CEF and mss-adjust

[shechter]

Hello,

Can cef and 'ip tcp mss-adjust' coexist?

Or better, where can I find a documnet wich states what ios commands can disable cef?

10x!

Dan

[raymong]

You can disable CEF globally or per interface

global - no ip CEF

interface - no ip route-cache CEF

[shechter]

Thank you for your reply.

I was not asking about how to disable cef. I asked what ios commands can cause cef to stop working implicitly (with out intention to disable cef)

So, does the command ‘ip tcp mss adjust’ disables cef?

10x!

Dan

[tbaranski]

I seriously doubt it disables CEF - I suspect that at worst, packets for which the command is used (TCP SYNs) may not be CEF'd, but all other packets should be fine.

(Russ White may be able to speak on this further. Perhaps if I say something like "BGP OSPF EIGRP IS-IS" it will get his attention.)

The problem with making a list of exactly what is supported in the CEF path and what isn't, as nice as such a thing would be, has been that it changes all the time between IOS versions. But as CEF becomes more and more the norm and CEF-incompatible features become less common, maybe we can start to hope for a list of what features *aren't* CEF-capable at an IOS Train level of granularity (rather than on a per-release basis).

[ruwhite]

Adjusting your TCP MSS will not disable CEF--in fact, it has little to do with CEF, since CEF isn't (typically) used for locally orignated packets, and adjusting the TCP MSS is only going to impact locally originated packets. They should be able to co-exist just fine.

:-)

Russ.W

[tbaranski]

http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017cf54.html#1035606 makes it sound like the command applies to all packets going through a given interface, regardless of origin, though I've never tried using it.

(I should clarify my "all other packets should be fine" comment earlier: I'd imagine the added task of checking a bit in each TCP packet's TCP header can be done in CEF mode, since policy routing has been CEF-capable for a good while now.)

[ruwhite]

Hmmm... Very interesting--it _does_ apply to the packets passing through the router, rather than packets being sent to/transmitted by the router. So, this would need to be supported in CEF for CEF to work when it's configured. But, poking around internally, I see that it is supported in CEF, and it's also supported on most if not all of the hardware platforms.

:-)

Russ.W

[shechter]

Thank you for your replies.

I want to use 'ip tcp mss-adjust' to eliminate fragmentation on ipsec tunnels via gre.

You mentioned "poking around internally, I see that it is supported in CEF". Can you point me to where can I poke for that data?

10x!

Dan

[ruwhite]

No, not really. I looked at the internal Cisco web sites, and didn't find a specific reference, but found some defects on aspects of the support for mss adjust which indicate it's supported in the CEF path. I then checked the source code, and the code is there, so I gather it is supported. I didn't test it in the lab, though.

:-)

Russ.W

[shechter]

Thank you for your replies.

I want to use 'ip tcp mss-adjust' to eliminate fragmentation on ipsec tunnels via gre.

You mentioned "poking around internally, I see that it is supported in CEF". Can you point me to where can I poke for that data?

10x!

Dan