Multilayer Switching: More Routed Ports and Comparisons with SVIs

Remember the ‘follow the path’ technique with troubleshooting at Layer 3.

Router On A Stick is not scalable. (Doh!)

SVI Checklist

  • Create the VLAN before the SVI. The VLAN must be active when the SVI is created – that VLAN will not be dynamically created at that time.
  • Theoretically, you need to open the SVI with no shutdown just as you would open a physical interface after configuring an IP address
  • The SVI and VLAN have an association, but they’re not the same thing..
  • The only SVI on the switch by default is the SVI for VLAN 1, intended to allow remote switch administration and configuration.

SVIs are a great way to allow interVLAN communication, but you must have a routing protocol configured in addition to the SVIs.

Fallback Bridging – (Uncommon in the real world)

CEF has a limitation in that IPX, SNA, LAT, and AppleTalk are either not supported by CEF or, in the case of SNA and LAT, are nonroutable protocols. If you’re running any of these on an CEF-enabled switch, you’ll need fallback bridging to get this traffic from one VLAN to another.

Fallback bridging involves the creation of bridge groups, and the SVIs will have to be added to these bridge groups.

To create a bridge group:

MLS(config)# bridge-group 1

To join a SVI to a bridge group:

MLS(config)#interface vlan 10
MLS(config-if)#bridge-group 1

SVI Advantages

  • No single point of failure
  • Faster than ROAS
  • Don’t need to configure a trunk between the L2 switch and the router

If you have an L3 switch, you’re much better off using SVIs for inter-VLAN communication rather than ROAS.

A black hole in routing is the result of an SVI remaining up when there are actually no “up/up” interfaces in that VLAN except for those connected to network monitors or similar devices.

To avoid this, we can exclude such ports from the “up/up” calculation with the switchport autostate exclude command. Using that interface-level command on ports like the one previous described will exclude that port from the “up/up” determination.

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