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Layer 2 redundancy between switches

Posted on 2011-09-06
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Last Modified: 2014-07-25

I need your expert help to implement this network change.

We have 2 Nortel 8600 (in layer 2 mode) switches in a zone connected between using IST link. We need to replace these 2 switches with new Cisco3750E.

My question is how to implement it on Layer 2 redundant mode. I believe the existing two has an IST (Inter Switch Trunk) link in between to provide the redundancy, if I am not wrong.

I need a similar configuration for Cisco 3750 switches.

An existing configuration
================

Switch1
========
#
# MLT CONFIGURATION
#

mlt 1 create
mlt 1 add ports 1/7-1/8
mlt 1 name "switch2_IST"
mlt 1 perform-tagging enable
mlt 1 ist create ip 10.10.10.41 vlan-id 200
mlt 1 ist enable

#
# STG CONFIGURATION
#

stg 1  add ports 1/1-1/2,1/7-1/8
stg 1  group-stp disable

Switch2
========
#
# MLT CONFIGURATION
#
mlt 1 create
mlt 1 add ports 1/7-1/8
mlt 1 name "switch1_IST"
mlt 1 perform-tagging enable
mlt 1 ist create ip 10.10.10.42 vlan-id 200
mlt 1 ist enable
# STG CONFIGURATION
#

stg 1  add ports 1/1-1/2,1/7-1/8
stg 1  group-stp disable


 
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Question by:Jaison_nj
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13 Comments
 
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Expert Comment

by:klodefactor
ID: 36493597
Would stacking the 3750E switches provide the level of redundancy you require?  Cisco's "1:N Master Redundancy" means that any switch in the stack can serve as the master.

--klodefactor
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by:
iwaxx earned 168 total points
ID: 36494102
Cisco's equivalent to Nortel "Layer2 Redundancy/IST/SMLT" is called VSS (Virtual Switching System).

And Nortel's IST link equivalent is VSL (Virtual Switching Link).
Unfortunately, VSS is not available on 3750E, only on 6500 to my knowledge.

The goal is to connect a third switch with Multichassis Etherchannel (MEC), (Nortel's equivalent is SMLT), allowing ports to be aggregated towards different physical chassis that conform a single "virtual switch" entity.
Check that link for more information:
http://www.cisco.com/en/US/products/ps9336/products_tech_note09186a0080a7c72b.shtml

Comparing to stacking, you can have your two ERS8600 or 6500 localized in two different datacenters, and all switches attached still believe there are only one.

So, as it's not available, one answer would be stacking, but if there's a great distance between your two 3750E, you'll have to connect your switches to your 3750Es with Spanning Tree, without being able of dual active links.
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Assisted Solution

by:Ernie Beek
Ernie Beek earned 166 total points
ID: 36494277
If I understand correctly you are interconnecting the two switches through two 'bundled' ports right?

With the 3750's you can do that by stacking them through the backplane. The advantage is that the backplane speed is 32Gbps and the two switches will be seen as one logical switch.

The second way is to use an etherchannel between the two switches. Here you bundle a number of ports to connect the two switches together. This can be layer two or layer three. Have a look at : http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.1_19_ea1/configuration/guide/swethchl.html

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Assisted Solution

by:SeeMeShakinMyHead
SeeMeShakinMyHead earned 166 total points
ID: 36494508
To get the level of redundancy you are requesting, stack the 2 3750 cisco switches and create a port-channel.  One of the port on the port-channel will go to one switch and the other port will go to the other switch.  On both ports, put them in the same channel-group.  Example below:

interface gi1/0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 1 mode on
interface po1
 switchport trunk encapsulation dot1q
 switchport mode trunk

interface gi2/0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 1 mode on

This will put int's gi1/0/1 and 2/0/1 in port-channel 1.  Alernatively, you can you the 10G ports if you require the higher bandwidth or put the X2 1G modules in the SFP slots if you need to use fiber due to longer distances.  Concept is the same.  

Also, if you need to prune what VLAN's are allowed (if you don't want certain VLAN's on the trunk for broadcast or security reasons), you can use "switchport trunk allowed vlan x, y, z-n" and apply that command to the port-channel interface.

Good luck!!  Let me know if I can be of further assistance.


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by:SeeMeShakinMyHead
ID: 36494515
sorry, meant GBIC slots - coffee not working yet :)
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Expert Comment

by:eeRoot
ID: 36495445
The problem with stacking is that the secondary switch will get its config from the primary.  So if the primary switch goes down due to a mistake in the config or a incorrectly typed command, it may take the secondary switch down with it.  If you use HSRP, then the switches are independent of each other.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_37_se/configuration/guide/swhsrp.html
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Expert Comment

by:SeeMeShakinMyHead
ID: 36495575
I believe the goal is to not mistype your config and put all changes through change mgmt :)  However, you can use HSRP between the two switches if you would like (stacking is better option unless distance b/t the two switches is an issue).  Did you want to make the switches L2 or L3.  That will later the configs a bit.  If you make them L3, then you don't have to worry about spanning-tree, but VLAN's will be independant on each switch.  If you make them L2, then you can trunk the two switches together with a port-channel b/t the two switches and use HSRP.
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Author Comment

by:Jaison_nj
ID: 36507688
The old Nortel 8600 switches are pure L2 and they have different configuration port wise. Like one IBM X series server has a Quad NIC card where 2 ports are connected to the Switch1 and other 2 ports to the Switch2.

I need to convert them with CIsco 3750 same as L2 with high availability.

I think HSRP works on L3 not on L2. So what would be the ideal solution.
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Expert Comment

by:eeRoot
ID: 36508224
If you want a L2 solution, then spanning tree or stacking would be the 2 best options.  Stacking would be faster, but with spanning-tree the switches are truly separate and not dependent on each other
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Expert Comment

by:SeeMeShakinMyHead
ID: 36509720
ok, so don't want to do routing on the 2 3750's then.  Then HSRP is out of the quetion then.  You could do spanning-tree if you don't want to stack the switches but make sure you use rapid spanning-tree (it is much faster as it doesn't go through all the spanning-tree states).  I personally would stack them if the switches are going to be right next to each other physically.  Spanning-tree can be a little tricky as you don't want a leaf switch becoming the root.  You should set the priority so that the two switches on the top will be designated root and backup.  
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Expert Comment

by:klodefactor
ID: 36511166
Stacking connections typically have much higher bandwidth than a single GigE port between switches.  You could always bond multiple GigE connections (e.g. EtherChannel) but that makes things more complicated again.

If your switches already have stacking modules or have stacking ports built in, I'd stack them.

--klodefactor
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