Using The EtherSeries Bridge and Encrypted Tunnel with Cisco Switches

Introduction

This technical note applies to the DCB Ethernet Bridge (ESB Products) and the DCB Ethernet Tunnel (ET products). It describes a problem where the DCB products seemingly hang up or appear overloaded when a Cisco switch or router is present on the network Symptoms might also include extremely low throughput on the link. This note may also apply to other managed switch devices.

Problem Description

The DCB Ethernet Bridge and Ethernet Tunnel are often used to bridge two network segments across a low speed serial link. When doing so, the user must design the network such that bridged traffic won't exceed the capability of the serial link. Otherwise excessive delays and dropped packets will result in a failure of the network applications.

Cisco switches and Cisco routers with internal switches typically have the Cisco Discover Protocol and the Spanning-Tree protocol enabled. These protocols use Ethernet level broadcasts and can generate a significant amount of traffic that will be bridged across the serial link. On a slow serial link, this could possibly exceed the full link capacity. In addition, if a Cisco switch is located at each end of the bridged network, the resulting communications between the two switches impose even more load.

Solutions

If the Cisco Discovery Protocol or the spanning-tree protocol are generating too much traffic for your bridged link, there are several remedies. In most cases, it is possible to disable these protocols. In fact, most Cisco switch products allow these protocols to be disabled on a port by port basis. This will allow continue use of these protocols on the other ports, but disable it on the port that the bridge is downstream of. Alternately, you can tune the parameters for these protocols to reduce the amount of traffic they generate.

Example

Below is an example of disabling these protocols on a Cisco series 1700 router with an internal switch card. This example may not apply to your specific product, so please refer to your specific product documentation.

To disable Cisco Discover Protocol and Spanning-Tree on all ports:

router> enable

password:

router# config terminal

router (config)# no cdp run

router (config)# no spanning-tree vlan 1 {see note below}

router (config)# end

router# copy running-config startup-config

To disable the protocols only on switch port 2.

router> enable

password:

router# config terminal

router (config)# interface fastethernet 2

router (config-if)# no cdp enable

router (config-if)# no spanning-tree vlan 1 {see note below}

router (config)# end

router# copy running-config startup-config

Note:

You must disable spanning-tree for each defined VLAN. In this example we are selecting VLAN1. Use the show spanning-tree command to see what VLANs have been defined.


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