Reasons for choosing a Layer 3 design
The reasons for choosing a Layer 3 design for blade server switching are outlined below. It is
worth noting that in most discussions of network architecture, the blade servers would be connected to a
which connects directly to servers as opposed to client (desktop, mobile computer) computers. Most discussions of network architecture recommend that server access switches be Layer 3 devices.
Layer 3 switching keeps more traffic within the BladeCenter chassis.
In any design where the blade servers are assigned to multiple VLANs, routing is required for servers which are not on the same VLAN to communicate with each other. (An example of such a design would be using blade servers as WebSphere® Web and application servers and placing the Web servers on one VLAN and the application servers on a different VLAN.)
With a Layer 3 configuration on the GbESM, servers on different VLANs can communicate via the switch module which is inside the BladeCenter chassis. If only Layer 2 switching is used within the chassis, then traffic between, for example, WebSphere Web and application servers would leave the chassis on the external links and flow through one or
more external devices until it reached a Layer 3 switch (router). It would then flow through one or more additional devices until it returned to the IBM Eserver BladeCenter, crossing the external links for a second time.
The benefits of keeping traffic within the chassis include greater security - because there are no patch panels which can be tampered with - and lower latency, since the traffic traverses a smaller number of switches between source and destination.
Layer 3 switching allows more efficient use of external connections
The key issue here is that use of Layer 3 allows the network to run without the use of Spanning Tree Protocol (STP). Spanning tree works by blocking links which would create a topology which includes a loop; connections from a GbESM to two or more upstream
switches which are connected to each other fall into this category. The consequence is that up to half of the links from the GbESM would be blocked during normal operations and would not carry traffic. Only if the active link(s) failed would the blocked links be used to carry traffic.
Layer 3 routing not only allows all of the uplinks to be active but also allows the GbESM to send traffic to a given destination on the best path to that destination.
7.8.1Layer 3 sample configuration with static routing and VRRP
This example is an extension to the basic configuration described in 7.6, “Basic Layer 2 entry topology” on page 69, providing a mesh topology between the two GbESM switches and the upstream Core switches. Trunking (link aggregation) is used in this configuration in the same way it was used in the advanced Layer 2 configurations shown in 7.7, “Advanced Layer 2 topology sample configurations” on page 80.
See Figure
VLAN 35 is configured and connects both of the GbESM switches to Core switch 1 (address
.245). VLAN 46 is configured to connect both of the GbESMs to Core switch 2 (address .246). This configuration uses VRRP and hot standby to provide high availability.
Chapter 7. Nortel Networks L2/3 GbESM configuration and network integration 109
