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fib in cef

sarahr202
Level 5
Level 5

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!

1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame
Hall of Fame

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

View solution in original post

2 Replies 2

Giuseppe Larosa
Hall of Fame
Hall of Fame

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

thanks a lot Giuseppe!

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