Siemens RUGGEDCOM ROX II User Manual page 648

Chapter 14
Network Redundancy
Section 14.1.1
VRRP Concepts
This section describes some of the concepts important to the implementation of the Virtual Router Redundancy
Protocol (VRRP) in RUGGEDCOM ROX II.
CONTENTS
• Section 14.1.1.1, "Static Routing vs. VRRP"
• Section 14.1.1.2, "VRRP Terminology"
• Section 14.1.1.3, "Connection Synchronization"
Section 14.1.1.1
Static Routing vs. VRRP
Many network designs employ a statically configured default gateway in the network hosts. A static default
gateway is simple to configure, requires little if any overhead to run, and is supported by virtually every IP
implementation. When the Dynamic Host Configuration Protocol (DHCP) is employed, hosts may accept a
configuration for only a single default gateway.
Unfortunately, this approach creates a single point of failure. Loss of the router supplying the default gateway, or
the router's WAN connection, results in isolating the hosts that rely upon the default gateway.
There are a number of ways to provide redundant connections for the hosts. Some hosts can configure alternate
gateways while others are intelligent enough to participate in dynamic routing protocols such as the Routing
Information Protocol (RIP) or Open Shortest Path First (OSPF) routing protocol. Even when available, these
approaches are not always practical due to administrative and operation overhead.
VRRP solves the problem by allowing the establishment of a virtual router group, composed of a number of routers
that provide one gateway IP. VRRP uses an election protocol to dynamically assign responsibility for the gateway to
one of the routers in the group. This router is called the Master.
If the Master (or, optionally, a condition) fails, the alternate (or backup) routers in the group elect a new Master.
The new master owns the virtual IP address and issues a gratuitous ARP to inform the network of where the
gateway can be reached.
Since the host's default route and MAC address does not change, packet loss at the hosts is limited to the amount
of time required to elect a new router.
Section 14.1.1.2
VRRP Terminology
Each physical router running VRRP is known as a VRRP Router. Two or more VRRP Routers can be configured to
form a Virtual Router. Each VRRP Router may participate in one or more Virtual Routers.
Each Virtual Router has a user-configured Virtual Router Identifier (VRID) and a Virtual IP address or set of IP
addresses on the shared LAN. Hosts on the shared LAN are configured to use these addresses as the default
gateway.
Each router in the Virtual Router Group has a specific priority, which is a number between 1 and 255. The router
with the highest priority (or highest number) is elected the Master, while all other routers are considered Backups.
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RUGGEDCOM ROX II
CLI User Guide
VRRP Concepts