Routed Access Design

[SE zimm]

I currently have at the distribution layer two 6509's configured with HSRP. I have 10 2950's and 5 3750g switches in the access layer using RSTP. I want to move to a routed access design but don't have the funding currently to replace all of the 2950's. So my question is does anybody run a mixed routed access and switched access layer? Would there be any issues with running a mixed network?

[Leo Laohoo]

If it can't be done once, don't do it.

Depending on how it's done.

6500 (routed link) < ---- > (routed link) 3750 (layer 2) < ---- > (layer 2) 2950

6500 is the core/redundant switch, 3750 is the access switch and  2950 as the access switch which hangs off from the 3750.

[SE zimm]

So the redundant 6509's are the distribution and the 2950 does not hang off the 3750. Its more of the 3750's support one floor and 2950 support a different floor. So the 2950 has 2 uplinks to the distribution layer and the 3750 has the same. So I am sure that this can be done but I was wondering if it was bad practice to have a mixed network with routed access and switched access?

[Richard Burts]

Matt

I think that most of us would agree that ideally the implementation of a layer would be consistent across the layer (all routed if you adopt the routed approach, or all switched). But if you can not implement it consistently over the entire access layer I do not think that it would be bad to start with what you have got. I do not see it as a problem to start using routing for the 3750s while you keep the 2950s as switching platforms. Over time, as you are able, replace the 2950s to achieve a consistently routed access layer.

In thinking about this a bit more perhaps we should ask for some more information about the network which might impact the decision about a partially routed and partially switched access layer. For example: are there any VLANs where the same VLAN (the same broadcast domain) is present in a 2950 and in a 3750? This would certainly complicate adoption of the mixed environment.

HTH

Rick

[Jon Marshall]

matt.zimmerman@gmail.com
So the redundant 6509's are the distribution and the 2950 does not hang off the 3750. Its more of the 3750's support one floor and 2950 support a different floor. So the 2950 has 2 uplinks to the distribution layer and the 3750 has the same. So I am sure that this can be done but I was wondering if it was bad practice to have a mixed network with routed access and switched access?

Matt

To expand on Ricks point.

With the 3750 switches you can use routed to the 6500 layer switches and to manage the actual 3750 switches themselves you can use loopbacks rather than a common L2 management vlan. This presupposes that the vlans you have on one 3750 are not needed on any of the other 3750 switches, the 6500s or the 2950s ie. you can isolate each vlan(s) to the relevant 3750 switches.

With the 2950 switches you would need a management vlan for the switches but if you could then limit the vlan(s) on each 2950 switch you could route them off the 6500 switches but only allow the relevant vlan(s) per 2950 plus the common management vlan across the L2 uplinks. If you then purchase 3750s to replace them it is a simple matter to migrate the L3 vlan interface(s) for the data vlans to the 3750s and use a loopback per 3750 to manage the switch.

For all of the above you need to make sure that you do not need a common L2 vlan across multiple access-layer switches.

Jon

[cowetacoit]

i've been down this road before and moving to a routed access layer in a phased approach works great. we eventually phased out all of

our layer 2 connections by replacing a 2960 or making the link from layer 2 to layer 3 on a 3560/3750 one at a time. Its very easy to move your SVI off of the core and onto a 3560/3750 and making a layer 3 p2p as you replace your 2960s.