13.5.3 Switching due to an upstream network failure in Layer 3 redundancy switching

For the upstream network, configure IP routing and do not use GRSP to control the network. When you use Layer 3 redundancy switching, IP routing detects failures in the upstream network and performs path switching as necessary.

Two GSRP switches must be connected to the upstream network. To assure continued communication with the upstream network, secure a communication path between the GSRP switches. By doing so, traffic can pass through the active backup switch even if the master GSRP switch fails due to a failure such a port failure.

The following figures provide an overview of the configuration necessary to handle failures occurring on the upstream network and an example communication path to be used in case of a failure.

Figure 13-13: Settings corresponding to upstream network failures

Figure 13-14: Communication path when an upstream network failure occurs

<Structure of this section>

(1) Upstream network settings

Use the following methods to allow both the master and backup GSRP switches to communicate with the upstream network through the ports or VLANs on the GSRP switches.

Configure the ports on the GSRP switches to which the upstream network is connected as ports not under GSRP (gsrp exception-port configuration command).
Use the GSRP VLAN group-only control functionality (gsrp limit-control configuration command) to configure the VLANs on the GSRP switches to which the upstream network is connected as VLANs not belonging to any VLAN group and therefore not under GSRP control.

Assign IP addresses to the ports and VLANs, and configure IP routing to connect to the upstream network.

Configure IP routing so that both GSRP switches are able to communicate with the upstream network. In addition, configure dynamic monitoring of dynamic routing or static routing to detect failures in the upstream network.

Normally, both GSRP switches directly communicate with the upstream network. If a link between one switch and the upstream network fails, the failed switch uses the direct link with the other switch to continue communication with the upstream network. This becomes possible by configuring IP routing to assign a lower priority to the route to the upstream network via the neighbor GSRP switch. For static routing, configure dynamic monitoring to periodically check for the arrival of packets and to detect failures.

(2) Configuring GSRP Switches

Because the upstream network is connected to both GSRP switches, the backup GSRP switch might receive packets from the upstream network. To relay these packets to the master GSRP switch, configure a Layer 3 communication path between the GSRP switches.

The GSRP switches are thus connected by a direct link and exchange GSRP Advertise frames over the GSRP-managed VLAN. You can also configure a VLAN other than a GSRP-managed VLAN and IP routing on this direct link to relay packets between the GSRP switches. However, if you do so, configure IP routing to assign a lower priority to this communication path when it is used to directly forward traffic from downstream to the upstream network.