What's better - an SVI or a routed interface in a 6500?

Unanswered Question
Mar 7th, 2008
User Badges:

If I want to use a single 10/100/1000 port on a blade in a 6500 (IOS) to be the uplink to a 3750 L2 switch and there are two ways I can do it.


1. int g2/1

switchport

switchport access vlan 50

switchport mode access

int vlan 50

ip address 192.168.1.1 255.255.255.0


2. int g2/1

ip address 192.168.1.1 255.255.255.0


#2 is certainly simliar, but is there a 'best practice' for this? What are the pros and cons of either option? Again there won't be a case where I would need to add any other 6500 port to the same subnet/vlan.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.5 (4 ratings)
Loading.

It would depend upon your future needs. The SVI is more flexible as you can add, move, change interfaces to the SVI at will. Although I'd shrink the network down to 28 bits or so. If you go with the routed interface, there's no need for more than a 28 bit mask, and a 30 bit mask is closer to the norm. The biggest Pro is when you'll need to move it. The SVI is the better choice.


On our data center network, we use the SVI method. It has saved us more times than I can count.

Istvan_Rabai Fri, 03/07/2008 - 12:06
User Badges:
  • Gold, 750 points or more

Hi Jim,


An example when it can be useful to use SVIs, if you want to have redundant uplink ports from the L2 switch to the 6500.


The g2/1 and and another, let's say g2/0 may be configured as trunks carrying several vlans. Spanning-tree would provide a loop-free topology and link redundancy.


Cheers:

Istvan

If there is no requirement for other ports to be a member of this VLAN then why even introduce a VLAN?

If you use a SVI then you must create a VLAN. That changes VTP and STP designs, if applicable.

I would not introduce a VLAN if you do not need it. Keep the link L3 if it is only going to be used as a point to point link.

If this is your core network device, best practice is to keep L2 off of your core as much as possible.


Additionally your port would no longer be a switch port which eliminates BPDU's, DTP, etc.




lamav Fri, 03/07/2008 - 12:49
User Badges:
  • Blue, 1500 points or more

I agree with Edward.


If an L3/routed connection is what you're looking for, configure the port accordingly -- and use a /30 mask. Maybe even a /31, which is the new trend.


The SVI method was used during the days of CatOS, where every switchport could only be configured as an L2 port.


HTH


Victor

jkeeffe Fri, 03/07/2008 - 13:40
User Badges:

Mind if I ask what you mean by using a /31 mask? You only get the subnet and the broadcast - how do you get IPs for the two interfaces?

jkeeffe Mon, 03/10/2008 - 07:55
User Badges:

Do you know of any problems with this solution that could occur? I can't think of any - there is no reason really to have broadcasts on a point to point, but then I don't think of all the possibilities.


Jon Marshall Fri, 03/07/2008 - 15:56
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Jim


Doesn't make a lot of difference in that you still have L2 on that link and STP whether you make it L3 routed on the 6500 or an SVI.


As Victor says you could make it a routed link at both ends but then obviously your 3750 is no longer a L2 switch only.


One thing i would do if the 6500 port is routed is to turn on portfast on the 3750 switch port that connects to the 6500 as you have no need to go through the listening/learning/forwarding etc. of normal STP. And by doing this the port can forward a lot more quickly than if you had an SVI on the 6500.


Jon



Actions

This Discussion