- Bronze, 100 points or more
I have few questioins about fib in cisco express fowrarding . please assume cisco mulilayer switch for my questions.
1)When will the next hop entry list " receive" in fib table?
I observed for default route the next hop entry is " receive".
2)Will all defaults route entries have next- hop as "receive" regardless of how the default route is learnt e.g by routing protocol,by ip route 000000 command,?
3) for every prefix which has next-hop entry as "receive", Will layer 3 engine use routing table?
4)For every prefix in fib which has "punt" adjacency in adjaceny table,would layer 3 engine use routing table to foward it?
thanks a lot and have a nice day!
these questions are really detailed.
Here are my attempts of answering your questions
Good starting points are:
Useful commands are the following:
show adjacency detail
to see if the neighbor entries are present and valid
show cef interface detail
show ip cef
show ip cef exact-route
show ip cef 10.4.4.4 platform
the following link helps
Next Hop-The next hop that is associated with this Prefix
The possible Next Hop values are:
receive-The prefix that is associated with MSFC interfaces
This entry contains a prefix with a mask of 32 that corresponds to the IP address of the Layer 3 (L3) interfaces.
attached-The prefix that is associated with a connected network
The IP address of the next hop
drop-All packets that match an entry with a drop are dropped.
the prefixes should be all /32 and be = ip addresses on the MSFC
0.0.0.0/32 has received
in the link
Cat6500-A#show ip cef
Prefix Next Hop Interface
0.0.0.0/0 126.96.36.199 FastEthernet2/48
receive indicates a L3 address of the MSFC
If this is true it is natural that traffic to these /32 prefixes has to be sent to the main cpu for process switching (example: a BGP packet with destination loop5 on the router).
4) punt = sent to cpu for process switching
Punt adjacency. Represents destinations that cannot be switched in the normal path and that are punted to the next-fastest switching vector.
So receive should be used only for /32 prefixes that are ip addresses of the MSFC itself.
Hope to help