distribution layer design

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Sep 2nd, 2008
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Hi all, A quick question, when adding a distribution layer to my network, I am joining the access to the dist and the dist to the core, however why do I need to link the distribution layer swiches together? what is the benefit of this ?

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carl_townshend Tue, 09/02/2008 - 03:06
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I have looked at the doc, thank for that

However can you list the benefits of having the link in between the dist switches ?

Joseph W. Doherty Tue, 09/02/2008 - 04:20
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Can you provide a reference where this is suggested?

Joseph W. Doherty Tue, 09/02/2008 - 07:12
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Ah, it threw me when core's were part of the original question. If we're talking about access to distribtion, and the distribtion cross links are for L2 on the access side, as shown by your diagram, Marwan's post addresses it.

Otherwise, links between distribution might be incase there was only one core device, but if it fails, still allow a backup path. Or, in some routing topologies, such as certain OSPF ABRs, they can be useful.

Marwan ALshawi Tue, 09/02/2008 - 05:01
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hi Carl

if u wondering why we need this link between two Dist Swtches basedon the layered network u mentioned

the resone is as follow

as we know the Dist switches will run a routing protocol or in general will make routing between vlan connected to it

and if we have two Dist switches and each Dist switch has two redundant links to the access switches

but there is othere topology of this access Dist design called routed access layer

in this case the Dist layer will run as L3 device while the Access layer will roun as L2/L3 device in other words will take the old Dist switch role

and now the uplink between the access layer to Dist layer will be routed interface L3 port with ip address


now the uplink fromthe Dist to the core layer L3 port as well

and Dist switch send only summary route from the access layer switches so the core layer have no idea about access layer interface only has summary route so in case of any access-to-Dist link the core will have no idea and will continue send the packect to the same Dist switch

so by adding the L3 link between the two Dist switches

in case of this access-dist link failur the even if the core continued sending the packet to the dist with link down with access the dist switch can use this link to forward the packet to the other dist switch that connected to the redundant access-switch link

also this works with normal access-dist-core topology

according to cisco srnd:

Because the distribution nodes send summarized information towards the core, an individual distribution node does not advertise loss of connectivity to a single VLAN or subnet. This means that the core does not know that it cannot send traffic to the distribution member where the link has failed. Adding an L3 link between the distribution switches allows the distribution node that loses connectivity to a given VLAN or subnet to reroute traffic across the distribution-to-distribution link. The address space selected for the distribution-to-distribution link must be within the address space being summarized to be effective.

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