Spanning vlans across access switches in distribution block.... please help

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Feb 3rd, 2009
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Hi All


Can someone please explain why Cisco states that in a Campus Hierarchical modle if Vlans are spanned across Access switches in a distribution block, then the Distrubution to distribution link should be Layer 2. Is this really necesary or just a recommendation, and if so why? Can't this link be a L3 link when spanning vlans across Access switches in distribution block, as I understand the benefit of having a L3 distribution to distribution link so that SPT is avoided.


Please help

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Elly Bornstein Tue, 02/03/2009 - 11:29
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Hmm, I am fairly sure the recommendation on Campus Hierarchical model is to have L3 between distribution switches.


It really depends on what first hop redundancy protocol you plan to run GLBP or HSRP or VRRP. Depends how far you need your vlans to span across the access layer.


GLBP is a pretty cool protocol, and it requires either a L3 link between distribution switches or blk link between distribution switches to get the full effect.


You have a link where it says that you need L2?

Sebastian Helmer Sat, 02/07/2009 - 00:39
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Hello,


The cisco recommended design is L3 links, but these is only possible if you have no vlans you need to span over the hole network.


It depends on your topology or what you want achieve.

If you need for one or more vlan's spanned the LAN, you need to use a layer 2 connection between all switches and between distribution too.


In my company we have for example a few vlans for restricted areas, like device management or else, so we can't use L3 Links in the distribution area because these vlan's are terminated at the firewall. I think these is good thing.


I would recommend you if you don't have to span one or more vlan's across the network to use L3 Links, specially in the case of redundancy way's. So you need no spanning-tree, but need to use other protocols like GLBP or else. The works faster and are not so confusing (for some people) as STP.


best regards,

Sebastian

ejohanss Sat, 02/07/2009 - 09:21
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Hello,


Say you have a topology where 5 access switches are connected in a ring. Assume sw1 is connected to distr1 and sw5 is connected to distr2. Lets say you have a L3 link between distr1 and distr2 and that traffic destined to a host on the ring is always routed via distr1 then if distr1 has a serious failure then the network will reconverge and use distr2 as backup. So far all is well however lets say that we instead have a failure on the link between sw2 and sw3 then all traffic to the ring will still be sent to distr1 which will forward the packet down the ring via sw1 unfortunately all hosts on sw3,sw4 and sw5 will be unreachable. If you instead make the link between distr1 and distr2 L2 then spanning-tree will resolve the issue with the failing link between sw2 and sw3 and allow traffic to reach all hosts on the ring.


Best Regards,

Erik


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