Chapter 15. VRRP

Table of Contents

The FB2500 supports VRRP (Virtual Router Redundancy Protocol), which is a system that provides routing redundancy, by enabling more than one hardware device on a network to act as a gateway for routing traffic. Hardware redundancy means VRRP can provide resilience in the event of device failure, by allowing a backup device to automatically assume the role of actively routing traffic.

15.1. Virtual Routers

VRRP abstracts a group of routers using the concept of a virtual router, which has a virtual IP address. The IP address is virtual in the sense that it is associated with more than one hardware device, and can 'move' between devices automatically.

The virtual IP address normally differs from the real IP address of any of the group members, but it can be the real address of the master router if you prefer (e.g. if short of IP addresses).

You can have multiple virtual routers on the same LAN at the same time, so there is a Virtual Router Identifier (VRID) that is used to distinguish them. The default VRID used by the FB2500 is 42. You must set all devices that are part of the same group (virtual router) to the same VRID, and this VRID must differ from that used by any other virtual routers on the same LAN. Typically you would only have one virtual router on any given LAN, so the default of 42 does not normally need changing.

Note

You can use the same VRID on different port groups without a clash in any way in the FB2500. However, you cannot use the same VRID on different VLANs on the same port group, as the internal switch in the FB2500 will only track the MAC address to one port at a time. You may also find some switches and some operating systems do not work well and get confused about the same MAC appearing on different interfaces and VLANs. As such it is generally a good idea to avoid doing this unless you are sure your network will cope. i.e. use different VRIDs on different VLANs.

At any one time, one physical device is the master and is handling all the traffic sent to the virtual IP address. If the master fails, a backup takes over, and this process is transparent to other devices, which do not need to be aware of the change.

The members of the group communicate with each other using multicast IP packets.

The transparency to device failure is implemented by having group members all capable of receiving traffic addressed to the same single MAC address. A special MAC address is used, 00-00-5E-00-01-XX, where XX is the VRID or VRRPv2, and 00-00-5E-00-02-XX for VRRPv3.

The master device will reply with this MAC address when an ARP request is sent for the virtual router's IP address.

Since the MAC address associated with the virtual IP address does not change, ARP cache entries in other devices remain valid throughout the master / backup switch-over, and other devices are not even aware that the switch has happened, apart from a short 'black-hole' period until the backup starts routing.

When there is a switch-over, the VRRP packets that are multicast are sent from this special MAC, so network switches will automatically modify internal MAC forwarding tables, and start switching traffic to the appropriate physical ports for the physical router that is taking up the active routing role.

Note

You can disable the use of the special MAC if you wish, and use a normal FireBrick MAC. However, this can lead to problems in some cases.


14.3. Packet Dumping		15.2. Configuring VRRP