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core dist ,access

Level 5
Level 5


If I have core , aggregation ,  access . Access  is layer 3 ( vlan interface on access switch ).

In that case  what is the role of aggregation 


7 Replies 7

Deepak Kumar
VIP Alumni
VIP Alumni

I can't predict your network design with missing information as count of switches/devices/servers/type of network etc.


What is common use case of a aggregation/Distri layer:

The aggregation (or distribution) layer aggregates the uplinks from the access layer to the data center core. This layer is the critical point for control and application services. Security and application service devices (such as load-balancing devices, SSL offloading devices, firewalls, and IPS devices) are often deployed as a module in the aggregation layer. This design lowers TCO and reduces complexity by reducing the number of components that you need to configure and manage.


In traditional multilayer data center design, service devices that are deployed at the aggregation layer are shared among all the servers. Service devices that are deployed at the access layer provide benefit only to the servers that are directly ...

Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!

Hi @Deepak Kumar 

The topology below 

I am using cat 9500 x 48 as core and  aggregation . There are total 60 stacks . That is the only reason  the aggregation came in to the picture . Because I cannot plug all the access switches to  one switch . 


core dist aggregation.JPG

My access layer is layer 3 all the svi's are configured on access layer,  in that case in aggregation what will be the configuration 




My deployment Core will be deployed in central distribution where you have large-scale deployment spread campus across different areas. where you have fiber reachability and other cable issues around. ( OS2 or OM4)


If they are all less than specific standard distance (were you looking to deploy)


then you can deploy Collapsed Core, in this Cat 9500 48 port /x 2 = SVL,  If you do 60 stacks. (Dual home you need 120 ports for Access Layer).



My access layer is layer 3 all the svi's are configured on access layer,  in that case in aggregation what will be the configuration 

If you Do not like Layer 2 Extention all over Campus, I will deploy Access Layer as Layer 3 to eliminate Layer 2 broadcast. 


Again Layer 2 vs Layer3 feature and License model you need to look., if you looking Cat 9300 as access, essential License works, since you have only one default route to Core or Dist.


below guide explain pros and cons :


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i think you already described why would you need distri switches in first phase. let me add a few more points as you might have some ACL, firewalls, packet filtering at Distri level. but mostly we keep Core switches for fast routing/switching between your  distri/WAN/Server farm network. You distri will also advertise a summary route toward to the core switches. 


Is it possible to terminate Disri layer?

Yes, it is possible and you can go with collapsed core network design where Distri and Core are on same switch. but many factors may not suitable for this as your access switches are distributed among many buildings and fiber cost or accessibility may not allow to pull all fiber cables between buildings. Technical design also can be a part of it. 



Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!



basically, if you make an access switch a layer 3 switch, it eliminates any sort of high speed uplink to the aggregation layer switch. In that design, there is no real purpose anymore to any sort of aggregation. In general, it is bad practice to use an access switch as a layer 3 device. What exactly are you trying to design ?


Hi @Georg Pauwen 

The reason for layer 3  access  layer  to avoid common stp issues . 

My design is collapsed core . and  our requirement matches with 2x cat9500x48 switch .

Unfortunately we have around 60 IDF . So I cannot connect all edge switches to the  cat9500 due to the port density .

So I increased the quantity from 2 to 4 


The reason to choose access layer to layer3  ,  I saw  the below cisco presentation 

l2 l3.JPG

Joseph W. Doherty
Hall of Fame
Hall of Fame

"In that case what is the role of aggregation "

Including your later postings, likely its role is "needless".  (NB: often with modern equipment, you can scale pretty large using a 2 layer topology, although there are some who still believe there's some pluses to a 3 layer topology, for example, to provide some possible fire breaks if something truly runs amok.)

How many core and aggregation devices will there be?

Is it possible you could just make all your core and aggregation devices into a "Collapsed Core"?  If not too many such devices, could they be full meshed?

BTW, personally, over the decades I've often recommended Collapsed Core networks, for what many would usually recommend 3 layer networks, but since my initial recommendations, I find it interesting, Fabric Networks, with their leaf and spline topologies, also are more like Collapsed Core networks.  Or, perhaps, Nexus with FEXs, or the discontinued Catalyst IAs.  I.e. all movement away from traditional 3 layer topologies.

Basically, the object of a Collapsed Core would be to balance at load on each core device and/or minimize data transfers between them.

With your existing core and aggregation 9500s, you're very close to having a Collapsed Core already, except, perhaps, in name only.


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