Cisco Systems RJ-45-to-AUX manual Adjacency Table, CEF Search Engine

∙Per−destination load balancing—Enabled by default when you enable CEF. It allows multiple paths to be used for load sharing. Packets destined for a given destination or source host are guaranteed to take the same path, although multiple destinations are available.

∙Per−packet load balancing—Uses a round−robin approach to determine what path individual packets will take over the network. Per−packet load balancing ensures balancing when multiple paths are available to a given destination. This method allows packets for a given destination to take different paths. However, per−packet load balancing does not work well with data such as Voice over IP and video; these types of data packets need a guarantee that they will arrive at the destination in the same sequence they were sent.

The Adjacency Table

The adjacency table maintains a one−to−one correspondence to the FIB. All entries in the FIB are maintained in the adjacency table. A node is said to be adjacent if the node can be reached in one hop. CEF uses the adjacency table to apply Layer 2 address information determined by such protocols as Address Resolution Protocol (ARP) when the next hop must use the physical hardware address of the interface. The adjacency table provides the Layer 2 information necessary to switch the packet to its next point destination; the table is updated as adjacencies are discovered.

The adjacency table contains the MAC address for routers that map to Layer 2 to Layer 3 addresses. It uses the IP ARP to populate neighbors gleaned from IP and Internetwork Packet Exchange (IPX) updates, indexed by interface and address. For each computed path, a pointer is added for the adjacency corresponding to the next hop. This mechanism is used for load balancing where more than one path exists to a destination.

Using host−to−route adjacencies, a few other types of adjacencies are used to expedite switching in certain instances. Let’s look at these instances and the conditions in which other adjacencies are used:

∙Null adjacency—Packets destined for a Null0. The Null0 address is referred to as the bit bucket. Packets sent to the bit bucket are discarded. This is an effective form of access filtering.

∙Glean adjacency—A node connected directly to more than one host, such as a multihomed PC. In this situation, the router or switch maintains a prefix for the subnet instead of the individual host. If a packet needs to reach a specific host, the adjacency table is gleaned for the information specific to that node.

∙Punt adjacency—Packets that need to be sent to another switching layer for handling. This is done when a packet needs special handling, or when the packets need to be forwarded to a higher switching layer.

∙Discard adjacency—Packets that are sent to the bit bucket and whose prefix is checked. The Cisco 12000 GSR is the only Cisco device using this type of adjacency.

CEF Search Engine

The CEF search engine can make either Layer 2−based or Layer 3−based switching decisions. The FIB places incoming packets into the internal memory. From there, the first 64 bytes of the frame are read. If a Layer 2 adjacency resolution needs to be made, the microcode sends the search engine the relevant source MAC address, destination MAC address, or the Layer 3 network destination. The search engine then conducts a lookup of the CAM table for the corresponding information. CEF uses the search engine to find the MAC address or the longest match on the destination network address. It does this very quickly and responds with the corresponding rewrite information; it then stores this information in the CAM table.

The CEFA now knows the port−of−exit for the packet, based either on its MAC address or on the Layer 3 IP or IPX network numbers. The packet is now transferred across the switching fabric to its point of destination to be sent to its next hop. The destination interface prepares the packet prior to exiting the switch. Figure 4.3 shows the CEFA components.

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