cef on cisco 2950 switch

Answered Question
Dec 6th, 2008

hi every body!

I was reading about cef . I was wondering about switching based on layer 4 informations.

1) Can switch use cef to switch packets based on layer 4 informations( port numbers).

2) cisco 2950 is layer 2 switch, it is not multilayer switch. However my book by cisco press (CCNP BCMSN)says " cef is supported on 2950 switch. Is it correct?

If it is correct, how cef is used to switch frame as 2950 being layer 2 switch , does not have ip routing table which is usually used by cef .

thanks alot!

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 12 months ago

Hello Sarah,

1) I think CEF is best suited for layer3: there are switches that are able to perform L4-L7 switching but they use or can use other methods to build their tables. The main problem with L4-L7 switches is that size of tables increases.

I think that for performing this kind of switching a traffic driven approach could help in containing the size of the tables: CEF is topology driven it tries to preempt all possible flows if the L4 ports should be taken in account.

However, if we think of Netflow flow definitions it includes the source and destination ports as well.

There are service modules for L4-L7 load-balancing like CSM and ACE that can be installed on a C6500 chassis.

We use them, but the main device/supervisor is only aware to forward traffic to the virtual server IP address, the service module makes the load balancing decision towards the real servers.

In a C6500 you could use ip slb but it is software based.

2) C2950 and CEF

CEF is used to populate forwarding tables in mutlilayer switches: it is not applicable to a layer2 only device.

The CAM table is enough for a L2 only device: even if CEF could be enabled it would stay idle.

Packets to the switch TCP/IP stack has to be process switched (for example a telnet session an SNMP get)

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 12/07/2008 - 04:18

Hello Sarah,

1) I think CEF is best suited for layer3: there are switches that are able to perform L4-L7 switching but they use or can use other methods to build their tables. The main problem with L4-L7 switches is that size of tables increases.

I think that for performing this kind of switching a traffic driven approach could help in containing the size of the tables: CEF is topology driven it tries to preempt all possible flows if the L4 ports should be taken in account.

However, if we think of Netflow flow definitions it includes the source and destination ports as well.

There are service modules for L4-L7 load-balancing like CSM and ACE that can be installed on a C6500 chassis.

We use them, but the main device/supervisor is only aware to forward traffic to the virtual server IP address, the service module makes the load balancing decision towards the real servers.

In a C6500 you could use ip slb but it is software based.

2) C2950 and CEF

CEF is used to populate forwarding tables in mutlilayer switches: it is not applicable to a layer2 only device.

The CAM table is enough for a L2 only device: even if CEF could be enabled it would stay idle.

Packets to the switch TCP/IP stack has to be process switched (for example a telnet session an SNMP get)

Hope to help

Giuseppe

Joseph W. Doherty Sun, 12/07/2008 - 05:24

With regard to your first question, I don't recall reading about CEF's support beyond L3, yet I do recall reading something about policy cache configuration isn't needed when running CEF, and since PBR can use L4 information, perhaps there is some CEF support for L4.

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