The challenge of combining L3 switching with true routing

Given that Layer 3 switches are designed to provide only a core of routing functions, L3 switch vendors commonly advise their customers to move the existing "traditional" routers to the edge of the network. This gives customers wire-speed ASIC-based switching at the core of the network and relegates the router to handling legacy protocols such as IPX at the edge. To hear the switch vendors tell it, this is an easy move. But don't be fooled.

Customers typically organize their routed networks into separate VLANs. Traffic from the different networks is then identified across interswitch links through 802.1q VLAN tagging. That way, even though the switches can't decode the network layer information (and, therefore, can't determine the ports associated with a particular network), they can still direct the traffic to the appropriate group of ports based on VLAN information.

In this configuration, the router at the edge of the network is required to forward traffic between these VLANs, and there are really only two ways to do it. First, the router could connect to an interswitch trunk link and route between different VLANs on a single port (so-called one-armed routing). However, this requires that the router support 802.1q - something few traditional routers do. (Remember that 802.1q was developed for switches, not routers.)

In the second approach, the router could simply transfer traffic between different physical interfaces, one per network. This follows the traditional multiport router model. The obvious drawback of this approach is that it introduces the added cost of multiple router ports and switch ports, since separate switch and router ports are required for each network connection.

In the end, customers who plan to deploy routers at the edge of their networks should be prepared either to dedicate several switch and router ports for a traditional router, or to allow for significant time to locate a router that supports 802.1q VLAN frame tagging.

Next week, we'll consider issues surrounding assigning VLANs to different networks, deciding when it's a luxury and when it's an absolute necessity.

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The many uses of the word "switching":
First you have to define it. Network World Network World on High Speed LANs, 4/15/98.

The pros and cons of Layer 2 routing assist:
A look at combining Layer 2 and 3 switching. Network World Network World on High Speed LANs, 4/21/98.

Switching grows up:
Layer 3 switching completes a circle, giving us pause to relect on where switching has brought us and where it's headed. Network World, 5/4/98.

Virtual LANs:
An Intel paper that introduces the technology.

Route Once, Switch Many:
Paper from NetReference that discusses the routing/switching issue.