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How Layer 3 Switching Works
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In this section of the chapter, we will discuss how layer 3 switching works First, we will cover the difference between layer 3 switching and routing Then, we will cover MLS's basic data-forwarding paradigm Finally, we will take a look at the interactions between switching devices and routing devices in a multilayer-switching environment
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Routing Versus Layer 3 Switching
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First, you need to know that MLS makes a distinction between routing and layer 3 switching Basically, when a router routes a packet, it performs the following actions:
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Receives a packet destined for its MAC address, but a remote IP address Looks in its routing table to determine where to send the packet Modifies the source MAC address in the packet, removing the sender's MAC address and inserting the router's MAC address Modifies the destination MAC address in the packet, removing its MAC address and inserting the next hop MAC address Decrements the IP TTL by at least 1 Recomputes the frame and IP checksums Forwards the packet out the appropriate interface
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A layer 3 switch does things a little differently First, layer 3 switching is based on the understanding that packets do not live in a vacuum In other words, you are unlikely to send a single packet to a remote host, and that remote host is unlikely to send only one packet back In most cases, you will both transmit and receive several packets to and from a remote host Routers (in the classical definition) do not follow this principle Every packet a router receives goes through the full process of looking in the table, changing the headers, and being forwarded This process is time-consuming A layer 3 switch, on the other hand, believes in "route once, switch many" In other words, if you send 4,000 packets from your PC to a specific remote host (say, wwwciscocom), you have to do all of that routing work for only the first packet in the communication The rest just use the information gained from routing the first packet Thus, the entire communication (except for the first packet) is much faster than if you routed each individual packet Note The classical definition of routing is known as process switching By default, most Cisco routers are configured to process switch packets However, some models are also able to use other switching mechanisms (such as fast switching and Cisco Express Forwarding) that more closely resemble the layer 3 switching "route once, switch many" rule For more information on switching mechanisms in routers, please visit http://wwwciscocom/ univercd/cc/td/doc/ product/software/ios121/ 121cgcr/switch_c/xcprt1/ xcdovipshtm
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MLS Data Forwarding
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In Cisco-ese, the communication stream is known as a flow A flow can be described in three ways:
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A communication to a given destination (known as destination IP) A communication to a given destination from a specific host (known as source destination IP) A communication to a given destination from a specific host using specific layer 4 protocols and port numbers (known as IP flow) Note Flows are unidirectional That is, if PC1 sends a packet to PC2, the packet stream from PC1 to PC2 is a single flow If PC2 responds back to PC1, that is classified as a separate flow
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The type of flow used by the switch (known as the MLS-SE, or multilayer switchingswitching engine) is known as the flow mask In most cases, the switch uses the destination IP mask However, if you are using access lists (discussed later in this section, as well as in 27), or if you need more specific accounting or logging data (in other words, you want to know who is eating up all of your bandwidth and which application they are using), you may choose to use one of the other masks The flow mask is primarily used to determine which information about flows is entered into the switching cache (known as the MLS cache) For instance, in Figure 20-2, PC1 (19216811) is communicating with Server1 (10111) through a layer 3 switch For the first packet, the layer 3 switch notices that the destination MAC is for the router, so it looks in its MLS cache for the destination IP address
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Figure 20-2: The MLS-SE cannot find the destination in the MLS cache Because an MLS entry was not found for the destination IP address, it forwards the packet to the router (known as the MLS-RP, or multilayer switching-route processor) for processing (shown in Figure 20-3)
Figure 20-3: The MLS-SE forwards the packet to the MLS-RP The router routes the packet and forwards it back to the MLS-SE (switch), which adds the destination to the MLS cache (shown in Figure 20-4) Because this example is using only the destination IP flow mask, the only information added is the destination IP, destination MAC address, and destination port
Figure 20-4: The MLS entry added to the MLS cache When PC1 sends the next packet, the MLS-SE looks in its cache, finds the entry for the destination, and rewrites the packet with the new information, without all of the additional overhead associated with routing the packet See Figure 20-5
Figure 20-5: The MLS process for subsequent frames in the flow Consequently, when PC 2 sends a packet to Server1, the communication will match the flow already entered in the table; and it will be switched, not routed, to the destination (shown in Figure 20-6)
Figure 20-6: The MLS process for frames from another source to the same destination So, the basic process for layer 3 switching is as follows:
The switch examines the destination MAC address If it is for any of the switch's (MLS-SE) configured routers (MLS-RP), layer 3 processing begins If not, the packet is layer 2 switched If the packet is to be layer 3 processed, the switch looks for an entry in the MLS cache for the destination IP address
If an MLS entry is found, the packet is rewritten using the information in the entry, the TTL is decremented, and the checksums are recomputed If an MLS entry is not found, the frame is forwarded to the appropriate MLS-RP; and it routes the packet and returns the packet to the MLS-SE, which writes a new cache entry The frame is forwarded Note In MLS, only the destination IP is checked for a match, regardless of the flow mask used This point is important because some Cisco documentation about this subject can be misleading
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