Gratuitous Address Resolution Protocol (GARP)

[abtt-39]

Hello,

I am going to migrate our telephony to SIP trunk

We will have 2 SIP fiber trunk links, with 2 AVAYA SBC (VM)

The 2 ISP routers ( same ISP but two different fibers, one active and one in "stand by") will be connected to my core switch, I created a VLAN with a dedicated SVI for this.
The 2 SBC Vms will be on this same VLAN. The IPBX server, which is also a VM, is on another VLAN. The IP phones are still on other dedicated VLANS. Inter vlan routing is active.

I looked at some documentation for AVAYA and read this:

Note:
High availability requires Gratuitous Address Resolution Protocol (GARP) support on the connected network elements.
When the primary Avaya SBCE fails over, the secondary Avaya SBCE broadcasts a GARP message to announce that the secondary Avaya SBCE is now receiving requests.
The GARP message announces that a new MAC address is associated with the Avaya SBCE IP address.
Devices that do not support GARP must be on a different subnet with a GARP-aware router or L3 switch to avoid direct communication.
For example, to handle GARP, branch gateways, Medpro, Crossfire, and some PBXs/IVRs must be deployed in a different network from Avaya SBCE, with a router or L3 switch.
If you do not put the Avaya SBCE interfaces on a different subnet, after failover, active calls will have a one-way audio. Devices that do not support GARP continue sending calls to the original primary Avaya SBCE.

All IP addresses configured on the Network Configuration screen are shared between both HA devices in HA deployment mode.
The HA devices are also configured with private, default IPs that are used to replicate signaling and media data between each other.
The configured interfaces are inoperative on the standby or secondary device until the device becomes active or primary.
When the devices failover, the active device sends a GARP message to update the ARP tables of the neighboring HA device to begin receiving traffic.

I don't really know what they call "GARP"? What does this mean for my network equipment? The core switch is an L3 switch.
The IP phones are connected to switches L2, L3 and L2 are connected in TRUNK.
Currently, everything works except that we are going to replace the link from ISDN to TRUNK SIP.

Should I configure something on the core switch?

[M02@rt37]

Hello @abtt-39,

By definition, GARP is a networking protocol used to update ARP tables in devices on a local area network. When a device sends out a GARP message, it's essentially announcing its presence and associating its MAC address with its IP address.

In the context of your AVAYA SBC setup for SIP trunking with high availability, GARP is crucial for seamless failover and proper communication during a failover event.

Check if your L3 core switch support GARP. Most modern networking equipment should support GARP, but it's always good to confirm this.

Check also if L3 core allow GARP messages to propagate between the VLANs where AVAYA SBC and related devices are located.

[abtt-39]

My core switch is a stack of 3850, version 16.12.05b

What command allows me to check this?

[David Ruess]

Hello,

If you are familiar with First Hop Redundancy Protocols they use the same mechanism of GARP. You can take HSRP for example. Lets say you have 2 switches/routers acting as Gateways for your devices to leave the network. One device is in Active mode replying to ARP requests and the other is in Standby just waiting for the Active node to fail. Lets also say they are connected to the same switch on different ports (Active on G0/0 and Standby on G0/1). HSRP uses the SAME MAC and the SAME IP on both devices so the transition is seamless. What's NOT seamless is the forwarding of packets when the Active goes down. The switch could still think that gateway MAC is out port G0/0 even when the Active router has failed. So the Standby device steps in and becomes Active and sends a Gratuitous ARP on its port connected to the switch G0/1 so the switch can update its CAM table and continue forwarding packets without interruption.

Hope that helps

-David

[M02@rt37]

Totally agree with you @David Ruess.

In scenarios where you have multiple devices acting as gateways or routers, FHRP mechanisms like HSRP ensure high availability and load balancing. The use of GARP helps in updating ARP and CAM tables, allowing for smooth transitions and preventing packet loss during failover events.

[abtt-39]

I put a simplified diagram, in this architecture, I imagine that there is an hsrp configured on the routers (I don't have control over it but they asked me to provide them with 2 LAN IPs + 1 IP I gave 3 IPs of VLAN 92. With virtual IP 10.39.92.222.
But I wonder. The SVI of VLAN 92 has the IP 10.39.92.254, I use the IPs of the SVI configured on the core switch as a gateway for the equipment (PC, Servers, Phone etc...) therefore in *.254. But in my diagram, which gateway should I put for the SBCs? 10.39.92.254 or 10.39.92.222?

[M02@rt37]

@abtt-39 

Gateway should be 10.39.92.254

[abtt-39]

Hello,

In my case, if i do a #sh running-config all | i gratuitous

ip gratuitous-arps non-local

WHat does that means exactly?