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New Member

ATM interface MTU

Hi,

We have 7206 with ENHANCED ATM PA - OC3 adapter.

I parsed "sh int atm 1/0 "thru the "Otput nterpreter":

https://www.cisco.com/cgi-bin/Support/OutputInterpreter/home.pl

and got the warning (see the EOM for the outputs) that the MTU is high (the default MTU for this interface is 4470. Is there a reason to change the MTU size in this case. We are running PPPeE on ATM for our ADSL custommers.

Thanks a lot,

Val

sh int atm 1/0 output:

ATM1/0 is up, line protocol is up

Hardware is ENHANCED ATM PA

MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit, DLY 80 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ATM, loopback not set

Keepalive not supported

Encapsulation(s): AAL5

4096 maximum active VCs, 188 current VCCs

VC idle disconnect time: 300 seconds

0 carrier transitions

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters 00:08:35

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: Per VC Queueing

5 minute input rate 121000 bits/sec, 43 packets/sec

5 minute output rate 492000 bits/sec, 62 packets/sec

29909 packets input, 8937192 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

3 input errors, 3 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

39014 packets output, 35151690 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

Otput nterpreter warning:

Interface ATM1/0 (up/up)

WARNING: The MTU is high.

TRY THIS: On AIP, use the smallest MTU you can, preferably 1500. Set it on all

subinterfaces and also on the main interface.

The MTU is very important on AIP, since it directly affects the number of

available buffers in MEMD and on the AIP itself.

VIP-based ATM port adapters are not as affected, since they use fixed size

particles instead of MTU size buffers and VIPs are much faster in draining

packets from MEMD.

Be careful - changing MTU on a 7500 will cause a CBUS restart.

During CBUS restart, all the interfaces will go down/up, since SRAM buffers are

reassigned to all the interfaces. The router will not switch any packets for

approximately 20-30 seconds.

4 REPLIES
Cisco Employee

Re: ATM interface MTU

For PPPoE connection, MTU size should be 1492 as documented at

http://www.cisco.com/warp/public/794/router_mtu.html

So if you are terminating the PPPoE connection on the aggregator 7200, you need to have "mtu 1492" under the "interface virtual-template 1"..See following url for config on 7200

http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t1/dtpppoe.htm#28158

New Member

Re: ATM interface MTU

TEJAL,

Thank you for the reply.

Yes, MTU of the "virtual-template" is 1492.

My question was about the ATM interface.

Anyway, thanks a lot again,

Val

Cisco Employee

Re: ATM interface MTU

MTU for ATM interface is defaults to 4470..(maximum mtu on that interface). You should not modify the mtu size at atm layer..At ip protocol later mtu should be 1492....

Changing the MTU value with the mtu interface configuration command can affect values for the protocol-specific versions of the command (ip mtu for example). If the value specified with the ip mtu interface configuration command is the same as the value specified with the mtu command and you change the value for the mtu command, the ip mtu value automatically matches the new mtu value. However, changing the value for the ip mtu command has no effect on the value for the mtu command.

New Member

Re: ATM interface MTU

Tejal,

thanks again for the detailed answer. The reason I'm asking these questions about MTU is the problem I have with our ADSL customers. Please see this my post:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.ee81a01

Thanks,

Val

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