Reset Search



RSTP loop on default VLAN that does not participate in RSTP.

« Go Back


TitleRSTP loop on default VLAN that does not participate in RSTP.
AnRSTP loop in thenetwork is causing high LP CPU. After disabling one of the links between the Campus and CER device as shown below, loop was subsided.

It is advised to enable RSTP on the default VLAN, put the layer 3 ports as route-only onCERas long term best practice.
Software Release: All
Fixed in Version: N/A
Here are some of the facts on the behavior of the box

1)CER, by defaultoperates in"no route-only" and all the ports by default belong to default VLAN.
In this case, we havea port configured for IP address, meaninglayer 3 port.This layer 3 port canstillreceive layer 2 packets andflood out to other ports in default VLAN.You can seeRSTP BPDUs and also Cisco PVST+ BPDUs onlayer 3 portswhich are part of default VLAN.

2) As you see below, you can configure layer 3 port as route-only. If the BPDU came from a no 'route-only' interface, it will still egress it out on layer port and may still cause problems.

NOTE: By default MLXoperates in 'route-only' mode and CER operates in 'no route-only' mode.
Some of the recommendations and best practice are as below.
  • The best practice is to have the layer 3 ports configured for "route-only" under interface config. This will drop the BPDU right when itingresses and does notegress out on default VLAN
  • Enable RSTP on default VLAN to prevent loop if some of the ports are not configured as 'route-only'
  • Add a global levelcommand "no dual-mode-default-vlan"
Additional notes



Was this article helpful?



Please tell us how we can make this article more useful.

Characters Remaining: 255