Hi,
I am trying to understand exactly why there are interlinks (either trunks, etherchannel or layer 3 router links) between the switches in the access (or distribution) layer of Cisco's 3 layer network design model. The design states that a server can have dual NIC's which can be teamed, with one NIC conneting to switch SW-A and one to switch SW-B, which in turn connect to the disctribution switches....see the attached design.
I need to know why these links are there as, from what I can see, if any of the links or switches fail then there is still a route out without needing to use the interlink. From an earlier question I asked, these interlinks are there to allow VSS (Virtual Switching System) to be employed when using 6500 seried switches or to monitor a heartbeat between the pair, but is that the only reason? What about when there are layer 3 links between these switches in a smaller deployment?
Also, why would you choose to use layer 3 links over a layer 2 technology such as etherchannel in this situation?
Thanks for your time,
Ally
small site
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Thanks for your answer but what I mean is why are they there in the first place? Do these switches really need to talk to each other and if the answer is yes, they do, then why is that? If any of the links or switches fail the traffic will continue to flow so don't these switches actually introduce a loop that would need spanning tree if it was a layer 2 link or a use a routing protocol if layer 3.
ashexpert4u,
I realise it's for redundancy but again I still really get why they are there. If there is a failure on either of the links from the access switch to the dist' switches then the traffic will still flow out. If there is a failure of the same link then a failure of the router further down the path then that would be the only way for these links to be used, which is highly unlikely, even for organisations such as banks etc.
Is there any other situation where these links would be used?
Sorry to be so persistant but I just don't see the need.
Ally
Hi ally0000, we use this solution at my work, although the interlink is etherchannel and this solution was just to fulfill a design policy. And this is that when you have a WAN link failure on SW1, you want the traffic from the local LAN to still flow to SW1 and then through the interlink to SW2 and then uses the back up WAN link on SW2.
The only time the link from access to SW2 is used is if SW1 completely fails or the link from access to SW1 fails.
So it is a matter of design policy and i hope this is clear.
602650528, thanks for the reply and I understand your answer, thanks for that also. What about when the design is like that in the MAN design I've just uploaded, it would take 2 link failures and a failure of one of the core switches, SW-01 or SW-02?
Ally
MAN Design
Well, what I mean is when would these links between the switches ever be used? It doesn't really matter if they are L2 or L3, I just don't see when they would be used and that is my question. I just don't see the reason for them to even exist, I mean people say they are for resiliency but how do they increase resiliency. Looking at the the MAN diagram I just uploaded, the two links from SW-B to SW-01 and SW-02 would both need to fail before this link was used.
If you have any way of clarifying their use then that would be great.
Thanks
Ally
No i don't agree ally0000 if you think the design doesn't make sense but i agree with you that the link will only be used when the two links from SW-B to SW-01 and SW-02 both fail before this link was used. You are looking at issues from the best practise angle which is not always the case in reality and even in CCIE exam, best practise is not used; you just have todo things as you are instructed as as the company/network policy stated.
suppose the network/company policy states that SWA should still be the default gateway for your network even if both of its WAN links go down, this means the only route available will be the etherchannel interlink.
You will need to track those two SWA WAN links and make HSRP still use SWA as the active router.
602650528, thanks again for your prompt reply, much appreciated. So the real reason for this link is to fulfil a design or company policy, I guess that makes sense. I suppose to install these links doesn't increase the cost of the implementation too much but allows for increased availablilty of the devices so if that's the only reason then I guess thats OK.
Thanks again for your time
Ally
Business Accounts
Answer for Membership
by: ashexpert4uPosted on 2009-08-17 at 07:05:27ID: 25114548
>Also, why would you choose to use layer 3 links over a layer 2 technology such as etherchannel in this situation?
We would use the Layer 3 link for the routing purposes. Layer2 dooes not allows routing between the networks- It's basically only the switching device. For instance the use of VLAN's . VLAN's needs routing to send traffic between the VLAN's and henceforth it's only be possible if it's an layer 3 device. I hope this helps.
-Ash