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Problem with MSTP and Edge Network



I was hoping someone could offer some help with this issue please?

Please see attached diagram for an overview of the network.

I have a core/distribution network made up of 4 layer 3 switches. The 4 core switches are all in the same MSTP region and the inter-switch links carry all vlans.        

I want to introduce an Edge network made up of 2 layer 3 switches. Connectivity between the edge and the core will be through layer 3 uplinks (routed). The reason for this is so that there is an IP layer on the edge network where ACL/QoS etc can be applied, creating a layer 3 boundary between the core and edge.

OSPF is used on the Core and on the Edge switches.

The problem I am having is this.

When I connect the uplinks from the edge switches to the core, MSTP always blocks all but one port connected to the edge switches. I understand that this is because there can only be one active path into the MSTP region from outside. For throughput, I wanted to have both links from each edge switch active, so that the traffic could be routed across both links.

I have considered adding the edge switches to the MSTP region so that I can create an MST-instance for each vlan and tune MSTP to have both paths open, but i'm not sure if this is the best option.

Could anyone advise on what would be considered best practice in this situation?



HTH Paul ****Please rate useful posts****
2 Replies 2

Reza Sharifi
Hall of Fame Expert Hall of Fame Expert
Hall of Fame Expert


On your edge1 and edge2 switches you have layer-3 routed links connecting to the core, but you also a corresponding vlan (100, 101, 102 and 103).  If the links are layer-3 routed, then there is no need for vlan and no link should be blocked.


Hi Reza,

The edge switches aren't Cisco, so they don't support the '(no) switchport' commands.I can only assign IP addresses to vlans, not directly to an interface.

I've had to create the 101-104 vlans so I can assign a /30 network on the uplinks.



HTH Paul ****Please rate useful posts****
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