Connecting Distribution Switches

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Oct 24th, 2007
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Two Quick Questions:

I am talking about the LAN Campus/user module.

What is the value in connecting routed distribution layer switches with an L2 trunk between them?

What is the value in connecting routed distribution layer switches with an L3 trunk between them?

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lamav Thu, 10/25/2007 - 04:00
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By the way, I already know that the L3 crosslinks between distro switches make HSRP possible. But why else have L3 crosslinks?

And what about L2?

Where are all the experts? :-)

ankbhasi Thu, 10/25/2007 - 04:11
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Hi Lamav,

L3 links gives you more control then l2 links in the sense of spanning your vlans across switches. You can restrict your STP on dist. switch itself instead of having stp moving across switches.



lamav Thu, 10/25/2007 - 04:19
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ankur, why would I use L2 trunks between the distro switches? is there any value in doing that?

I see networks where they have BOTH L2 and L3 crosslinks.

Kevin Dorrell Thu, 10/25/2007 - 05:12
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Isn't it the other way round?

If you have L2 links between your distribution switches then you can have their routers on the same subnet so you can run HSRP.

If you have L3 links, then you have to go for a routing solution.

If I am wrong, can you explain why I am wrong?


Kevin Dorrell


lamav Thu, 10/25/2007 - 05:51
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Yes, you are right. I got confused and didnt think it through. You need to have an L2 trunk configured between the distro switches so that the SVIs for all the configured vlans have the active router, standby router and the VIP on the same subnet. And if you have an L3 link without the L2 link between distro switches, then you will have a discontiguous subnet situation.

OK, how about a sanity check on THAT?

If what I said is correct, then why have the L3 links between the distro switches?

Kevin Dorrell Thu, 10/25/2007 - 07:06
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L3 links between the distribution switches tend tindicate that the whole architecture is layer-3, even the links to the access switches. Effectively, each access switch (router) sees two distribution layer routers on the end of point-to-point layer-3 links, and chooses the best routes from each to reach the destinations.

The cross-link is there because there might be routes present on one of the distribution routers and not on the other. It ensures full connectivity in the event of any link failure.

Kevin Dorrell


nguyenvinnie Fri, 10/26/2007 - 07:11
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I have a question, my distribution switches are connected to each other by layer 2 fiber cables and the other day when I tried to change these 2 layer 2 ports to layer 3 by removing "switchport" off from their interfaces but that didn't work well, only one of the Vlan interface show up and active in the "show standby". When I put them back to "switchport" layer 2, HSRP works OK. Did I do it right? Do I need an IP address on them to make them layer 3 ports?


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