Routed interface or SVI

Hi,

Been wondering for a while now but what's the difference of connecting a switch and a router together through an SVI or a L3 interface?

a) You assign a /30 to both sides in the same subnet
b) You create a /30 SVI and add a particular interface into that vlan where you will connect your router and assign the other ip to the router

You will probably get some unwanted broadcast in that domain, but apart from that what are the pros cons?

A

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cxzar20

Spanning-Tree will block links in the SVI unless you use port channel.

vinbuck

That's not entirely true....

Two uplinks from a switch to two different routers in the same VLAN will be in a forwarding state. There has to be a loop in the topology to block a link. The OP is talking about putting in a /30 and doesn't mention multiple links. This depends on your hardware and what flexibility you want to have. You can drop out multiple links in the same VLAN/Subnet if you use an SVI.

For a /30, it all comes down to design preference as both types of routed links will be handled by CEF in a similar fashion with similar performance on most Cisco multilayer switches.

Good article on this here

Convergence Delays: SVI vs Routed Interface - Packet Life

ColbyG

Routed links are usually cleaner, and leave slightly less room for mistakes down the road. I only use SVIs if I need multiple devices on routed links, say two routers and two switches, then I'll use an SVI and /29.

Forsaken_GA

ColbyG wrote: »

Routed links are usually cleaner, and leave slightly less room for mistakes down the road. I only use SVIs if I need multiple devices on routed links, say two routers and two switches, then I'll use an SVI and /29.

This is the same way I run. An SVI for a point to point routed link is pointless, you're using a port for layer 3 access either way. If more than two devices are going to exist on the subnet, then I use an SVI.

Forsaken_GA

Actually, now that I'm thinking about this.... if your router is running a trunk link back to the switch, then you'd use an SVI as opposed to a routed port on the switch. I've seen this on a few occasions where the router was limited on ports, and a link was trunked because it needed to do double duty as a transit link and a management link, and the management traffic needed to be isolated.

MrBishop

The names point you in the direction of were you should implement them in the network. SVI's for running traffic over the trunk links connecting switches together and router ports running from the switch to the router. Just clicked with me and this thread cleared it up!

Akiii

vinbuck wrote: »

That's not entirely true....

Two uplinks from a switch to two different routers in the same VLAN will be in a forwarding state. There has to be a loop in the topology to block a link. The OP is talking about putting in a /30 and doesn't mention multiple links. This depends on your hardware and what flexibility you want to have. You can drop out multiple links in the same VLAN/Subnet if you use an SVI.

For a /30, it all comes down to design preference as both types of routed links will be handled by CEF in a similar fashion with similar performance on most Cisco multilayer switches.

Good article on this here

Convergence Delays: SVI vs Routed Interface - Packet Life

Great article Sir!

cxzar20

vinbuck wrote: »

That's not entirely true....

Two uplinks from a switch to two different routers in the same VLAN will be in a forwarding state. There has to be a loop in the topology to block a link. The OP is talking about putting in a /30 and doesn't mention multiple links. This depends on your hardware and what flexibility you want to have. You can drop out multiple links in the same VLAN/Subnet if you use an SVI.

For a /30, it all comes down to design preference as both types of routed links will be handled by CEF in a similar fashion with similar performance on most Cisco multilayer switches.

Good article on this here

Convergence Delays: SVI vs Routed Interface - Packet Life

That is right, I was bringing up the point about SVI in addition to the issue raised by the OP. If you have multiple links from a device coming into another with an SVI then spanning tree considerations have to be made. An example of this would be multiple uplinks to a stacked switch SVI. Even though they may go to two separate physical switches it acts as one logical and thus one of those links will be blocked if it isn't root.