Failover in ring configuration

[bschear]

I have 7 3750 and 1 3560 at 8 locations which is not being connected by a fiber ring so that switch number 1 connects to 2 and could also connect to switch 8. I am looking for the best configuration so I can have failover back around the ring if connectivity is lost between two switches.

[Giuseppe Larosa]

Hello Brian,

L2 solutions using Rapid STP (Rapid PVST or 802.1s MST) have been reported.

The suggestion coming from Cisco STP expert Francois Tallet is to increase the max-age parameter to be sure that in case of failure the switches can build a single topology.

(max-age is also used to decide if a received message is acceptable or not)

Standard timers values are for up to 7 switches and max-age is 20 seconds increase it to 24 seconds and you should be fine.

The addition of STP loop guard can be of help in case of misbeaving links (undirectional links, by the way UDLD can be too slow for this with Rapid STP).

Choice the primary and secondary root bridge using commands to modify bridge priority.

L3 solutions using dynamic routing protocols are possible as well and using EIGRP or OSPF combined with ip fast convergence can provide sub-second convergence.

Hope to help

Giuseppe

[bschear]

Thank you. I know configuring STP is one option. Let me give a little bit more of the current configuration to see what people think fits the situation best. If anyone has any configuration examples that match the specific situation that always seems really helpful to see a configuration of a similar situation or how it should be configured. There is a backbone vlan lets say vlan 50 that all the ports which connect the switches to each are in with just the commands switchport mode access and switchport access vlan 50. IP Routing is enabled on each of the switches and each switch has at least two other VLans on it, one for data and one for voice. So for example switch one would have vlan 2 for data, vlan 3 for voice, and vlan 50 that the ports connecting the switches are in. The switch has static routes to networks in the other locations pointing to the address in vlan 50 on the other switch which then routes it to the appropriate vlan in that location.

[Giuseppe Larosa]

Hello Brian,

if you use static routes you rely on STP for convergence.

I would use a dynamic routing protocol in addition to rapid STP this would allow all devices to detect possible failures at layer3. In a LAN segment until a valid ARP entry exists for the static route IP next-hop (up to 4 hours with default timers) a static route is considered valid even if the remote device has failed.

This is clearly a non desirable behaviour wasting ring bandwidth for traffic that cannot be received.

Hope to help

Giuseppe