First-Hop Gateway Configuration Module

The first-hop Gateway configuration module implements mechanisms used to implement a shared router IPv4 address on a stub access network.

The module supports statically configured anycast gateway IPv4 address and VRRPv3 for IPv4 and IPv6.

Platform Support

The module is supported on these platforms:

Operating system

Anycast

VRRPv3

VRRPv3
IPv6

Arista EOS

✅

✅

✅

Aruba AOS-CX

✅

✅

✅

Cisco Catalyst 8000v

❌

✅

✅

Cisco CSR 1000v

❌

✅

✅

Cisco IOSv/IOSvL2

❌

✅

✅

Cisco Nexus OS

❌

✅

✅

Cumulus Linux

✅

✅

✅

Dell OS10

✅ ❗

✅ ❗

✅ ❗

FRRouting

✅

✅

✅

Nokia SR OS

✅

✅

✅

Nokia SR Linux

✅

❌

❌

VyOS

❌

✅

✅

Global Parameters

The module supports the following global parameters:

  • gateway.protocol (default: anycast) – the first-hop gateway resolution protocol. The supported values are anycast and vrrp.

  • gateway.id (default: -2) – the IP address within the subnet used for the gateway IP address. The gateway.id could be a positive number (a fixed IP address within a subnet using the gateway module) or a negative number specifying the offset from the end of the subnet. The default value specifies a subnet’s last valid IP address (-1 is the broadcast address).

Interface Parameters

You can specify gateway: True or a dictionary of valid parameters on individual interfaces (nodes-on-a-link data).

Specifying a gateway interface attribute on a link that does not have a gateway attribute automatically enables the Gateway configuration module on the connected link but turns it off for other nodes connected to the same link, ensuring the Gateway module is active only on the nodes where you explicitly enabled it.

Similarly, you can use the gateway: False setting to turn off the default gateway functionality on individual interfaces on a link on which the Gateway configuration module is enabled.

Reserved IP Addresses

A positive gateway.id value could generate IP addresses that overlap with node interface addresses on subnets using ID-based address allocation.

The gateway module tries to prevent that and reserves the node ID values specified in the global, link, or global VLAN gateway.id values.

Tip

If you want to use one of the first IP addresses in the high-availability subnets as the gateway IP address, set the global value of the ‌gateway.id parameter unless you have an excellent reason to do something else.

Anycast Gateway

The gateway configuration module supports IPv4 anycast gateways, which use MAC and IPv4 addresses shared between multiple nodes attached to the same segment.

Implementation Notes

  • The default netlab shared MAC address is 0200.cafe.00ff.

  • The gateway IP is configured to respond to ping requests

  • Vendors use different names for anycast gateways: VARP (Arista), VRR (Cumulus), passive VRRP (Nokia)

  • Arista EOS uses the same shared MAC address on all interfaces. Do not set the link-level gateway.anycast.mac parameter in topologies using Arista EOS.

  • Arista vEOS cannot use VRRP MAC address as a shared MAC address (default recommended by Cumulus Linux)

Anycast Parameters

Anycast implementation of shared first-hop IPv4 address supports these parameters that can be specified globally or on individual links or interfaces.

  • gateway.anycast.unicast (default: True) – configure node-specific unicast IP addresses even when using the anycast IP address.

  • gateway.anycast.mac – Static MAC address used for the anycast IPv4 address

Tip

  • Many implementations require unique unicast IPv4 addresses configured on interfaces with anycast IPv4 address. Set gateway.anycast.unicast to False only when absolutely necessary.

Virtual Router Redundancy Protocol (VRRP)

netlab supports a single VRRPv3 instance per subnet/interface. The VRRPv3 instance can provide shared IPv4 and IPv6 addresses.

Tip

More complex topologies, such as multiple VRRPv3 instances on the same subnet, could be deployed with a judicious application of interface parameters[1]. Still, you won’t be able to model multiple VRRPv3 instances per interface[2].

VRRP Parameters

netlab supports these VRRP parameters that can be specified globally or on individual nodes, links, or interfaces.

  • vrrp.group (default: 1) – VRRP group

  • vrrp.priority (interface parameter, integer)

  • vrrp.preempt (interface parameter, boolean) – VRRP preempting is enabled by default; you can disable it with vrrp.preempt: False.

No other aspect of VRRP (VRRP version, timers…) is manageable through netlab parameters; if you want to configure them, create a plugin to implement additional VRRP functionality.