sh ip cef adjacency ?

null - adjacency state for packets destined for the Null0 interface that will be dropped or silently discarded

glean - adjacency state for destinations attached via a broadcast network for which no MAC rewrite strings are available

punt - adjacency state for packets forwarded to L3 for processing, typically ip exceptions (packets with ip options)

drop - adjacency state that drops packets because they can't be cef-switched or punted to l3

discard - similar to drop, also applies to policy filtered (ie, acl's)

more from wiki: http://en.wikipedia.org/wiki/Cisco_Express_Forwarding


The adjacency table maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an ARP request for each table lookup. There are several types of adjacencies. Some are listed below:

• Cache adjacency: This type of entry contains the correct outbound interface and the correct MAC address for its FIB entry. The MAC address is the IP address's MAC address if the destination's subnet is directly connected to the router, or is the MAC address of the router that the packet needs to be sent to if the destination's subnet is not directly connected to the router currently processing the packet.
• Receive adjacency: This type of entry handles packets whose final destinations include the router itself. This includes packets whose IP addresses are assigned to the router itself, broadcast packets, and multicasts that have set up the router itself as one of the destinations.
• Null adjacency: Handles packets destined to a NULL interface. Packets with FIB entries pointing to NULL adjacencies will normally be dropped.
• Punt adjacency: Deals with packets that require special handling or can not be switched by CEF. Such packets are forwarded to the next switching layer (generally fast switching) where they can be forwarded correctly.
• Glean adjacency: This adjacency is created when the router knows that either the destination IP's subnet is directly connected to the router itself and it does not know that destination device's MAC address, or the router knows the IP address of the router to forward a packet to for a destination, but it does not know that router's MAC address. Packets that trigger this entry will generate an ARP request.
• Discard adjacency: FIB entries pointing to this type of adjacency will be discarded.
• Drop adjacency: Packets pointing to this entry are dropped, but the prefix will be checked.

dls1#sh ip cef adj null 0 172.16.1.2
% No adjacency for 172.16.1.2 on Null0
dls1#sh ip cef adj glean           
Prefix               Next Hop             Interface
172.16.1.0/24        attached             Vlan1
172.16.10.0/24       attached             Vlan10
172.16.20.0/24       attached             Vlan20
172.16.30.0/24       attached             Vlan30
192.168.1.0/24       attached             FastEthernet0/24
dls1#sh ip cef adj punt 
Prefix               Next Hop             Interface
dls1#sh ip cef adj drop
Prefix               Next Hop             Interface
240.0.0.0/4          drop
dls1#sh ip cef adj discard
Prefix               Next Hop             Interface
dls1#