Components Used
The information in this document is based on the software and hardware versions below.
Two Catalyst 3750G−24T switches running 12.1(14)EA1 EMI Software Release stacked together•
Catalyst 2950G−48 running 12.1(12c)EA1 EI Software Release•
Catalyst 2948G running 6.3(10) Software Release•
Note: The configuration from the Cisco 7200VXR is not relevant and, therefore, is not shown in this
document.
The information presented in this document was created from devices in a specific lab environment. All of the
devices used in this document started with a cleared (default) configuration. If you are working in a live
network, ensure that you understand the potential impact of any command before using it.
Background Theory
In a switched network, VLANs separate devices into different collision domains and Layer 3 (L3) subnets.
Devices within a VLAN can communicate with one another without requiring routing. On the contrary,
devices in separate VLANs require a routing device to communicate with one another.
L2 only switches require a L3 routing device (either external to the switch or in another module on the same
chassis). A new breed of switches, however, (for example, 3550 and 3750) incorporate routing capability
within the switch. The switch receives a packet, determines that it belongs to another VLAN, and sends the
packet to the appropriate port on the other VLAN.
A typical network design segments the network based on the group or function the device belongs to. For
example, the engineering department VLAN would only have devices associated with the engineering
department, while the finance VLAN would only have finance related devices. If routing is enabled, the
devices in each VLAN can talk to one another without all the devices being in the same broadcast domain.
Such a VLAN design also has the added benefit of allowing the administrator to restrict communication
between VLANs using access−lists. In our example, the engineering VLAN could be restricted (using
access−lists) from accessing devices on the finance VLAN.
IP Routing on a Stack of 3750s
On a stack of Catalyst 3750 switches, the software running on the master determines the capabilities of the
whole stack. If the master switch is running an EMI image, the whole stack will support the full set of
supported routing protocols (such as Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing
Protocol (EIGRP), and such) even if the other stack members are just running an SMI image. However, it is
advisable to have the same software running on the different stack members. If the stack master fails, you
would lose the extended routing capabilities if the other members run an SMI image as opposed to the EMI
image of the former master.
A stack of Catalyst 3750 switches appears to the network as a single router, independent to which of the stack
switches the routing peer is connected. A router will create a single adjacency with a stack of 3750 routers.
The stack master performs the following tasks:
Initialization and configuration of the routing protocols•
Generation of routing protocol messages•
Processing of received routing protocol messages•
Cisco − Configuring Inter−VLAN Routing with Catalyst 3750 Series Switches in a Stacked Configuration
