13.4.1 Overview

Layer 3 redundancy switching allows two switches to be switched over using the same IP addresses and same MAC address. This way, PCs can continuously send and receive traffic via the default gateway without stopping.

The following figure provides an overview of GSRP Layer 3 redundancy switching. In this example, the network containing the PCs is called the downstream network. The network that receives the IP packets forwarded from the downstream network is called the upstream network. GSRP master/backup switchover affects the downstream network.

Figure 13-8: Outline of GSRP Layer 3 redundancy switching function

<Structure of this section>

(1) Default gateway IP address

When you use GSRP to provide redundancy for the default gateway, assign the same IP address to the same VLAN on each paired GSRP switch. On the master GSRP switch, VLANs are enabled. The master GSRP switch forwards IP packets as the default gateway. On the backup GSRP switch, VLANs are disabled. The backup GSRP switch does not forward IP packets.

(2) MAC addressing for the default gateway

When you use GSRP to provide redundancy for the default gateway, a GSRP-specific virtual MAC address is used as the MAC address of the default gateway. A different virtual MAC address is assigned to each VLAN group ID.

The master switch periodically sends a GSRP control frame (a frame for virtual MAC address learning) containing its virtual MAC address as the source MAC address to the lower-level LAN switches so that they can learn the virtual MAC address of the master switch.

The figure and table below describe the format and components of the virtual MAC addresses used by GSRP.

When the VLAN group ID is not greater than 8, a virtual MAC address is generated by using the method described below.

Figure 13-9: How to generate a virtual MAC for GSRP Layer 3 redundancy switching facility (VLAN grouping ID is 8 or less)

Table 13-5 How to generate a virtual MAC for GSRP Layer 3 redundancy switching facility (VLAN grouping ID is 8 or less)
Item	Code
GSRP group ID	Sets 0 to 3 for GSRP grouping ID1 to 4. For Layer 3 redundancy switching, GSRP grouping ID must be between 1 and 4.
VLAN group ID	Sets 0 to 7 for VLAN grouping ID1 to 8.
Fixed (3 bits)	The three least significant bits are fixed at 7.

If VLAN group ID is 9 or higher, assign virtual MAC addresses in the range 0000.8758.1311 to 0000.8758.1350 to ID 9~64 of VLAN group in order.

(3) Transmit VLAN of frames for virtual MAC address learning and transmission interval

A virtual MAC address-learning frame is sent in units of VLAN ports that belong to VLAN group in the master status at the specified transmit interval. The number of frames (transmission rate) to be transmitted per second is determined so that frames can be transmitted to the target VLAN port at the set transmission interval. The transmission rate automatically fluctuates within 100pps according to the following formula. Frames are not transmitted when the send rate exceeds 100pps.

[Formula for calculating the transmit rate of frames for learning virtual MAC addresses]

Transmit rate (pps) = number of VLAN ports/transmission interval (seconds)

For example, if the sending interval is set to 5 seconds for VLAN ports to be transmitted 200(VLAN ports), the sending rate will be 40pps.

Note that when the transmission rate is calculated and 100pps is exceeded, there are VLAN ports that do not transmit frames for virtual MAC addresslearning.