help! about the three-layer architecture model

[wangyan0726]

the textbook says when the core layer consists of layer 2 switches(only two switches), if the two switches are connected with each other, then, the bridge loops will occur, who can tell me why? thanks!

[lgijssel]

The textbook should say: "may occur". This is to identify a key difference with a L3-core.

In a L2-core the switches will need to run spanning tree as they normally have multiple connection paths. The sole fact that multiple paths exist also means that a loop MAY occur although not under normal circumstances.

No spanning-tree is needed in a L3-core as we are using a routing protocol and load balancing.

Another reason is that Cisco rather likes to sell you a L3 solution because it means more $$$.

This is not in the textbook of course;-)

Regards,

Leo

[jsayer]

There are lots of other reasons to use a L3 core, such as faster convergence and multicast control. In a L2 core, there are almost always multiple physical loops, especially since most carry the core VLAN across the link between the distribution switches. Spanning tree then has to block some of the paths to create a loop-free forwarding path. While this in itself is not a really bad thing, it means that recovery from a failed link will depend on spanning-tree timers (typically 30-50 seconds). In a L3 core, spanning-tree does not get involved in the recovery process since there is no loop within any given VLAN. This means that the routing protocol determines the recovery time - typically within a few seconds, depending on the routing protocol used.

Regards,

John