What is a collapsed core, and when would you use one instead of a separate core layer?
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A collapsed core merges the core and distribution layers into one redundant pair of layer 3 switches. Access switches uplink to the pair, and the pair also connects the WAN, internet edge and servers. It fits a single building or small campus where one pair can terminate every access uplink. You add a separate core when you have several distribution blocks, usually several buildings, and meshing them directly gets impractical.
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Why it works for small sites
In one building the distribution blocks are just a few closets. A dedicated core would only connect one distribution pair to itself. Merging the layers saves two switches, the links between them and one layer of routing, with no real loss of function.
When to split out a core
- Many distribution blocks. Without a core, n blocks need n(n-1)/2 block-to-block connections to be fully meshed: 4 blocks need 6, 8 blocks need 28. With a core each block connects only to the core, so you need n.
- Several buildings. Fiber from every closet in every building to one pair gets long and expensive, and that pair becomes a site-wide failure domain.
- Port density. The pair runs out of 10G or 25G ports for access uplinks.
- Change risk. Every change to the collapsed pair touches everything. A separate core can stay untouched while you work on one block.
How I build the pair
- Two independent switches with a routed link between them, SVIs with HSRP or VRRP, and the STP root aligned with the active gateway.
- Or a virtual pair (StackWise Virtual on Catalyst 9000, VSS on older Catalyst 6500 and 4500) so each access switch connects with one multichassis EtherChannel and nothing blocks.
They will ask next
Would you use StackWise Virtual for the collapsed core pair?
It is attractive: one logical switch, one gateway, no FHRP, and access uplinks bundled with nothing blocked by STP. The cost is a single control plane. A software bug, a bad upgrade or a dual-active event when the virtual links fail can hit both chassis at once. Two independent switches keep two control planes at the price of FHRP and STP tuning. For a small office the virtual pair is usually fine, with dual-active detection configured and an upgrade plan agreed.
Traps
- Saying a collapsed core has no redundancy. It is still a pair; only the layers are merged.