What is the difference between an SVI and a routed port on a layer 3 switch, and when do you use each?
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An SVI (interface Vlan10) is a virtual routed interface for a whole VLAN. It serves many layer 2 ports
and is the gateway for hosts. A routed port (no switchport plus an IP) turns one physical port into a
plain router interface: no VLAN, no spanning tree, no DTP. I use SVIs for host gateways and routed ports
for links between layer 3 switches, because a routed port goes down the moment the link fails and
there is no layer 2 path that could loop.
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Configuration
interface Vlan10
description Users gateway
ip address 10.10.0.1 255.255.255.0
!
interface GigabitEthernet1/0/48
description Routed uplink to CORE1
no switchport
ip address 10.0.0.1 255.255.255.254Why routed uplinks converge faster
- An SVI stays up while any port in its VLAN is up and forwarding. If an uplink in a "transit VLAN" fails but another port in that VLAN is up, the SVI never goes down and the routing protocol waits for its dead timer.
- A routed port goes down with the link. OSPF or EIGRP drop the neighbor and reroute immediately.
- No spanning tree runs on a routed port, so there is no blocked link and no loop risk between the layer 3 switches.
Where SVIs still belong between switches
A layer 2 trunk between two distribution switches that share VLANs (for HSRP and for stretched access VLANs) still needs SVIs on both. The routed uplinks go north to the core.
They will ask next
Can a routed port carry several VLANs?
On platforms that support layer 3 subinterfaces, yes:
interface Gi1/0/48.100withencapsulation dot1Q 100. Otherwise you trunk and use SVIs.Can you bundle routed ports?
Yes. Configure the members and the port-channel with
no switchportand put the IP on the port-channel interface.
Traps
- Building every point-to-point uplink as a dedicated VLAN with SVIs. It pulls spanning tree and autostate into the routed core for nothing.