What would be your approach while configuring a Cisco Catalyst Switch?
Switches are basically layer 2 or layer 3 devices that can offer a variety of functionalities. Following terms explain the main features you need to configure, when you implement a switch in a network:
Virtual LAN
There are two things you need to consider to remove from your network. One is the repeated unicast packets and the second the broadcast packets. Switch ports can normally block repeated unicast transmissions, but a switch cannot block broadcast packets since it is a layer 2 device and we need a layer 3 device to do it. A router is a layer 3 device and it could normally block broadcast packets. The only way you can block broadcast in a layer 2 switch is by introducing Virtual LANs. We can create as many VLANs in a network as per the requirement. In a normal situation we create VLANs by departments or by functionalities etc. For eg; you can create VLANs for your company departments such as finance, marketing, engineering etc. This means all the people from different departments would be connected to the same switch or group of switches, but they will be in different VLANs, so basically a broadcast which generates from a PC in finance VLAN cannot travel to marketing VLAN as it will be filtered at the switch.
Trunking
There are two main protocols used for switch to switch trunking and those are the ISL (Inter Switch Link, a proprietary protocol from Cisco which is currently not used) and the second one is the 802.1Q (called Dot1q, which is mainly used). Both have the ability to trunk packets from multiple VLANs to maintain different VLANs on the same network.
Etherchannels
Etherchannels can be used logically to combine multiple port bandwidths together as a single bundled one. This bundled pipe can be used either for trunking or for connecting FEC (Fast Etherchannel) or GEC (Gigabit Etherchannel) connections to Servers or high speed workstations supporting Etherchannels. Cisco uses PAGP (Port Aggregation Protocol, Cisco proprietary) and other vendors use LACP (Link Aggregation Control Protocol, open architecture) to establish Etherchannels.
Spanning-Tree
When you think about redundancy with switches (means using more than one trunk from one switch to the other), STP (Spanning Tree protocol) has to be enabled. This is to block any broadcast loops which may generate in the network due to redundancy links. Normally without STP, both trunk ports will be in forwarding state and the packets will travel through both links. This will make a broadcast storm and that will collapse the network within very short time. STP is defined with IEEE 802.1D and there are different and enhanced version of STP such as CST (Common Spanning Tree - used with non Cisco switches), PVST (Per VLAN Spanning Tree - Cisco Proprietary), PVST+, RSTP (Rapid Spanning Tree - IEEE 802.1W) and MSTP (Multiple Spanning Tree). The different types of STP will give you different kinds of functionalities that depend on your design plans.
Spanning-tree port fast
Normally the host ports in a switch (the ports that connect to the Work stations) are considered as broadcast loop free as it does not maintain a redundancy connection. For this reason, we do not need these ports to be initialized through the conventional STP states (blocking, listening, learning and forwarding) which are used to catch any loops that can occur. Cisco recommends to use spanning-tree port fast command to non-trunk ports (such as ports to connect to work stations).This will skip the listening and learning states and the port will directly move from blocking to forwarding state. This is also a core requirement for DHCP clients as the STP initialization will block the DHCP request pulses before the port comes to the forwarding state.
You can configure a lot of other features that will help you control your network in a better way, but the above mentioned feature configurations are very important for every switch implementation if you plan about broadcast segmentation, inter switch connectivity and redundancy.
From our Tech Guru,
Jayagiri Nair - Technical Consultant, Comstor Middle East, jnair@me.comstor.com
