Describing High Availability in Multilayer Switching
Layer 2 and 3 redundancy alignment

When implementing strategies for failover at the access and distribution layers it is important that the failover paths and timers are aligned between Layer 2 failover protocol (STP) and Layer 3 failover protocol (HSRP or GLBP). This would be most significant if the link between the distribution switches was a Layer 2 link and therefore hosting a redundant Layer 2 path for the VLANs in the Access layer. Although it is a Layer 3 link, alignment of the protocols is still a best practice in the event that a rogue switch is placed on the network.

In Figure , the distribution switch configured as the HSRP Active router for VLANs 12 and 120 is also configured as the STP primary root for the same VLANs. The second distribution switch serves as the HSRP standby and STP secondary root for those VLANs.

Likewise, the other distribution switch is configured as the HSRP Active router for VLANs 11 and 110 and is also configured as the STP primary root for the same VLANs. The second distribution switch serves as the HSRP standby and STP secondary root for VLANs 11 and 110.

It is important that the timers of STP and HSRP agree providing failover at recovery at the nearly the same time. This would require the implementation of RSTP on all access and distribution switches.

Autostate Layer 3 Convergence during Layer 2 Failure
The autostate feature notifies a switch or routing module VLAN interface (Layer 3 interface) to transition to up and up status when at least one Layer 2 port becomes active in that VLAN.

Autostate also senses the STP forwarding state of ports associated with VLAN id, this will prevent routing protocols and other features from using the VLAN interface as if it were fully operational.

To operate correctly there should not be any local ports with the VLAN id that are NOT offering a connection directly to the access switch which has that VLAN configured.

  • Trunk links which have the VLAN id are assumed to provide a path the and will keep interface up
  • Access ports with the VLAN id will also keep VLAN interface up.

An example of a problem would be if a trunk link to an access switch which only had VLAN 12 and 14 associated with it, had its trunk configured to carry all VLANs. This trunk would appear to the autostate process to provide a path every active VLAN and hence local VLAN interfaces for 12 and 14 would never be shutdown because this trunk appears to provide a path.

Affect of Layer 3 Failure with Autostate
Using the trunk range command will ensure appropriate action of the VLAN interface to a loss of physical connectivity. Having discussed the process of autostate we can now discuss the effects of a failure on IP traffic. For the following discussion we will assume that the distribution nodes are summarizing.

When the Layer 2 trunk between SW A and SW C fails, physical connectivity to VLAN 11 is lost on SW A. This is because the trunks are properly configured so autostate will detect that there is no longer any ports active for VLAN 11 and the VLAN 11 interface will shutdown on SW A and the directly connected route to VLAN 11 will be removed from the routing table.

This has the benefit of

  • The distribution switch will replace its directly connected route to VLAN 11 with the route to VLAN 11 being advertised by SW B across the Layer 3 link.
  • When return path traffic arrives on the distribution switch SW A destined for VLAN 11, it will be routed toward the access layer through SW B.
  • Because summarization is taking place, no external network routing update has been propagated into the core.

If the VLAN interface had not shutdown, then the IP return path traffic would have be lost at SW A. This is sometimes referred to as being ‘black holed’