Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Community Member

Switch Virtual Interface (SVI)

What is the purpose of an SVI and can I have two SVIs on the same vlan, but different switches trunked together?

SW#1:

inter vlan 150

ip address 192.168.0.1 255.255.255.0

SW#2:

inter vlan 150

ip address 192.168.0.2 255.255.255.0

Thanks in advance.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Gold

Re: Switch Virtual Interface (SVI)

David

Your follow up question was how a host would know which gateway to use. And the answer is that each host would be configured with a default gateway. Who ever configured the host would choose 192.168.0.1 or they would choose 192.168.0.2. And whatever gateway was configured will be the one that the host uses.

I would guess that what your CCIE was talking about was to systematically alter configuring the hosts so that half of them use 192.168.0.1 and the other half use 192.168.0.2. This would produce load balancing where half of the hosts use one switch while the other half use the other switch. This is essentially a manual solution to the question of how to load balance.

HTH

Rick

11 REPLIES
Hall of Fame Super Gold

Re: Switch Virtual Interface (SVI)

David

The purpose of an SVI is to be a layer 3 interface for processing a particular VLAN on the switch.

Yes it is quite possible to have interface vlan 150 on both switch 1 and switch 2, as you show, as long as each SVI has a unique address within the subnet.

HTH

Rick

Community Member

Re: Switch Virtual Interface (SVI)

Rick,

If a host with a single gateway is connected to VLAN 150, how will it know which SVI to route out?

An CCIE told me that it would be good for load balancing? Sorry, I'm just trying to get a grasp.

Re: Switch Virtual Interface (SVI)

David,

Using HSRP as an example, a virtual ip address is assigned to your SVIs and these are used as the gateway for your hosts:

SW1#

int vlan 150

ip address 192.168.1.2 255.255.255.0

standby 150 ip 192.168.1.1

standby 150 preempt

int vlan 150

ip address 192.168.1.3 255.255.255.0

standby 150 ip 192.168.1.1

standby 150 preempt

The hosts use 192.168.1.1 as their gateway, and there is one switch that's a primary and the other is the standby. If one switch goes down, the other will answer for it.

In the case of GLBP, it can load balance links whereas HSRP doesn't (it can, but not without creating more ips and having hosts use different gateways).

HTH,

John

HTH, John *** Please rate all useful posts ***

Re: Switch Virtual Interface (SVI)

David,

I completely misread your post, so if you received my response, please disregard. I thought you were asking if two SVIs on the SAME switch could exist for the same vlan with ip addresses in the same subnet. Rick is right, and what you have will work. In fact, that's the way it should be in the case of using HSRP.

Sorry for the confusion :)

John

HTH, John *** Please rate all useful posts ***
Blue

Re: Switch Virtual Interface (SVI)

David:

"Rick,

If a host with a single gateway is connected to VLAN 150, how will it know which SVI to route out?"

You can implement a redundancy solution, where the hosts on a LAN will not point to any particular router's SVI IP address, but instead to a virtual IP address that both routers may have control over.

The most common solution is known as HSRP, in which both routers are placed in a redundnacy "standby" group, but only one will control the virtual IP address (VIP) at any given time. If that router fails, the other takes over.

This is why you do not point the hosts on the LAN to a particular SVI, but to the HSRP virtual IP address that they both share. In other words, the default gateway of the hosts will be the VIP.

In the case of load balancing, there is another first hop redundancy protocol known as GLBP, which allows all routers in the redundnacy group to receive user traffic and forward it simultaneously. This utilizes network bandwidth more efficiently since both routers can use their connections and forwarding capability at the same time, instead of one at a time, as is the case with HSRP.

HTH

Victor

Community Member

Re: Switch Virtual Interface (SVI)

Rick, John, and Victor.

I understand HSRP and I pointed out to the CCIE engineer. He said NO HSRP, but load balancing. I ask him to clarify, but his response was like a foreign language to me.

Re: Switch Virtual Interface (SVI)

David,

The only load balancing protocol that also provides redundancy is GLBP.

John

HTH, John *** Please rate all useful posts ***
Blue

Re: Switch Virtual Interface (SVI)

David:

I cannot speak for your CCIE. I wasnt there. I can only guess that if he mentioned "load balancing" in the scenario with which you present us, he was probably referring to GLBP.

Read my post.

From above post:

"In the case of load balancing, there is another first hop redundancy protocol known as GLBP, which allows all routers in the redundnacy group to receive user traffic and forward it simultaneously. This utilizes network bandwidth more efficiently since both routers can use their connections and forwarding capability at the same time, instead of one at a time, as is the case with HSRP."

Thanks

Victor

Hall of Fame Super Gold

Re: Switch Virtual Interface (SVI)

David

Your follow up question was how a host would know which gateway to use. And the answer is that each host would be configured with a default gateway. Who ever configured the host would choose 192.168.0.1 or they would choose 192.168.0.2. And whatever gateway was configured will be the one that the host uses.

I would guess that what your CCIE was talking about was to systematically alter configuring the hosts so that half of them use 192.168.0.1 and the other half use 192.168.0.2. This would produce load balancing where half of the hosts use one switch while the other half use the other switch. This is essentially a manual solution to the question of how to load balance.

HTH

Rick

Blue

Re: Switch Virtual Interface (SVI)

David:

"I would guess that what your CCIE was talking about was to systematically alter configuring the hosts so that half of them use 192.168.0.1 and the other half use 192.168.0.2. This would produce load balancing where half of the hosts use one switch while the other half use the other switch. This is essentially a manual solution to the question of how to load balance."

If this is what the CCIE suggested, he should be defrocked. :-)

Yes, this would give you some primitive form of load sharing, but, for one thing, it would depend on who the heavy talkers are.

You also would have no redundancy for the LAN hosts.

Victor

Community Member

Re: Switch Virtual Interface (SVI)

I think you nailed it on the spot.

That would be the only reason I could think of, but wanted to see verify that I wasn't losing my mind.

902
Views
5
Helpful
11
Replies
CreatePlease to create content