bridging and switching methods and performance issues 287 A 8 1 4 6 C D B 3 Figure 6.5 A weighted network graph. Kruskal’s algorithm can be expressed as follows: 1. Sort the edges of the graph (G) in their increasing order by weight or length. 2. Construct a subgraph (S) of G and initially set it to the empty state. 3. For each edge (e) in sorted order: If the endpoints of the edges (e) are disconnected in S, add them to S. Using the graph shown in Figure 6.5, let’s apply Kruskal’s algorithm as follows: 1. The sorted edges of the graph in their increasing order by weight or length produces the following table: Edge Weight/Length A-C 1 B-D 3 C-B 4 C-D 6 A-B 8 2. Set the subgraph of G to the empty state. Thus, S = null. 3. For each edge add to S as long as the endpoints are disconnected. Thus, the first operation produces: A C S = A,C or 288 chapter six The next operation produces: A S = (A,C) + (B,D) or C B D The third operation produces: A S = (A,B) + (B,D) + (C,B) or C B D Note that we cannot continue as the endpoints in S are now all connected. Thus, the minimum spanning tree consists of the edges or links (A, B) + (B, D) + (C, B) and has the weight 1 + 4 + 3, or 7. Now that we have an appreciation for the method by which a minimum spanning tree is formed, let’s turn our attention to its applicability in transparent bridge-based networks. Similar to the root of a tree, one bridge in a spanning tree network will be assigned to a unique position in the network. Known as the root bridge, this bridge is assigned as the top of the spanning tree, and because of this position, it has the potential to carry the largest amount of intranet traffic due to its position. Because bridges and bridge ports can be active or inactive, a mechanism is required to identify bridges and bridge ports. Each bridge in a spanning tree network is assigned a unique bridge identifier. This identifier is the MAC address on the bridge’s lowest port number and a two-byte bridge priority level. The priority level is defined when a bridge is installed and functions as a bridge number. Similar to the bridge priority level, each adapter on a bridge that functions as a port has a two-byte port identifier. Thus, the unique bridge identifier and port identifier enable each port on a bridge to be uniquely identified. Path Cost Under the spanning tree algorithm, the difference in physical routes between bridges is recognized, and a mechanism is provided to indicate the preference for one route over another. That mechanism is accomplished by the ability bridging and switching methods and performance issues 289 to assign a path cost to each path. Thus, you could assign a low cost to a preferred route and a high cost to a route you only want to be used in a backup situation. Once path costs are assigned to each path in an intranet, each bridge will have one or more costs associated with different paths to the root bridge. One of those costs is lower than all other path costs. That cost is known as the bridge’s root path cost, and the port used to provide the least path cost toward the root bridge is known as the root port. Designated Bridge As previously discussed, the spanning tree algorithm does not permit active loops in an interconnected network. To prevent this situation from occurring, only one bridge linking two networks can be in a forwarding state at any particular time. That bridge is known as the designated bridge, while all other bridges linking two networks will not forward frames and will be in a blocking state of operation. Constructing the Spanning Tree The spanning tree algorithm employs a three-step process to develop an active topology. F irst, the root bridge is identified. To accomplish this, each bridge in the intranet will initially assume it is the root bridge. To determine which bridge should actually act as the root bridge, each bridge will periodically transmit bridge protocol data unit (BPDU) frames that are described in the following section. BPDU frames under Ethernet version 2 are referred to as HELLO frames or messages and are transmitted on all bridge ports. Each BPDU frame includes the priority of the bridge defined at installation time. As the bridges in the intranet periodically transmit their BPDU frames, bridges receiving a BPDU with a lower priority value than its own cease transmitting their BPDUs; however, they forward BPDUs with a lower priority value. Thus, after a short period of time the bridge with the lowest priority value is recognized as the root bridge. In Figure 6.3b we will assume bridge 1 was selected as the root bridge. Next, the path cost from each bridge to the root bridge is determined, and the minimum cost from each bridge becomes the root path cost. The port in the direction of the least path cost to the root bridge, known as the root port, is then determined for each bridge. If the root path cost is the same for two or more bridges linking LANs, then the bridge with the highest priority will be selected to furnish the minimum path cost. Once the paths are selected, the designated ports are activated. 290 chapter six In examining Figure 6.3a, let u s now use the cost entries assigned to each bridge. Let us assume that bridge 1 was selected as the root bridge, since we expect a large amount of traffic to flow between Token-Ring 1 and Ethernet 1 networks. Therefore, bridge 1 will become the designated bridge between Token-Ring 1 and Ethernet 1 networks. Here the term designated bridge references the bridge that has the bridge port with the lowest-cost path to the root bridge. In examining the path costs to the root bridge, note that the path through bridge 2 was assigned a cost of 10, while the path through bridge 3 was assigned a cost of 15. Thus, the path from Token-Ring 2 via bridge 2 to Token- Ring 1 becomes the designated bridge between those two networks. Hence, Figure 6.3b shows bridge 3 inactive by the omission of a connection to the Token-Ring 2 network. Similarly, the path cost for connecting the Ethernet 3 network to the root bridge is lower by routing through the Token-Ring 2 and Token-Ring 1 networks. Thus, bridge 5 becomes the designated bridge for the Ethernet 3 and Token-Ring 2 networks. Bridge Protocol Data Unit As previously noted, bridges obtain topology information by the use of bridge protocol data unit (BPDU) frames. Once a root bridge is selected, that bridge is responsible for periodically transmitting a ‘‘HELLO’’ BPDU frame to all networks to which it is connected. According to the spanning tree protocol, H ELLO frames must be transmitted every 1 to 10 seconds. The BPDU has the group MAC address 800143000000, which is recognized by each bridge. A designated bridge will then update the path cost and timing information and forward the frame. A standby bridge will monitor the BPDUs, but will not update nor forward them. If the designated bridge does not receive a BPDU on its root port for a predefined period of time (default is 20 seconds), the designated bridge will assume that either a link or device failure occurred. That bridge, if it is still receiving configuration BPDU frames on other ports, will then switch its root port to a port that is receiving the best configuration BPDUs. When a standby bridge is required to assume the role of the root or designated bridge, the HELLO BPDU will indicate that a standby bridge should become a designated bridge. The process by which bridges determine their role in a spanning tree network is iterative. As new bridges enter a network, they assume a listening state to determine their role in the network. Similarly, when a bridge is removed, another iterative process occurs to reconfigure the remaining bridges. bridging and switching methods and performance issues 291 Although the STP algorithm procedure eliminates duplicate frames and degraded intranet performance, it can be a hindrance for situations where multiple active paths between networks are desired. In addition, if a link or device fails, the time required for a new spanning tree to be formed via the transmission of BPDUs can easily require 45 to 60 seconds or more. Another disadvantage of STP occurs when it is used in remote bridges connecting geographically dispersed networks. For example, returning to Figure 6.2, suppose Ethernet 1 were located in Los Angeles, E thernet 2 in New York, and Ethernet 3 in Atlanta. If the link between Los Angeles and New York were placed in a standby mode of operation, all frames from Ethernet 2 routed to Ethernet 1 would be routed through Atlanta. Depending on the traffic between networks, this situation might require an upgrade in the bandwidth of the links connecting each network to accommodate the extra traffic flowing through Atlanta. Since the yearly cost of upgrading a 56- or 64-Kbps circuit to a 128- Kbps fractional T1 link can easily exceed the cost of a bridge or router, you might wish to consider the use of routers to accommodate this networking situation. In comparison, when using local bridges, the higher operating rate of local bridges in interconnecting local area networks normally allows an acceptable level of performance when LAN traffic is routed through an intermediate bridge. Protocol Dependency Another problem associated with the use of transparent bridges concerns the differences between Ethernet and IEEE 802.3 frame field compositions. As noted in Chapter 4, the Ethernet frame contains a type field that indicates the higher-layer protocol in use. Under the IEEE 802.3 frame format, the type field is replaced by a length field, and the data field is subdivided to include logical link control (LLC) information in the form of destination (DSAP) and source (SSAP) service access points. Here, the DSAP and SSAP are similar to the type field in an Ethernet frame: they also point to a higher-level process. Unfortunately, this small difference can create problems when you are using a transparent bridge to interconnect Ethernet and IEEE 802.3 networks. The top portion of Figure 6.6 shows the use of a bridge to connect an AppleTalk network supporting several Macintosh computers to an Ethernet network on which a Digital Equipment Corporation VAX computer is located. Although the VAX may be capable of supporting DecNet Phase IV, which is true Ethernet, and AppleTalk if both modules are resident, a pointer is required to direct the IEEE 802.3 frames generated by the Macintosh to the right protocol on the VAX. Unfortunately, the Ethernet connection used 292 chapter six Dec phase IV Apple talk Ethernet NIC Ethernet Ethernet B M M M IEEE 802.3 IEEE 802.3 Length InformationInformation Information DSAP SSAP Control Type Frame differences Legend: = Workstation = Macintosh Figure 6.6 Protocol differences preclude linking IEEE 802.3 and Ethernet networks using transparent bridges. A Macintosh computer connected on an IEEE 802.3 network using AppleTalk will not have its frame pointed to the right process on a VAX on an Ethernet. Thus, the differences between Ethernet and IEEE 802.3 networks require transparent bridges for interconnecting similar networks. by the VAX will not provide the required pointer. This explains why you should avoid connecting Ethernet and IEEE 802.3 networks via transparent bridges. Fortunately, almost all Ethernet NICs manufactured today are IEEE 802.3–compatible to alleviate this problem; however, older NICs may operate as true Ethernets and result in the previously mentioned problem. Source Routing Source routing is a bridging technique developed by IBM for connecting Token-Ring networks. The key to the implementation of source routing is the bridging and switching methods and performance issues 293 use of a portion of the information field in the Token-Ring frame to carry routing information and the transmission of discovery packets to d etermine the best route between two networks. The presence of source routing is indicated by the setting of the first bit position in the source address field of a Token-Ring frame to a binary 1. When set, this indicates that the information field is preceded by a route information field (RIF), which contains both control and routing information. The RIF Field Figure 6.7 illustrates the composition of a Token-Ring RIF. This field is variable in length and is developed during a discovery process, described later in this section. 2 bytes Up to 16 bytes Control Ring # Ring # Ring # Bridge # Bridge # Bridge # 12 bits 4 bits BBB L LL L L LF LF LF LF RRRD Field format B are broadcast bits Bit settings Designator Bit settings 0XX 10X 11X Nonbroadcast All-routes broadcast Single route broadcast L are length bits which denote length of the RIF in bytes D is direction bit LF identifies largest frame Size in bytes 000 001 010 011 100 101 110 111 516 1500 2052 4472 8191 Reserved Reserved Used in all-routes broadcast frame R are reserved bits Figure 6.7 Token-Ring route information field. The Token-Ring RIF is vari- able in length. 294 chapter six The control field contains information that defines how information will be transferred and interpreted and what size the remainder of the RIF will be. The three broadcast bit positions indicate a nonbroadcast, all-routes broadcast, or single-route broadcast situation. A nonbroadcast designator indicates a local or specific route frame. An all-routes broadcast d esignator indicates that a frame will be transmitted along every route to the destination station. A single-route broadcast designator is used only by designated bridges to relay a frame from one network to another. In examining the broadcast bit settings shown in Figure 6.7, note that the letter X indicates an unspecified bit setting that can be either a 1 or 0. The length bits identify the length of the RIF in bytes, while the D bit indicates how the field is scanned, left to right or right to left. Since vendors have incorporated different memory in bridges which may limit frame sizes, the LF bits enable different devices to negotiate the size of the frame. Normally, a default setting indicates a frame size of 512 bytes. Each bridge can select a number, and if it is supported by other bridges, that number is then used to represent the negotiated frame size. Otherwise, a smaller number used to represent a smaller frame size is selected, and the negotiation process is repeated. Note that a 1500-byte frame is the largest frame size supported by Ethernet IEEE 802.3 networks. Thus, a bridge used to connect Ethernet and Token-Ring networks cannot support the use of Token-Ring frames exceeding 1500 bytes. Up to eight route number subfields, each consisting of a 12-bit ring number and a 4-bit bridge number, can be contained in the routing information field. This permits two to eight route designators, enabling frames to traverse up to eight rings across seven bridges in a given direction. Both ring numbers and bridge numbers are expressed as hexadecimal characters, with three h ex characters u sed to denote the ring number and one hex character used to identify the bridge number. Operation Example To illustrate the concept behind source routing, consider the intranet illus- trated in Figure 6.8. In this example, let us assume that two Token-Ring networks are located in Atlanta and one network is located in New York. Each Token-Ring and bridge is assigned a ring or bridge number. For sim- plicity, ring numbers R1, R2, and R3 are used here, although as previously explained, those numbers are actually represented in hexadecimal. Simi- larly, bridge numbers are shown here as B1, B2, B3, B4, and B5 instead of hexadecimal characters. bridging and switching methods and performance issues 295 A A A A A A A B R1 R1 R1 R1 R1 R1 R1 B1 B2 B2 B1 B1 R3 R3 R3 0 0 C D B5 B5 R2 R2 R2 B4 B4 B4 R2 B3 B3 B3 B3 New York Atlanta Figure 6.8 Source routing discovery operation. The route discovery process results in each bridge entering the originating ring number and its bridge number into the RIF. When a station wants to originate communications, it is responsible for finding the destination by transmitting a discovery packet to network bridges and other network stations whenever it has a message to transmit to a new destination address. If station A wishes to transmit to station C, it sends a route discovery packet containing an empty RIF and its source address, as indicated in the upper left portion of Figure 6.8. This packet is recognized by each source routing bridge in the network. When a source routing bridge receives the packet, it enters the packet’s ring number and its own bridge identifier in the packet’s routing information field. The bridge then transmits the p acket to all of its connections except the connection on which the packet was received, a process known as flooding. Depending on the topology of the interconnected networks, it is more than likely that multiple copies of the discovery packet will reach the recipient. This is illustrated in the upper right corner of Figure 6.8, in which two discovery packets reach station C. Here, one packet contains the sequence R1B1R1B2R30 — the zero indicates that there is no bridging in the last ring. The second packet contains the route sequence R1B3R2B4R2B5R30. Station C then picks the best route, based on either the most direct path or the earliest arriving packet, and transmits a response to 296 chapter six the discover packet originator. The response indicates the specific route to use, and station A then enters that route into memory for the duration of the transmission session. Under source routing, bridges do not keep routing tables like transparent bridges. Instead, tables are maintained at each station throughout the network. Thus, each station must check its routing table to determine what route frames must traverse to reach their destination station. This routing method results in source routing using distributed routing tables instead of the centralized routing tables used by transparent bridges. Advantages There are several advantages associated with source routing. One advantage is the ability to construct mesh networks with loops for a fault-tolerant design; this cannot be accomplished with the use of transparent bridges. Another advantage is the inclusion of routing information in the information frames. Several vendors have developed network management software products that use that information to p rovide statistical information concerning intranet activity. Those products may assist you in determining how heavily your wide area network links are being used, and whether you need to modify the capacity of those links; they may also inform you if one or more workstations are hogging communications between networks. Disadvantages Although the preceding advantages are considerable, they are not without a price. That price includes a requirement to identify bridges and links specifically, higher bursts of network activity, and an incompatibility between Token-Ring and Ethernet networks. In addition, because the structure of the Token-Ring RIF supports a maximum of seven entries, routing of frames is restricted to crossing a maximum of seven bridges. When using source routing bridges to connect Token-Ring networks, you must configure each bridge with a unique bridge/ring number. In addition, unless you wish to accept the default method by which stations select a frame during the route discovery process, you will have to reconfigure your LAN software. Thus, source routing creates an administrative burden not incurred by transparent bridges. Due to the route discovery process, the flooding of discovery frames occurs in bursts when stations are turned on or after a power outage. Depending upon the complexity of an intranet, the discovery process can degrade network [...]... Size (Bytes) Ethernet 15 26 72 Fast Ethernet 15 26 72 Gigabit Ethernet 15 26 72 10 Gigabit Ethernet 15 26 72 50% Load 100% Load 4 06 7440 812 14,880 4050 74,400 8120 148,800 40 ,60 0 744,000 81,200 1,488,000 4 06, 000 7,440,000 812,000 14,880,000 310 chapter six central processor to support the additional frame processing associated with their higher operating rate We can extend our analysis of Ethernet frames... 15 26- byte frames Here, the time per frame becomes: 9 .6 µsec + 15 26 bytes × 8 bits/byte or 9 .6 µsec + 12,208 bits × 100 nsec/bit or 1.23 msec Thus, in one second there can be a maximum of 1/1.23 msec or 812 maximumsize frames For a minimum frame size of 72 bytes, the time per frame is: 9 .6 µsec + 72 bytes × 8 bits/byte × 100 nsec/bit or 67 .2 × 10 6 sec Thus, in one second there can be a maximum of 1 /67 .2... degrade significantly when use exceeds between 60 to 70 percent of that rate Ethernet Traffic Estimation An Ethernet frame can vary between a minimum of 72 bytes and a maximum of 15 26 bytes Thus, the maximum frame rate on an Ethernet will vary with the frame size Ethernet operations require a dead time between frames of 9 .6 µsec The bit time for a 10-Mbps Ethernet is 1/107 or 100 nsec Based upon the preceding,... pair of remote bridges connected by a 9 .6- Kbps line The time per frame for a 72-byte frame at 9 .6 Kbps is: 9 .6 × 10 6 + 72 × 8 × 0.0001041 s/bit or 0.0599712 seconds per frame Thus, in one second the number of frames is 1/.0599712, or 16. 67 frames per second Table 6. 3 compares the frame-per-second rate supported by different link speeds for minimum- and maximum-size Ethernet frames As expected, the frame... speed available for the transmission of data is 1.5 36 Mbps TABLE 6. 3 Link Speed versus Frame Rate Frames per Second Link Speed Minimum Maximum 9 .6 Kbps 16. 67 0.79 19.2 Kbps 33.38 1.58 56. 0 Kbps 97.44 4 .60 64 .0 Kbps 111.17 5.25 2815.31 1 36. 34 14,880.00 812.00 1.5 36 Mbps 10.0 Mbps bridging and switching methods and performance issues 311 Predicting Throughput Until now, we have assumed that the operating... preamble and 6 bytes for the destination address) before being able to initiate a search of its port-destination address table At 10 Mbps we obtain: 9 .6 µs + 14 bytes ∗ 8 bits/byte ∗ 100 ns/bit or 9 .6 × 10 6 + 112 ∗ 100 ∗ 10−9 or 20.8 ∗ 10 6 seconds Here 9 .6 µs represents the Ethernet interframe gap at an operating rate of 10 Mbps, while 100 ns/bit represents the bit duration of a 10-Mbps Ethernet LAN... secretarial activity = 3,112 × 3 = 9,3 36 bps Total estimated network activity = 49,5 46 bps ∗ Note: Bit rate is computed by 3 ,60 0 seconds/hour Frequency Bit Rate∗ 1,500 480,000 120,000 2,000 1/hour 1/hour 2/hour 2/hour 4 1, 067 533 9 1 ,61 3 1,500 320,000 30,000 3,000 2/hour 2/hour 2/hour 4/hour 7 1,422 134 27 1,590 1,500 64 0,000 12,000 3,000 4/hour 2/hour 8/hour 6/ hour 14 2,844 214 40 3,112 by multiplying... 1 /67 .2 × 10 6 or 14,880 minimum-size 72-byte frames Since 100BASE-T Fast Ethernet uses the same frame composition as Ethernet, the maximum frame rate for maximum- and minimum-length frames are ten times that of Ethernet That is, Fast Ethernet supports a maximum of 8120 maximum-size 15 26- byte frames per second and a maximum of 148,800 minimum-size 72-byte frames per second Similarly, Gigabit Ethernet uses... support by an order of magnitude beyond the frame rate of Gigabit Ethernet For both Gigabit and 10 Gigabit Ethernet the maximum frame rates are for full-duplex operations Table 6. 2 summarizes the frame processing requirements for a 10-Mbps Ethernet, Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet under 50 percent and 100 percent load conditions, based on minimum and maximum frame lengths Note... associated with Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet commonly preclude the ability to upgrade a bridge by simply changing its adapter cards Due to the much greater frame processing requirements associated with very high speed Ethernet networks, bridges are commonly designed to support those technologies from the ground up to include adapters and a TABLE 6. 2 Ethernet Frame Processing . of Ethernet frames, such as Ethernet, IEEE 802.3, Novell’s Ethernet- 802.3, and Ethernet- SNAP. The latter two frames represent variations of the physical IEEE 802.3 frame format. Ethernet and Ethernet- 802.3. IV Apple talk Ethernet NIC Ethernet Ethernet B M M M IEEE 802.3 IEEE 802.3 Length InformationInformation Information DSAP SSAP Control Type Frame differences Legend: = Workstation = Macintosh Figure 6. 6. thernet 2 in New York, and Ethernet 3 in Atlanta. If the link between Los Angeles and New York were placed in a standby mode of operation, all frames from Ethernet 2 routed to Ethernet 1 would be routed