Active-Active Router Redundancy (L3 Anycast Gateway)

Implementing active-active First Hop Router Redundancy to provide active/active unicast IP routing, Arista recommends L3 Anycast Gateway (a.k.a. vARP – Virtual ARP).

The primary benefit being that all configured routers are active and able to perform routing. L3 Anycast Gateway also provides rapid failover in the event of a link or switch failure while enabling the sharing of IP forwarding load between both switches. L3 Anycast Gateway requires configuring the same virtual-router IP address on the appropriate VLAN interfaces of both peers, as well as a global unique virtual-router MAC address. VARP functions by having both switches respond to ARP requests and GARP for a configured IP address with the “virtual-router” MAC address. This address is receive-only MAC address and no packet is ever sent with this address as its source. If ‘ip routing’ is enabled, received packets will be routed with the following process. When the DMAC of a packet destined to a remote network matches the configured “virtual-router” MAC address, each MLAG peer locally forwards the traffic to it’s next hop destination.

Below is an example configuration of L3 Anycast Gateway. The example uses 00:1c:73:00:00:99, which is a MAC address that will not occur naturally.

Switch1:

Switch1#config t Switch1(config)#interface vlan 10
Switch1(config-if-Vl10)#ip address 10.10.10.2/24
Switch1(config-if-Vl10)#ip virtual-router address 10.10.10.1
Switch1(config-if-Vl10)#interface vlan 20
Switch1(config-if-Vl20)#ip address 10.10.20.2/24
Switch1(config-if-Vl20)#ip virtual-router address 10.10.20.1
Switch1(config-if-Vl20)#exit
Switch1(config)#ip virtual-router mac-address 00:1c:73:00:00:99

Switch2:
Switch2#config t Switch2(config)#interface vlan 10
Switch2(config-if-Vl10)#ip address 10.10.10.3/24
Switch2(config-if-Vl10)#ip virtual-router address 10.10.10.1
Switch2(config-if-Vl10)#interface vlan 20
Switch2(config-if-Vl20)#ip address 10.10.20.3/24
Switch2(config-if-Vl20)#ip virtual-router address 10.10.20.1
Switch2(config-if-Vl20)#exit
Switch2(config)#ip virtual-router mac-address 00:1c:73:00:00:99 

L3 Anycast Gateway provides significant benefits over traditional FHRP protocols (e.g. VRRP, HSRP) with rapid fail-over and allowing all configured routers to perform routing.

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