need to implement a new network

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Jan 28th, 2008
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I have a dilemma,

I need to implement a new network a Core, distribution and access Layer

The core is exciting from two 3845 routers talking EIGRP the backbone, and connected to the distribution layer via gigabit Ethernet to two 3750G12S-E switches(the 2 distribution switches share a crosslink between them ,the access switches are connected with gigabit Ethernet to each distribution switch,


My question is what is a better configuration?

A router on a stick approach (sub interfaces on the 3845 routers and with VTP configuration,

Or EIGRP on the 3750G switches and the 3845 routers and VTP configuration

Thank you for your answer

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Tony.henry Tue, 01/29/2008 - 16:42
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Hard to answer with little back ground information. Which means that in the end it depends on the requirements.

I'd rather see segregation between core and distribution/access layers and that usually means I'd let the 3750G do the local routing the 3845 do WAN routing and summarise the site.



igonda Sun, 02/03/2008 - 11:16
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Hi Tony,

Sorry for the slow response i had problems with login in,

I've attached a drawing this way it's easier to communicate this way,

the 3845 Is communicating with the WAN(EIGRP), gi0/1 interface on the3845 has sub interfaces configured ( each sub interface is a vlan),and is attached to the 3750,

HSRP is also is configured on every sub interface.

The 3750 and the access switches are all in the same VTP domain,

Is there any way to make this design better ?

thanks again


Tony.henry Sun, 02/03/2008 - 14:11
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No problems.

Layer 2 looks OK. Stacking the switches wouldn't be a bad option, I think it would help with spanning tree issues should something go wrong. I'm not sure it would improve things greatly though it would improve management of the network.

Layer 3 I'm having problems with, I'd rather you place all your local routing on the 3750's and have the 3845 aggregate the site do the WAN routing. As Joseph suggested.

Where is the server farm? Is there a requirement for WAN users to get to the farm? What percentage of the LAN users access something off the WAN?



Joseph W. Doherty Tue, 01/29/2008 - 17:14
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I suggest you stack your 3750s and use them as a collapsed L3 distribution/core. Reserve your 3845s for WAN edge.

igonda Sun, 02/03/2008 - 11:19
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hI Joseph

I've uploaded a drawing, see my reply to Toni

I do not see the advantage of Stacking of the 3750 yet

Sorry for the late reply

Joseph W. Doherty Sun, 02/03/2008 - 12:53
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Assuming your 3750s on the edge are only doing L2, the major advantage of stacking the distribution/core 3750s, it avoids unicast flooding.

Other reasons are minor, such as one device to manage instead of two, increased bandwidth between the two, perhaps better L2 loadsharing since, I believe, 3270s don't support GLBP.


Whether you chose to stack or not, if you have an extra Ethernet interface on your 3845s, suggest they have separate links to both 3750s.

dongdongliu Tue, 01/29/2008 - 18:41
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maybe you can run route protocol between 3750 and 3845, and running stp betweec 3750 and access SW.

to stack 3750 is a good idea, I think, if you have bought stack cable.

igonda Sun, 02/03/2008 - 11:21
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hi ,thank you for your reply

this was my first approach , but does this not make things complicated ?

ive added an drawing (please see my comment to Toni)




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