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NETW206: Introduction to Switching

In practice, it is common to interconnect switches to achieve redundancy. When switches are interconnected, the network is redundant and cannot fail even if one of the connected links gets faulty. This is a common technique in computer networking. However, this interconnection introduces serious problem in the network known as Layer 2 Switching loop. Layer 2 traffic is traditionally classified as unicast, multicast and broadcast. Unicast, multicast, and broadcast are varieties of network communication mechanisms required for the normal operation of the network. In the case of multicast and broadcast in MAC, for instance, the switch needs to forward the Ethernet frame out all its ports. For unknown destination, MAC addresses also, the switch need to forward or flood all the ports with the packets except the source port to ensure that the Ethernet frame reaches the destination.

The illustration below can be used to describe the issues associated with Layer 2 Switching Loop.

An Ethernet Frame originating from the Workstation to the File server goes through Switch 4. Switch 4 will forward the packets to all its ports except the source port because for some reason, the MAC address of the destination device in this case the file server might not available in the MAC address destination. Switch one and 3 will receive a copy of the Ethernet frame and search for the destination MAC addresses in its MAC table, and if they fail to find the address, both switches will forward the Ethernet frame to all ports. This may cause the Ethernet frame to reach back to switch 4 through paths 1-3-4 or 3-1-4. This is called switch loop and the Ethernet will start circulating the network in the loop. Another issue is that the file server can receive multiple copies of the same Ethernet frame coming from different paths leading to additional network overheads. All these issues cause serious network performance.
They can be prevented in networks using Spanning Tree Protocol. One of the basic functions of switches is to eliminate Layer 2 switching loops. Spanning Tree Protocol prevents Layer 2 switching loop and broadcast storms in LANs. STP allows redundant links in the network to prevent complete network failure if an active link fails without the danger of Layer 2 Switching loops. RSTP is an advanced version with much better convergence time than STP while MSTP is an enhancement to RSTP and is the preferred spanning tree protocol for BIG-IP systems.
In the context of Devry University, Layer 2 loop can be prevented by configuring spanning tree mode rstp to active state. This will prevent packet looping in the network. Also, turning all non-trunking ports (ports not connected to other switches) as access switches only to prevent other switches from connecting to it will solve switch looping issues.
VLAN trunking protocol VTP as designed by Cisco is used to manage VLANs and operate in three modes namely; server mode, client mode and transparency mode. VTP is an automatic sharing of VLAN databases. Enabling a switch as a server or client will permit sharing of databases in the same VTP domain. This is useful for multiple configurations but present severe drawbacks. If a switch with a higher revision number is introduced into the VTP domain, and classified as a server, its databases will be deemed to be of higher quality than preceding revisions and the switch acceptance this will overwrite its VLAN database with current information.
In order to avoid this, making the switch a server that can update VLANs is a viable solution but password is used for each switch. Configurations for the password are:
conftvtp password XXXXX
Also, make the switch part of the domain Devry0123 so that it cannot accept advertisements from another switch. Only accept advertisements with the correct password and drop the rest.

Turn on version 2 on server to propagate to the rest

VTP pruning is implemented within a VTP management domain so that traffic intended for a particular VLAN is only passed to a switch across a trunk link if only the link has its VLAN ports configured. In the context of Devri University, turn on pruning features.
Another option is to make each switch a transparent switch. Each switch will accept another switch’s VLAN database but will not modify its own database. Configure as followsconftvtp mode transparent
802. 1dq (dot1q) is the de facto standard for VLANs that ensure interoperability of its implementations between switches and NICs from varied vendors. 802. 1dq (dot1q) must be used on trunk lines on both ends of the link. Also, turn on trunk line without negotiation.
These configurations will stop any rogue switches from accessing the network and erasing the VLAN databases. As noted, issues such as those experienced when a student connected a switch purchased from a local garage to the network, inadvertently bridging the network and causing Layer 2 loops will not be experienced again. Security of the University’s network will be enhanced.

Works Cited

Froehlich, Andrew. CCNA Voice Study Guide:. John Wiley & Sons, 2012.
Kennedy Clark, Kevin Hamilton. Cisco LAN Switching. Cisco Press, 1999.
Tiso, John. Designing Cisco Network Service Architectures (ARCH) Foundation Learning Guide: (CCDP ARCH 642-874). Cisco Press, 2011.