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New Member

Interface MTU

Hello,

There is a network in my job, where we have links E1, E3, POS, ATM and Ethernet(Fast and Giga). In this network, there is MPLS TE configured, and the main traffic is voip (90%) with low size packet.

We have many problems with CPU on routers 7513 and 3745, of course that there is a large amount traffic in this network, but I would like to know about MTU size.

All routers are with default size MTU in interfaces, POS, E3 an ATM 4470 and others 1500.

What kind recomendation does anyone give me about this diference between interfaces mtu size?

Thanks all

CLRGomes

8 REPLIES
Purple

Re: Interface MTU

Hi,

I'm not exactly sure that I understand your question but in any case... The largest packet you will be able send end-to-end in any network is limited by the size of the smallest interface MTU along the way. There are various reasons why different interface types have different MTU capabilities.. one of these is hardware capability. Another is protocol limitations e.g. ethernet used to have a maximum frame size of 1518 bytes and so a lot of ethernet hardware would not support anything greater than that.

My recommendation would be to try and configure the interfaces for the highest MTU supported on that hardware.

Hope that helps - pls do rate the post if it does.

Paresh

New Member

Re: Interface MTU

Hello Paresh,

Ok, I understand you, and your recommendation is to configure all interface with the same value and the highest supported, OK.

My question is, due the small packet, there are many packets in the routers, and cpu value is growing very quickly, and can MTU value help me to minimize CPU value?

Thanks

CLRGomes

Purple

Re: Interface MTU

The higher end-to-end MTU will help to ensure that there is less fragmentation in the network. However, the overall size of packets is going to depend on the type of traffic e.g IF your network is transporting a lot of voice traffic, there will be a lot of small packets being switched by your routers and the large MTU setting on your routers will not help at all. The only thing you are doing with a large MTU is ensuring that there is no unnecessary fragmentation (or dropping of packets with the DF bit set)...

Paresh

Silver

Re: Interface MTU

The handling of the small packet is depending on the router processing power. It is not direct related to MTU setting in router.

Even you setup large MTU but the packet originally is small, the router will not combine it to a large packet.

Therefore, if you find the CPU loading is high due to many packet (i.e. high PPS) then you have to consider to upgrade the router to higher performance model. Normally, the spec. of the router will tell you how many packet can be processed per second, it normally is the small size packet (e.g. 64 byte). So you can have an idea of the router processing power.

What is your router model ? Is there any service that enabled in the router to cause high CPU ? e.g. tunnel, IPSEC, IP accounting, etc. ?

Hope this helps.

New Member

Re: Interface MTU

Cisco 3745 (R7000) processor (revision 2.0) with 247808K/14336K bytes of memory.

Processor board ID X

R7000 CPU at 350MHz, Implementation 39, Rev 3.3, 256KB L2, 2048KB L3 Cache

2 FastEthernet interfaces

1 Serial interface

1 ATM interface

8 Channelized E1/PRI ports

1 Subrate T3/E3 port

DRAM configuration is 64 bits wide with parity disabled.

151K bytes of NVRAM.

125184K bytes of ATA System CompactFlash (Read/Write)

This is a router 3745, and this 90% voip traffic with small packet. there is only QoS configured and PBR to mark traffic with precedence value. But I think that there is any problem, because the traffic is not very high.

Please, look the file attached

Thanks all

Silver

Re: Interface MTU

Thanks for the info. According to the sh interface, the total PPS is around 116k PPS, it is not high that 3745 can support up to 225k PPS. And there is no large no. of packet drop.

So I agreed w/ you that there may be some process use CPU resource. Did you check the "sh proc" result ?

Below are some doc. for your reference :

http://www.cisco.com/en/US/tech/tk39/tk371/technologies_tech_note09186a0080093c91.shtml

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a00800a70f2.shtml

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_tech_note09186a00800a65d0.shtml

Hope this helps.

New Member

Re: Interface MTU

Hi Guys,

In context of MTU from from end to end,your traffic will be limited by the smallest MTU in path.

Paresh could you please see show proc cpu and send it to us so we can have look which process is hogging the CPU.

thanks

Jay

New Member

Re: Interface MTU

Hello Jay,

Next monday I will post the complete show proc cpu of 3745 router.

Please wait..

Thanks

CLRGomes

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