Switches, Routers, Bridges and LANs/Switching Technology
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Switching technologies are crucial to the new network design. Because the prices on layer 2 switching have been dropping dramatically, it is easier to justify the cost of buying switches for your entire network. This doesn't mean that every business can afford switch ports for all users, but it does allow for a cost-effective upgrade solution when the time comes. Layer 2 switching – Layer 2 switching is hardware based, which means it uses the Media Access Control (MAC) address from the host's network interface cards (NICs) to filter the network. Switches use Application-Specific Integrated Circuits. (ASICs) to build and maintain filter tables. It is OK to think of a layer 2 switch as a multiport bridge. Layer 2 switching provides the following: Hardware-based bridging (MAC) Wire speed High speed Low latency Low cost Layer 2 switching is so efficient because there is no modification to the data packet, only to the frame encapsulation of the packet, and only when the data packet is passing through dissimilar media (such as from Ethernet to FDDI). Use layer 2 switching for workgroup connectivity and network segmentation (breaking up collision domains). This allows you to create a flatter network design and one with more network segments than traditional 10BaseT shared networks. Layer 2 switching has helped develop new components in the network infrastructure:
Servers are no longer distributed to physical locations because virtual LANs can be created to create broadcast domains in a switched internetwork. This means that all servers can be placed in a central location, yet a certain server can still be part of a workgroup in a remote branch, for example.
Allows organization-wide client/server communications based on a Web technology. These new technologies are allowing more data to flow off of local sub-nets and onto a routed network, where a router's performance can become the bottleneck.
Limitations of Layer 2 Switching
Layer 2 switches have the same limitations as bridge networks. Remember that bridges are good if you design the network by the 80/20 rule: users spend 80 percent of their time on their local segment.
Bridged networks break up collision domains, but the network is still one large broadcast domain. Similarly, layer 2 switches (bridges) cannot break up broadcast domains, which can cause performance issues and limits the size of your network. Broadcast and multicasts, along with the slow convergence of spanning tree, can cause major problems as the network grows. Because of these problems, layer 2 switches cannot completely replace routers in the internetwork.
Layer 3 Switching –
The only difference between a layer 3 switch and a router is the way the administrator creates the physical implementation. Also, traditional routers use microprocessors to make forwarding decisions, and the switch performs only hardware-based packet switching. However, some traditional routers can have other hardware functions as well in some of the higher-end models. Layer 3 switches can be placed anywhere in the network because they handle high-performance LAN traffic and can cost-effectively replace routers. Layer 3 switching is all hardware-based packet forwarding, and all packet forwarding is handled by hardware ASICs. Layer 3 switches really are no different functionally than a traditional router and perform the same functions, which are listed here: Determine paths based on logical addressing Run layer 3 checksums (on header only) Use Time to Live (TTL) Process and responds to any option information Can update Simple Network Management Protocol (SNMP) managers with Management Information Base (MIB) information Provide Security The benefits of layer 3 switching include the following: Hardware-based packet forwarding High-performance packet switching High-speed scalability Low latency Lower per-port cost Flow accounting Security Quality of service (QoS) Layer 4 Switching – Layer 4 switching is considered a hardware-based layer 3 switching technology that can also consider the application used (for example, Telnet or FTP). Layer 4 switching provides additional routing above layer 3 by using the port numbers found in the Transport layer header to make routing decisions. These port numbers are found in Request for Comments (RFC) 1700 and reference the upper-layer protocol, program, or application. Layer 4 information has been used to help make routing decisions for quite a while. For example, extended access lists can filter packets based on layer 4 port numbers. Another example is accounting information gathered by NetFlow switching in Cisco's higher-end routers. The largest benefit of layer 4 switching is that the network administrator can configure a layer 4 switch to prioritize data traffic by application, which means a QoS can be defined for each user. For example, a number of users can be defined as a Video group and be assigned more priority, or band- width, based on the need for videoconferencing. However, because users can be part of many groups and run many applications, the layer 4 switches must be able to provide a huge filter table or response time would suffer. This filter table must be much larger than any layer 2 or 3 switch. A layer 2 switch might have a filter table only as large as the number of users connected to the network may be even less if some hubs are used within the switched fabric. However, a layer 4 switch might have five or six entries for each and every device connected to the network! If the layer 4 switch does not have a filter table that includes all the information, the switch will not be able to produce wire-speed results.
Multi-Layer Switching (MLS)
Multi-layer switching combines layer 2, 3, and 4 switching technologies and provides high-speed scalability with low latency. It accomplishes this high combination of high-speed scalability with low latency by using huge filter tables based on the criteria designed by the network administrator. Multi-layer switching can move traffic at wire speed and also provide layer 3 routing, which can remove the bottleneck from the network routers. This technology is based on the idea of route once, switch many. Multi-layer switching can make routing/switching decisions based on the following: MAC source/destination address in a Data Link frame IP source/destination address in the Network layer header Protocol filed in the Network layer header Port source/destination numbers in the Transport layer header There is no performance difference between a layer 3 and a layer 4 switch because the routing/switching is all hardware based.
LAN Switch Types
LAN switching is used to forward or filter frames based on their hardware destination. However, there are three different methods in which frames can be forwarded or filtered. Each method has its advantages and disadvantages, and by understanding the different LAN switch methods available, you can make smart switching decisions. There are three switching modes:
With the store-and-forward mode, the complete data frame is received on the switch's buffer, a cyclic redundancy check (CRC) is run, and then the destination address is looked up in the MAC filter table.
With the cut-through mode, the switch waits for only the destination hardware address to be received and then looks up the destination address in the MAC filter table.
FragmentFree is the default mode for the Catalyst 1900 switch; it is sometimes referred to as modified cut-through checks the first 64 bytes of a frame for fragmentation (because of possible collisions) before forwarding the frame.
Figure shows the different points where the switching mode takes place in the frame. The different switching modes are discussed in detail in the following sections.
Route Switch Modules (RSMs)
Route Switch Modules (RSMs) are also called internal route processors because the processing of layer 3 packets is internal to a switch. You need to add an RSM to a layer 2 device for example, a 5000 Catalyst switch--to be able to provide switching of layer 3 packets without a router. An RSM makes layer 2 switches a multi-layer switch and can integrate layer 2 and layer 3 functionality in a single box. The 5000 series uses the RSM or a Route Switch Feature Card (RSFC), and the 6000 series uses the Multilayer Switch Module (MSM) to perform this function. The RSM, RSFC, and MSM are configured in exactly the same way on the switch. The RSM is a module plugged directly into the switch, which runs the Cisco IOS in order to perform inter-VLAN communication. The 5000 series switch sees the RSM as a single trunked port and a single MAC address. In other words, it appears as a router on a stick to the switch. The RSM inter- face to the switch is through VLAN 0 and VLAN 1. VLAN 0 is not accessible to the administrator. The RSM uses two channels, and VLAN 0 maps to channel 0, which supports communication between the RSM and the Catalyst 5000 series default VLAN (VLAN 1). VLAN 1 maps to channel 1. The MAC address assigned to the RSM is from the Programmable Read Only Memory (PROM) on the line communication processor (LCP). This MAC address is used to identify the slot of the RSM and for diagnostics. The MAC addresses for VLAN 1 are assigned from a PROM that contains 512 MAC addresses. All routing interfaces except VLAN 0 use the base MAC address. The RSFC is a daughter card for the Supervisor Engine II G and Supervisor III G cards. The RSFC is a fully functioning router running the Cisco IOS. The MSM uses four full-duplex Gigabit Ethernet interfaces to connect to the switch and looks like an external router to the switch. These four inter- faces can be four separate links for four different VLANs, or they can be trunked and configured as one load-balanced link running EtherChannel and ISL or 802.1q. Subinterfaces are then used to configure each VLAN