fib in cef

Answered Question
Sep 11th, 2008
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hi everybody!

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!


Correct Answer by Giuseppe Larosa about 8 years 8 months ago

Hello Sarah,

these questions are really detailed.


Here are my attempts of answering your questions



Good starting points are:


http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/cef.html#wp1130844


http://www.cisco.com/en/US/docs/ios/ipswitch/command/reference/isw_s1.html


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



1) received


the following link helps


http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00804c5472.shtml#adjacency


#


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.





2)

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 14.1.24.1 FastEthernet2/48

0.0.0.0/32 receive



3)

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


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

Giuseppe


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Correct Answer
Giuseppe Larosa Fri, 09/12/2008 - 05:50
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Hello Sarah,

these questions are really detailed.


Here are my attempts of answering your questions



Good starting points are:


http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/cef.html#wp1130844


http://www.cisco.com/en/US/docs/ios/ipswitch/command/reference/isw_s1.html


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



1) received


the following link helps


http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00804c5472.shtml#adjacency


#


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.





2)

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 14.1.24.1 FastEthernet2/48

0.0.0.0/32 receive



3)

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


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

Giuseppe


sarahr202 Fri, 09/12/2008 - 07:58
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  • Bronze, 100 points or more

thanks a lot Giuseppe!

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