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Virtual Router Redundancy - VRR

Virtual Router Redundancy (VRR) enables hosts to communicate with any redundant router without reconfiguration, running dynamic router protocols, or running router redundancy protocols. This means that redundant routers will respond to Address Resolution Protocol (ARP) requests from hosts. Routers are configured to respond in an identical manner, but if one fails, the other redundant routers will continue to respond, leaving the hosts with the impression that nothing has changed.

The diagram below illustrates a basic VRR-enabled network configuration. The network includes several hosts, and two routers running Cumulus Linux configured with Multi-chassis Link Aggregation (MLAG):


A production implementation will have many more server hosts and network connections than are shown here. However, this basic configuration provides a complete description of the important aspects of the VRR setup.

As the bridges in each of the redundant routers are connected, they will each receive and reply to ARP requests for the virtual router IP address.

Multiple ARP Replies

Each ARP request made by a host will receive replies from each router; these replies will be identical, and so the host receiving the replies will either ignore replies after the first, or accept them and overwrite the previous identical reply, rather than being confused over which response is correct.

Reserved MAC Address Range

A range of MAC addresses is reserved for use with VRR, in order to prevent MAC address conflicts with other interfaces in the same bridged network. The reserved range is 00:00:5E:00:01:00 to 00:00:5E:00:01:ff.

Cumulus Networks recommends using MAC addresses from the reserved range when configuring VRR.

The reserved MAC address range for VRR is the same as for the Virtual Router Redundancy Protocol (VRRP), as they serve similar purposes.

Configuring a VRR-enabled Network

Configuring the Routers

The routers implement the layer 2 network interconnecting the hosts and the redundant routers. To configure the routers, add a bridge with the following interfaces to each router:

  • One bond interface or switch port interface to each host.

    For networks using MLAG, use bond interfaces. Otherwise, use switch
    port interfaces.

  • One or more interfaces to each peer router.

    Multiple inter-peer links are typically bonded interfaces, in order
    to accomodate higher bandwidth between the routers, and to offer
    link redundancy.

Example VLAN-aware Bridge Configuration

The example NCLU commands below create a VLAN-aware bridge interface for a VRR-enabled network:

cumulus@switch:~$ net add bridge
cumulus@switch:~$ net add vlan 500 ip address
cumulus@switch:~$ net add vlan 500 ip address-virtual 00:00:5e:00:01:01
cumulus@switch:~$ net add vlan 500 ipv6 address 2001:aa::1/48
cumulus@switch:~$ net add vlan 500 ipv6 address-virtual 00:00:5e:00:01:01 2001:aa::1/48
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

The NCLU commands above produce the following /etc/network/interfaces snippet:

auto bridge
iface bridge
    bridge-vids 500
    bridge-vlan-aware yes
auto vlan500
iface vlan500
    address 2001:aa::1/48
    address-virtual 00:00:5e:00:01:01 2001:aa::1/48
    vlan-id 500
    vlan-raw-device bridge

Configuring the Hosts

Each host should have two network interfaces. The routers configure the interfaces as bonds running LACP; the hosts should also configure its two interfaces using teaming, port aggregation, port group, or EtherChannel running LACP. Configure the hosts, either statically or via DHCP, with a gateway address that is the IP address of the virtual router; this default gateway address never changes.

Configure the links between the hosts and the routers in active-active mode for First Hop Redundancy Protocol.