Cisco expects CCNAs to have command of the tradeoffs involved when designing campus LANs. One key consideration is to understand the different behavior when separating, or segmenting, LAN segments with some switching device. All the concepts related to LAN segmentation using bridges, switches, and routers are included in other parts of this chapter or in other chapters. However, the comparisons made in this section are important and would be missed if this book simply covered the technical content when covering bridging, switching, and routing.
The basic workings of a bridge and a switch have already been covered in this chapter. For review, routing logic is covered first, followed by a comparison of segmentation using the three different device options.
Routing is covered more fully in other chapters. For comparison, the same example flow of a client connecting to a Web server is shown, this time with a router separating two Ethernet segments. This same example is shown for bridging and switching earlier in this chapter. Figure 4-13 illustrates a couple of key features of routing.
The flows in the figure match the numbers in this list, which explains the meaning and implications of the flows in the figure:
Step 1 The PC is preconfigured with the IP address of the DNS. The PC notices that the IP address is on a different subnet, so the PC wants to forward the packet to its default router. However, the PC does not know its default router’s MAC address yet, so it must use ARP to find that router’s MAC address.
Step 2 The router replies to the ARP request with its MAC address, 0200.4444.4444.
Step 3 The PC requests name resolution for the Web server by sending a packet with the destination IP address of the DNS. The destination MAC address in the frame sent by the PC is the router’s E0 MAC address. The router receives the frame, extracts the packet, and forwards it.
Step 4 The DNS returns the IP address of the Web server to the PC in the DNS reply.
Step 5 The PC does not know the Web server’s MAC address, so it sends an ARP broadcast to learn the MAC address. The router has no need to forward the ARP broadcast.
Step 6 The Web server replies to the ARP, stating that its MAC address is 0200.3333.3333.
Step 7 The PC can now connect to the Web server.
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Figure 4-13 Example Protocol Flows—Using a Router
The ARP broadcasts are not forwarded by the router. In fact, the logic in Step 1 begins with an ARP looking for the MAC address of the client’s default router—namely, the router’s E0 MAC address. This broadcast was not forwarded by the router, a fact that causes a router to be called a broadcast firewall. Comparing this to a transparent bridge or a LAN switch, this difference in broadcast treatment is the biggest advantage of routers.
0200.3333.3333 0200.4444.4444 0200.5555.5555 Web Client E0 E1 DNS DMAC = FFFF.FFFF.FFFF SMAC = 0200.1111.1111 ARP (Router-E0) 0200.1111.1111 1 DMAC = 0200.1111.1111 SMAC = 0200.4444.4444 ARP Reply 2 DMAC = 0200.4444.4444 SMAC = 0200.1111.1111 DNS Request 3 DMAC = 0200.1111.1111 SMAC = 0200.4444.4444 DNS Reply 4 DMAC = FFFF.FFFF.FFFF SMAC = 0200.1111.1111 ARP (Web) 5 DMAC = 0200.1111.1111 SMAC = 0200.3333.3333 ARP 6 DMAC = 0200.3333.3333 SMAC = 0200.1111.1111 Connect to Web 7 0200.2222.2222
(Note: Addresses of frames on left-side Ethernet)
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Table 4-7 lists several features relating to segmenting LANs with bridges, switches, and routers. Essentially, this chart summarizes features that could differ among the three devices. Table 4-8 lists features that describe how each device performs when compared to using a single Ethernet segment with no bridge, switch, or router. The two tables together provide the necessary details when comparing the three types of devices.
1. Routers can forward IP multicast packets, if configured to do so. However, this does not mean that the LAN multicast frame is forwarded.
Table 4-8 lists features that should be interpreted within the following context: “If I migrated from a single Ethernet segment to a network with two segments separated by a bridge/switch/ router, and if traffic loads and destinations stayed constant, the result would be _______.”
Table 4-7 Comparison of Segmentation Options
Feature Bridging Switching Routing
Forwards LAN broadcasts? Yes Yes No Forwards LAN multicasts? Yes Yes; can be
optimized with CGMP
No1
OSI layer used when making forwarding decision
Layer 2 Layer 2 Layer 3
Internal processing variants Store-and- forward Store-and- forward, cut- through, FragmentFree Store-and- forward
Frame/packet fragmentation allowed? No No Yes Multiple concurrent equal-cost paths to
same destination allowed?
No No Yes
Table 4-8 Comparison of a Single Segment to Multiple Segments
Feature Bridging Switching Routing
Greater cabling distances allowed Yes Yes Yes Decrease in collisions, assuming equal
traffic loads
Yes Yes Yes
Decreased adverse impact of broadcasts No No Yes Decreased adverse impact of multicasts No Yes, with CGMP Yes
Increase in bandwidth Yes Yes Yes
Filtering on Layer 2 header allowed Yes Yes Yes Filtering on Layer 3 header allowed No No Yes
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Certainly, the most important distinction among the three segmentation methods is their treatment of broadcasts and multicasts. Remembering the concepts of collision domains, broadcast domains, and how each device separates LANs into different domains is one key to understanding campus LAN design and troubleshooting.