White Papers

Force10 Resilient Ring Protocol (FRRP)
FRRP provides fast network convergence to Layer 2 switches interconnected in a ring topology, such as a metropolitan area
network (MAN) or large campuses. FRRP is similar to what can be achieved with the spanning tree protocol (STP), though even
with optimizations, STP can take up to 50 seconds to converge (depending on the size of network and node of failure) and may
require 4 to 5 seconds to reconverge. FRRP can converge within 150ms to 1500ms when a link in the ring breaks (depending on
network configuration).
To operate a deterministic network, a network administrator must run a protocol that converges independently of the network
size or node of failure. FRRP is a proprietary protocol that provides this flexibility, while preventing Layer 2 loops. FRRP
provides sub-second ring-failure detection and convergence/re-convergence in a Layer 2 network while eliminating the need for
running spanning-tree protocol. With its two-way path to destination configuration, FRRP provides protection against any single
link/switch failure and thus provides for greater network uptime.
Topics:
• Protocol Overview
• Implementing FRRP
• FRRP Configuration
• Troubleshooting FRRP
• Sample Configuration and Topology
• FRRP Support on VLT
Protocol Overview
FRRP is built on a ring topology.
You can configure up to 255 rings on a system. FRRP uses one Master node and multiple Transit nodes in each ring. There is no
limit to the number of nodes on a ring. The Master node is responsible for the intelligence of the Ring and monitors the status of
the Ring. The Master node checks the status of the Ring by sending ring health frames (RHF) around the Ring from its Primary
port and returning on its Secondary port. If the Master node misses three consecutive RHFs, the Master node determines the
ring to be in a failed state. The Master then sends a Topology Change RHF to the Transit Nodes informing them that the ring
has changed. This causes the Transit Nodes to flush their forwarding tables, and re-converge to the new network structure.
One port of the Master node is designated the Primary port (P) to the ring; another port is designated as the Secondary port
(S) to the ring. In normal operation, the Master node blocks the Secondary port for all non-control traffic belonging to this
FRRP group, thereby avoiding a loop in the ring, like STP. Layer 2 switching and learning mechanisms operate per existing
standards on this ring.
Each Transit node is also configured with a Primary port and a Secondary port on the ring, but the port distinction is ignored
as long as the node is configured as a Transit node. If the ring is complete, the Master node logically blocks all data traffic in
the transmit and receive directions on the Secondary port to prevent a loop. If the Master node detects a break in the ring, it
unblocks its Secondary port and allows data traffic to be transmitted and received through it. Refer to the following illustration
for a simple example of this FRRP topology. Note that ring direction is determined by the Master node’s Primary and Secondary
ports.
A virtual LAN (VLAN) is configured on all node ports in the ring. All ring ports must be members of the Member VLAN and the
Control VLAN.
The Member VLAN is the VLAN used to transmit data as described earlier.
The Control VLAN is used to perform the health checks on the ring. The Control VLAN can always pass through all ports in the
ring, including the secondary port of the Master node.
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