Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
Bronze

cef and attached subnet

Hi everybody

Let say we have a setup  as shown below:

Rest of network----R1-f1-199.199.199.0/24-LAN--H10(199.199.199.199.10)

R1 is using CEF:

Fib tables shows the pertinent entry as attached as shown below:

199.199.199.0/24  attached    f1

My question is: packets destined for hosts on 199.199.199.0 subnet say 199.199.199.10 will never be cef switched?  will those packets always be sent to main processor?

thanks and have a great weekend

Everyone's tags (3)
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

cef and attached subnet

Hello Sarah,

>> packets destined for hosts on 199.199.199.0 subnet say 199.199.199.10 will never be cef switched?  will those packets always be sent to main processor?

No, if this would happen CEF would be far less useful and far less efficient.

Host 199.199.199.10 is resolved by ARP process and once host MAC address is known, packet rewrite of IP packets with 199.199.199.10 destination can be performed.

We could talk of a neighbor cache, or adjacencies table that   is equal to ARP table roughly (it is a superset of ARP table if there are serial or other WAN links on the router)

We say that CEF is topology driven so one IP subnet would look like one CEF entry, but CEF has multiple tables including the equivalent of ARP table.

CEF can pick informations from all of these tables when preparing the packet rewrite info.

This means that from a CEF point of view an attached subnet is converted in N entries one for each host of the subnet that has a not expired entry in the ARP table of the router

Again, as in a previous thread  a show output may show less information of it can be misleading

I think that the show cef or sho ip cef shows also the attached hosts but I may be wrong.

Hope to help

Giuseppe

1 REPLY
Hall of Fame Super Silver

cef and attached subnet

Hello Sarah,

>> packets destined for hosts on 199.199.199.0 subnet say 199.199.199.10 will never be cef switched?  will those packets always be sent to main processor?

No, if this would happen CEF would be far less useful and far less efficient.

Host 199.199.199.10 is resolved by ARP process and once host MAC address is known, packet rewrite of IP packets with 199.199.199.10 destination can be performed.

We could talk of a neighbor cache, or adjacencies table that   is equal to ARP table roughly (it is a superset of ARP table if there are serial or other WAN links on the router)

We say that CEF is topology driven so one IP subnet would look like one CEF entry, but CEF has multiple tables including the equivalent of ARP table.

CEF can pick informations from all of these tables when preparing the packet rewrite info.

This means that from a CEF point of view an attached subnet is converted in N entries one for each host of the subnet that has a not expired entry in the ARP table of the router

Again, as in a previous thread  a show output may show less information of it can be misleading

I think that the show cef or sho ip cef shows also the attached hosts but I may be wrong.

Hope to help

Giuseppe

465
Views
0
Helpful
1
Replies
CreatePlease to create content