Spanning Tree

Unanswered Question
Mar 10th, 2008

I have a question regarding a simple Layer 2 setup. Current setup is ACCESS1 connects to CORE-1 and CORE-2. CORE-1 and CORE-2 both connect to an outbound GW/ROUTER. ACCESS1, CORE-1, and CORE-2 are all switches running a single VLAN.

My question is such: Being that CORE-1 and CORE-2 do not have any inter-switch links, there are no Spanning Tree Loops, so what happens b/c there are actually 2 uplinks. Are they both used and load balanced between or since the MAC address of the GW/ROUTER is learned out of a single uplink, the second would actually be in a "quasi-standby" state. Is this a way of "load balancing per destination IP address?"

I'm actually looking to migrate off this design, but want to understand what is actually going on right now.



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srue Mon, 03/10/2008 - 11:40

core1 and core2 connect to two seperate interfaces on the same router? are the ip addresses on these interfaces in the same network?

sundar.palaniappan Mon, 03/10/2008 - 12:13


As the previous poster said the core switches are connected to two different router IP interfaces, correct? If that indeed is the case then load sharing or STP has no relevance as the access switch would be learning the MAC address of the router interface(s) only from the core switch that's directly connected to that router interface.

Instead if you are bridging the two interfaces on the router then STP does come into play and one of the ports would be blocked somewhere to prevent any spanning tree loops.



jaye15394 Mon, 03/10/2008 - 12:43

One interface on the router that connects to a hub. The hub connects to both core1 and core2.

Ultimately, the question is... if there is a Layer 2 network that is looped (past the core somewhere) but all ports in the Access and Core1 and Core2 are in the forwarding state, how is that traffic treated?

Does that make sense?

A more detailed design is this:

Access1 connects to Core1 and Core2

Access2 connects to Core3 and Core4

Core1 connects to Core3

Core2 connects to Core4

An Access port on Access2 is in the blocking state. How is the traffic treated on the links connecting to Access1 when a host on Access1 is talking with a host on Access2. Is only one link used (first one to receive a response to an initial ARP request)? or are Both used somehow for load balancing?




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