Justifying Routing at the Edge

VTP, Portfast, Spanning Tree and all the other switch related stuff.
Guest

Justifying Routing at the Edge

Post by Guest » Sun Nov 26, 2006 8:06 am

We are currently creating all our VLANS on our core 6513 sup2 720. One of our VLANS has over 19 class c subnets on it. Last week after a power outage, the cores processor went up to 92% until I looked on the wire and saw that DHCP was going mad! We restarted the DHCP service and all was well again. The DHCP server is in the VLAN that has all those subnets. Im trying to convince my boss that it is wise to move all the vlans out to the edge onto the distribution switches and have a set of VLANS per building. Im sure my preferred scenario would have stopped this problem, which I think was a massive broadcast storm. Would you all agree? Jamie

Guest

Re:Justifying Routing at the Edge

Post by Guest » Sun Nov 26, 2006 8:30 am

jhancockuwic wrote: We are currently creating all our VLANS on our core (6513 sup2 720). One of our VLANS has over 19 class c subnets on it. Last week after a power outage, the cores processor went up to 92% until I looked on the wire and saw that DHCP was going mad! We restarted the DHCP service and all was well again. The DHCP server is in the VLAN that has all those subnets. Im trying to convince my boss that it is wise to move all the vlans out to the edge onto the distribution switches and have a set of VLANS per building. Im sure my preferred scenario would have stopped this problem, which I think was a massive broadcast storm. Would you all agree? Jamie Jamie It would be a very good idea not to have such large vlans ie. 19 class C subnets, as that is the root cause of your problem and not where the inter-vlan routing is being done. However If you have multiple buildings interconnected via the core 6500(s) then yes it would be a good idea to have each buliding use its distribution switch(s) to do the inter-vlan routing assuming of course that you have L3 capable distribution switches in each building ? Jon

Guest

Re:Justifying Routing at the Edge

Post by Guest » Sun Nov 26, 2006 9:57 am

  Hi Jon, Yeah I thought so, unfortunately Im the new kid so its not always easy to get my point across.  Going to try and put my point across tomorrow. Oh, and yeas we have layer 3 capable switches at the distribution. Jamie

Guest

Re:Justifying Routing at the Edge

Post by Guest » Sun Nov 26, 2006 11:21 am

Hello Jamie, having 19 IP subnets /24 sharing the same L2 broadcast domain is a bad idea as you have seen on that outage. When a broadcast is sent it needs to be processed by all devices including those that actually are not interested on it.also cpu usage on user PCs suffer from this.There is a relationship between number of hosts and broadcast level in a vlan/L2 broadcast domain. You should have a one to one corrispondence between one IP subnet and one vlan/L2 broadcast domain. if the total number of vlans is high and you cannot scale moving the L3 boundary towards access is a wise step. if you have only few vlans you can create more vlans easily and you move users to them Hope to helpGiuseppe

Guest

Re:Justifying Routing at the Edge

Post by Guest » Sun Nov 26, 2006 11:53 am

Hi Giuseppe Thanks for your help. We have over 20 vlans per site. Can you explaine this point please "if you have only few vlans you can create more vlans easily and you move users to them"? Jamie

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