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What is the different between clustering and ISL for a cisco switch?

I mean what is the cluster function of Cisco? Only for management? That means two switch in one cluster is still two switch?

What about ISL? If I connect two switch with ISL are they still treated as two separate switch? I ask this question because as you might already knew that 802.3ad only works on single switch scenario and I would like to know if I connect two switch using ISL and then connect the ethernet port of a dual port server each to a separate switch, can I enable 802.3ad then?

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migrated from stackoverflow.com Aug 7 '11 at 20:43

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1 Answer 1

ISL and 802.1q are two methods to run VLAN's between switches. ISL is a Cisco proprietary format, and 802.1q is an industry standard. If you connect ISL between two switches, you are just allowing yourself to create multiple "virtual" interfaces, one for each VLAN, and route traffic on it. The switches are still considered two separate switches, and it is not clustered.

To "cluster" a layer 3 switch or router, you can use VRRP or HSRP. For example, Hot Standby Router Protocol (HSRP) basically "clusters" an IP address. So if one router dies, the other router in the HSRP group will assume the "clustered" IP address (i.e. such as an outgoing gateway address) and continue working. This won't work on a layer 2 switch though, as it is not routing by IP address.

802.3ad is a way to aggregate multiple physical connections into one logical connection. Cisco sometimes calls this an "Etherchannel". So for example, you can configure five cat6 cables connected at gigabit speeds to be in one etherchannel, and it would give you, in essense, 5 Gb/sec speed between the switches. This will allow you to lose a cable here and there without going offline, just having a reduction in the potential speed. I wouldn't call this clustering though.

To answer your question, you can plug in a dual port from a server to a switch using 802.3ad. I have done this before with Intel NIC cards. The tricky bit here is making sure the server NIC is configured properly. You can test it by setting up a ping, and then pulling each network cable individually while making the sure ping is not interrupted.

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