Cisco router/ VLAN question

Posted on 2006-04-03
Last Modified: 2008-02-01
Hi All,

Our network is currently contained in one subnet,, across one site. We have an ISP managed Cisco 2500 router, then a PIX 506e Firewall, then 4  3Com 4228G switches which are connected to the patch panel/servers.

We have contractors and visitors to the office who sometimes need access to a printer and the internet (e.g. to check their OWA). We need to replace some of the 3Com switches anyway, so I was thinking of purchasing some Cisco 2950 switches and creating a seperate VLAN on for these contractors. Couple of questions -

i) If I wanted to enable inter-VLAN routing, are there any recommendations to what router I should purchased? We have only about 40 workstations in the office, so it wouldn't need to be heavy use.
ii) For visitors' printing, I could either add a printer to the visitors VLAN or allow them access to the print server on If I went with the second option and enabled inter-VLAN routing, to deny access to all other machines I was thinking of using access lists on the inter-VLAN router to allow only communications to the print server. Can you use access lists between VLAN's, or only between networks?
iii)We'll still keep about 2 of our existing 3Com switches for the time being. All hosts on the visitors VLAN will be connected to one 2950 only, so all ports on the 3Coms will be in the same (default) VLAN. But, are there any known issues between 3Com/ Cisco switches (autonegotiation etc)?

Many thanks in advance.
Question by:Dilan77
    LVL 50

    Accepted Solution

    i) Rather than get a 2950 and a router, I would go with a 3550 (or current model equivalent; 3750, 3650). That would combine the L2 and L3 function into a single box.
    ii) Since a VLAN is network, an ACL would be the way to control traffic between networks (VLANs).
    iii) I'm not a big fan of autonegotiation. Expecially between different vendors. Set speed and duplex manually and you shouldn't have any problems.

    LVL 14

    Assisted Solution

    i) donjohnston is pretty much on the money with the 3750 recommendation; you would want switch longevity.  Most devices are moving away from 10/100MB and are beginning to move towards 1 GB NICs.  
    Added L3 functionaility depending on your needs and the switch you buy would be advantageous.

    ii) A VLAN separates Layer 2 traffic of your switch fabric into logically distinct layer 2 flow groups.   The ACLs would be a Layer 3 function that can be applied to the interfaces ingressing or egressing those subnetworks (VLAN or otherwise) for the purpose of restricting specific traffic flow. So yeah, L3 between connected VLANs or subnetworks.

    iii) You may want to keep the flow set at half duplex, 100MB as the fastest option.  I had an issue back in 2001, where full duplex communication between Cisco and 3com did some funky things to a database app when data was updated between user and app.  We were forever repairing the database until we realized what was happening.  Otherwise,  donjohnston is right about autonegotiation between vendors.  The way they implement negotiation can cause odd side effects because of they way their algorithm interacts.

    LVL 2

    Author Comment

    Thanks for the replies.
    I can see myself having headaches keeping a mixed 3Com/ Cisco switched network, so may just replace everything with Cisco kit.

    At the moment, we have 4 3Com switches. If I was to replace these with 3 2950's and one 3550 (or 3750, 3650 etc), could I still have inter-VLAN routing, or do either all the switches have to be 3550s or all 2950's and one 'real' Cisco router?


    LVL 50

    Expert Comment

    by:Don Johnston
    Three 2950's and one 3550 would work fine.

    That's how I would do it.

    LVL 2

    Author Comment

    Thanks guys...appreciated the help.

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