dhcrelay --nl -d -i vx-40 -i vlan10 10.0.0.4 -U 10.0.1.2  %vlan20.
Program terminated with signal SIGSEGV, Segmentation fault.

To resolve the issue, manually edit the /etc/default/isc-dhcp-relay-default file to remove the space, then run the systemctl restart dhcrelay@default.service command to restart the dhcrelay service and apply the configuration change.

Considerations

  • The dhcrelay command does not bind to an interface if the interface name is longer than 14 characters. This is a known limitation in dhcrelay.
  • DHCP packets received on bridge ports and sent to the CPU for processing cause the RX_DROP counter to increment on the interface.

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