Which two tables are used by Cisco Express Forwarding? (Choose two.)
- ACL table
- Routing table
- MAC address table
- Adjacency table
Cisco Express Forwarding (CEF) is a Layer 3 switching technology based on information contained in the forwarding information base (FIB) and the adjacency table.
The FIB is conceptually equivalent to a routing table in that it contains information used in the packet-forwarding decision. It is derived from the routing table and is optimized for maximum lookup throughput. The adjacency table contains information about the adjacent route processors. The adjacency table contains the MAC information for the next-hop addresses for all FIB entries. A device is considered adjacent if it is reachable over a single Layer 2 connection. It is stored in DRAM. The adjacency table is derived from the ARP table.
CEF is a topology-based Layer 3 switching technology that is enabled by default on the latest Cisco products. The FIB table stores IP destination prefixes from the most specific to the least specific entry in the Ternary Content Addressable Memory (TCAM). The content of the FIB table is similar to the routing table and contains the forwarding information similar to what is found in the IP routing table. A single FIB entry can point to up to six adjacencies. When changes are made to the IP routing table, the FIB table is also updated.
The Layer 3 processor engine builds the FIB and adjacency tables in software. That information is distributed from the control-plane hardware to the data-plane hardware Application Specific Integrated Circuits (ASICs) at the port or line card. This enhances the Layer 3 forwarding operation by moving it from the software-based engine to the ASICs. Of course, there are exception packets that are still software-processed, such as:
- Non-conforming protocols
- Data link encapsulations
Packets sent to a destination address that the CEF-based switch does not yet have a valid MAC address for will be sent instead to the Layer 3 engine. The Layer 3 engine will then perform the ARP request. Packets that arrive requiring fragmentation are also first sent to the Layer 3 engine.
When the adjacency table is full, a CEF TCAM table entry points to the Layer 3 engine to redirect the adjacency. Therefore, it will be forwarded by routing.
With respect to CEF switching, there is no ACL table. Access Control Lists (ACL) are used to filter traffic, but are not contained in a table and are not involved directly in Cisco Express Forwarding.
The routing table is not directly involved by CEF, although the FIB is derived from the routing table.
Layer 2 Technologies
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