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Routing Between Switches

Posted on 2004-10-07
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Last Modified: 2010-04-17
Location 1 has 10 layer 2 switches.  Location 2 has 6 layer 2 switches.  Location 3 has 8 layer 2 switches.

Location 1 has VLAN1 and VLAN2.  Location 2 has VLAN2 and VLAN3.  Location 3 had VLAN2 and VLAN4.

Each VLAN is in its own subnet.

There is a 3750 at each location.  I want to do intervlan routing so workstations can connect across vlans at all locations.  

Is this possible if I use IP Routing on all 3750 switches and if so, how are the trunks configured?  Also, must the be ip addresses for all vlans on all switches or just for the vlans with corresponding subnets?

Thanks,
Oz
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Question by:cisdoz2
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by:lrmoore
ID: 12256002
Are all switches connected by fiber between locations?
With the 3750, you have an option to create a single switch stack. They don't have to be physically stacked, just logically. Then they all appear as just one switch with multiple "blades". Only one IP address, only one L3 vlan interface per vlan. Think of it as a single switch. Depending on the model of L3 switches, you may be able to add them to the same stack.
If you want to keep them distributed, make one the router for VLAN 1&2, another for VLAN 2&3, and another for VLAN4

If you "stack" them, then you don't have to worry about trunking or anything...
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by:cisdoz2
ID: 12259198
If the switches aren't physically stacked, wouldn't they be considered a cluster instead of a stack?
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by:lrmoore
ID: 12259349
Clusters are managed from one IP address with web interface, but still are independent of each other.
The 3750's have a different stacking technology rather than just the cluster management suite..
It's been a few months since I've played around with the 3750's, and I am getting old and gray and braincells are going awol on me, so I could be rusty.. You may have to use the stacking cables with them to put them in a stack, but I'm not sure....
Hmmm.. need ... more  .... coffee....
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Dr-IP earned 1500 total points
ID: 12259613
The neat things about the 3750 is you can stack them, and have them operate like they where one big switch, but since only one switch can operate as the master, if it goes down you have a major problem. If they where all in one location, you’d have the option of going into one of the slaves, and turning it into the master should the master switch die, but when they are spread out that isn’t so easy. It’s the reason I would configure each remote switch by it’s self, so that no mater what happens at least the campuses can operate on the local level with the lest disruption possible should one of the other switches go down.

As for the trunks, you configure them as normal, and since you want to allow all sites to see all VLAN’s you don’t prune them. When it comes to the virtual VLAN interfaces you should configure it only on the switch where the most people on that VLAN will be located. If you want to, you could try configuring hot router standby on the central switch at the demark and the local switch, I have never tried doing it on a 3750, but have it working that way on 6500’s, below is an example.

interface Vlan1
ip address 10.0.0.254 255.255.255.0
no ip redirects
standby 10 timers 5 15
standby 10 priority 110 preempt
standby 10 authentication Secret
standby 10 ip 10.0.0.1

interface Vlan1
ip address 10.0.0.253 255.255.255.0
no ip redirects
standby 10 timers 5 15
standby 10 priority 100
standby 10 authentication Secret
standby 10 ip 10.0.0.1

The first example is for the switch where the VLAN is native, and the second one is for the central switch. Priority sets which one should do the routing normally, and the preempt statement allows that switch to take over routing when it comes up should the other one be doing the routing. This all happens by the way really quick, so quick that must users will never even notice.  

 


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by:cisdoz2
ID: 12264417
Thanks Doc,
After considering the possibilities, I will configure each switch to do routing for its paticular campus to avoid the single point of failure scenario.
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Expert Comment

by:Dr-IP
ID: 12264691
Not taking in account of possible failures is one of the most common mistakes I see in designing networks. I know it’s not always practical to create a fully fault tolerant network, but you can usually minimize the impact of failures. I always think of what is the worst thing that could happen, and take it from there. Sometimes it makes for more work on the administrative side, but over the years it has served me well, as most years my network delivers five nines reliability. So keep in mind, if you can conceive of something going wrong, if you don’t plan contingences for it, it’s almost assured to happen to you.          
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