09-02-2008 01:55 AM - edited 03-06-2019 01:07 AM
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 ?
09-02-2008 02:32 AM
Read the below SRND for campus network design:-
http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/campover.html
09-02-2008 03:06 AM
I have looked at the doc, thank for that
However can you list the benefits of having the link in between the dist switches ?
09-02-2008 03:16 AM
You have read it all the way thru?? but you would have seen:-
http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/campover.html#wp708865
Which tells you what you need to know - and indicates the best design and why.
09-02-2008 04:20 AM
Can you provide a reference where this is suggested?
09-02-2008 05:12 AM
09-02-2008 07:12 AM
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.
09-02-2008 05:01 AM
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
ok
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.
if helpful Rate
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