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CEF drops packets due to ARP timeout

Core Issue

Cisco Express Forwarding (CEF) is a Layer 3 (L3) IP switching technology that provides superior performance compared to other switching technologies, especially in networks with dynamic traffic patterns. 

CEF maintains data structures called Forwarding Information Base (FIB) and adjacency tables. The FIB table mirrors the information in the routing table and is used to make forwarding decisions. The adjacency table contains the precomputed link-layer header for next hop devices. Based on the next hop interface, entries in the FIB table are mapped to entries in the adjacency table. A device is not able to CEF switch packets if the adjacency table is not populated with the required information.


If CEF drops packets at regular intervals, interspaced by periods of normal operation, it is probably due to the adjacency table being cleared periodically. This is caused by the aging of the Address Resolution Protocol (ARP) entry. Packets are not CEF switched for the duration in which the adjacency table is repopulated with the required next hop information. While ARP entries are refreshed by default every four hours, configuring a very small value of ARP timeout is disruptive to CEF operation.


To change the time in which an entry remains in the ARP cache, issue the arp timeout command in interface configuration mode.

For more information on CEF adjacency, refer to Troubleshooting Incomplete Adjacencies with CEF.

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‎06-22-2009 05:32 PM
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