L3 Switch SVI

[daniel4579329]

Hi im learning for the CCNA 

can someone explain to me the concept of SVI on L3 switch and how to configure 

and i have a couple of questions

1. if i give to one svi port ip address so i cant give an ip on the same subnet to an other port on that L3 switch ?

2. what is normal to put into the link a single pc or maybe switch that have multiple pc?

3. does the svi port is the default gateway of this port subnet ?

thanks fore the help (:

[Martin L]

1.  L2 Switch needs  SVI IP address (default vlan 1) can be access via telnet or ping so you can manage it *needs default gateway if to be access to/from remote networks. 

L3 switch needs SVI so you can enable routing to/from it.  PCs can use SVI as default gateway; Basically you are replacing (or not using) a router to route between different subnets (aka vlans)

yes, SVI must be on the same subnet with other L3 switch or router to exchange info

2. SVI x serves all PCs/devices that are on that vlan.  SVI named Interface vlan x will reach all hosts in vlan x; to reach hosts in vlan z, you need another SVI named vlan z and enabled routing (enabled by default) 

3. Yes, usually SVI is the default gateway for all devices that are in that vlan (matching svi number)

Regards, ML
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[Martin L]

here is example:
interface Vlan1
no ip address
shutdown
!
interface Vlan10
mac-address 00e0.f9cd.1b01
ip address 10.0.10.1 255.255.255.0
!
interface Vlan20
mac-address 00e0.f9cd.1b02
ip address 10.0.20.1 255.255.255.0
!
interface Vlan30
mac-address 00e0.f9cd.1b03
ip address 10.0.30.1 255.255.255.0
!
ip classless

sw1#show ip route
Gateway of last resort is not set

10.0.0.0/24 is subnetted, 3 subnets
C 10.0.10.0 is directly connected, Vlan10
C 10.0.20.0 is directly connected, Vlan20
C 10.0.30.0 is directly connected, Vlan30

[daniel4579329]

Thanks for the help

But what i dont understand is that we need to attribute these ports to the vlan (or maybe im wrong)

I feel very confused about this subject if you could explain to me the concept (to clear my mind ) i would appreciate it

[Martin L]

attribute these ports to the vlan?  ports must be assigned to vlan(s) on any switch.  note that ports are either assigned by default (all ports are already assigned to vlan 1) or by admin. 

Internally that is how L3 switch SVI routes packets between Vlans and builds routing table.  Traffic within the same vlan, stays inside and does not need SVI.  But to reach other vlans or remote network, you will need at least one SVI up up. 

instead of router doing routing (router-on-a-stink concept), L3 SVI switch will do routing. Also, L3 switch replaces l2 switch and a router In case you want to route packets;  So, you could have 1 device instead of 2 (switch and router).

Regards, ML
**Please Rate All Helpful Responses **

[Martin L]

what books do you have/use for your studies?  If you study for CCNA exam, I would recommend book by W. Odom from CiscoPress;  His Book is good even if you do not study for CCNA (or going above CCNA level). One of my online friends is Keith Barker, He has his own CCNA channel and Sunday group discussions.  

Also, go to youtube and search for videos on subject vlans, router-on-a-stck, ROAS, and alike.  Watching several videos or as many as you can find may give you clues to better understand technology concepts.

good luck

[daniel4579329]

1.can you please explain more the answer for question 2

2.how can there be a different svi on one svi port ?

Thanks for all the help (:

[Richard Burts]

I am a bit confused about where we are in the discussion. There have been multiple posts with various explanations. If I am understanding correctly question 2 was

2. what is normal to put into the link a single pc or maybe switch that have multiple pc?

I believe that the answer to this question depends on the network design. In some situations it makes good sense to have a single device and in most situations it makes good sense to have multiple devices. 

If this does not answer your question then perhaps you can provide some clarification.

[Jaderson Pessoa]

@daniel4579329 Hello my friend,

can someone explain to me the concept of SVI on L3 switch and how to configure 

Firstly i would like to  explain about broadcast.

Broadcast it is a method that propagate a message everybody except who is sending, a network like 10.0.0.0/8 has a big broadcast because there are a lot of address into these address space and if there is no techniques applied in environments like this, maybe it will use a lot of bandwidth without necessity. How to avoid it? you can use a techique know by VLAN.

VLAN it is a method to split broadcast domain in (virtual local area network), you can have an address space 10.0.0.0/8 and split it in smal networks to minimize broadcast domain.

A VLAN is created do define a broadcast domain, if you create a vlan to associate differents network, you will two options.

1. Create a vlan and associate it in a layer 3 device that will be used as gateway for this vlan, can be: (firewall, switch layer 3 and so on).

2. Create a layer 2 vlan and create a SVI that will be a Interface vlan in a layer 3 switch this Interface vlan can be used as gateway of end hosts in that vlan itself.

When you create a interface vlan, your intention it is to enable routing for this network, then this interface will be used as bridge (gateway) for other networks in your environmnet.

1. if i give to one svi port ip address so i cant give an ip on the same subnet to an other port on that L3 switch ?

R= If you want create a network as management for all devices in your environment. You will create vlan layer 2 and SVI and propagate this vlan in all devices that you need to manager, all devices will have a SVI with an ip address  whithin the same network

2. what is normal to put into the link a single pc or maybe switch that have multiple pc?

R= Depends of your requirements, it can be used in a switch that have multiple pc in differentes networks, in this case differents SVI.

3. does the svi port is the default gateway of this port subnet ?

R= Yes, normally SVI are created in the CORE/DISTRIBUTION  to used as gateway for end hosts. There are a lot of documentations about topologies... looks in to the linsk below, certainly will help you in your doubts.

https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Borderless_Networks/Unified_Access/Unified_Access_Book/UA_Design.html

Best regards,

[Richard Burts]

I am going to suggest a different approach to answering these questions. First let us start by reviewing some of the concepts involved:

- a vlan is a layer 2 entity.

- a vlan is a single broadcast domain. This means that any device connected in this vlan can communicate directly with any other device connected in this vlan.

- switches assign ports (or interfaces) to vlans. A switch port configured as an access port belongs to only a single vlan. A switch port configured as a trunk can belong to multiple vlans.

- a vlan could possibly be connected in several switches. switchA to switchB to switchC to switchD. These connections could be access port to access port or trunk port to trunk port. 

- a device connected to a particular vlan in switchA can communicate directly with another device connected to that vlan in switchD. They would arp for each other and communicate without needing any router or any other type of device.

- a device connected to a particular vlan can not communicate directly with a device in a different vlan (assuming that there have not been configuration errors that produce vlan mismatches between switches - and if there have been that kind of configuration error that produce vlan mismatches then in fact there is a single vlan that just has been assigned different names). If a device in vlan 2 wants to communicate with a device in vlan 3 then some device must provide inter vlan routing.

- a vlan interface sounds very similar to a vlan but it is quite different. While a vlan is a layer 2 entity a vlan interface is a layer 3 entity. A vlan interface provides layer 3 functionality for the devices in the layer 2 vlan (routing etc). A vlan interface is sometimes referred to as an SVI (switch virtual interface).

- there is generally a one to one relationship between vlan and IP subnet. In general one vlan will have one subnet and one subnet belongs to one vlan.

- it is possible to have 2 IP subnets belong in the same vlan (using the concept of secondary addressing).

- if a single IP subnet is assigned to 2 vlans then it is a problem.

With these things in mind let us answer the questions

1. if i give to one svi port ip address so i cant give an ip on the same subnet to an other port on that L3 switch ?

Cisco IOS will not allow 2 layer 3 interfaces on the same device to be assigned to the same subnet. So you can not give an ip on the same subnet to another port on that L3 switch.

2. what is normal to put into the link a single pc or maybe switch that have multiple pc?

First we need to understand when you talk about putting into the link are you using "link" the same as "vlan"?

It is certainly possible to put a single device into a vlan. It is more normal to put multiple devices into that vlan.

3. does the svi port is the default gateway of this port subnet ?

It is common for a vlan to have a single SVI. In the situation where a vlan has a single SVI it is common that this SVI is the default gateway for the devices connected in this subnet. But it is not required that the SVI be the default gateway. It is possible that the vlan could have the SVI on a switch and also be connected to a router and have the router act as the default gateway.

It is also possible that a vlan which connects on several switches (switchA, switchB, switchC) might have an SVI on each of the switches. It is possible that the SVI on each switch might act as the default gateway for devices connected on that switch. It would be more common if multiple switches have an SVI to run HSRP (Hot Standby Routing Protocol) on the SVIs so that one switch SVI would act as gateway but if a problem developed on that interface that the SVI on another switch could take over and act as the gateway.