Re: MTU Issues

Bryan ,

If am pinging through a computer how i can send a larger MTU though if am
having a packet capture (wireshark),The problem is not with the ping the
ping success rate is 100% on MTU till 1492 but when i do with 1493 the
success rate is 0% even though the DF bit is not set.

The customer says that the next hop to core that means ISP/PE is intimating
him icmp: 10.X.X.X unreachable  need to frag (mtu 1492). But not my
core nor my DS.WHY????

On Tue, Oct 6, 2009 at 11:48 PM, Bryan Bartik <bbartik_at_ipexpert.com> wrote:

> Yes, well the Fragmentation needed is a subset of the destination
> unreachable so you are right. Nevertheless it seems like an MTU issue.
> Whether you are using RR's or not shouldn't matter. Do you have a computer
> that you can send pings from while running a capture? It shouldn't be too
> hard.
>
>
> On Tue, Oct 6, 2009 at 1:40 PM, mike arnold <haynessmith70_at_gmail.com>wrote:
>
>>
>> Helllo,
>> Though i have a RR connected to my core and RR is peering MPBGP with core
>> and DS,but since my DS and core are directly connected so it does'nt make
>> such difference for presence of ip next-hopself command on core as my all
>> loopback are advertised in IGP (OSPF,)
>>
>> One thing to highlight mine core and DS is 10gig X2 module,
>>
>> Any more hints from any body,it's a strange issue for me,what else i can
>> do here.
>>
>>
>>
>> On Tue, Oct 6, 2009 at 11:21 PM, mike arnold
<haynessmith70_at_gmail.com>wrote:
>>
>>> i dont have packet capture,this message i won't be getting on my telnet
>>> session.i have read on internet i shld receive this
>>> message.
>>>
>>> Customer complaints that TCP files transfered from B to A with MTU 1500
>>> has no problem but from A to B it can't above 1492. He says that the
problem
>>> is with the customer carrier not with the backbone carrier.
>>>
>>> How the above para can be possible,traffic is passsing through the same
>>> interface as such what it is passing from to A to B
>>>
>>> On Tue, Oct 6, 2009 at 11:10 PM, Bryan Bartik
<bbartik_at_ipexpert.com>wrote:
>>>
>>>> You won't get a Destination Unreachable, you will get a Fragmentation
>>>> Needed and Don't Fragment Bit Set. Did you get a packet capture? Who is
>>>> sending you the message?
>>>>
>>>>
>>>> On Tue, Oct 6, 2009 at 1:01 PM, mike arnold
<haynessmith70_at_gmail.com>wrote:
>>>>
>>>>> Hello Dear,
>>>>>
>>>>> Yes it is a straight ethernet,and i have not changed anything on
>>>>> interface configs on DS/PE facing to customer A,it is default to
>>>>> 1500,customer is complaining that he can't transfer TCP files larger
than
>>>>> 1492 from A----B. But he says that he is able to transfer TCP files
from
>>>>> B----A with larger size than 1492. He says that there is no problem
with
>>>>> backbone carrier but with the customer carrier.
>>>>>
>>>>> When i do a extended ping from mine DS/PE to customer B with a DF bit
>>>>> set the ping sucess is 0% for larger MTU also i dont recieve any error
>>>>> saying ICMP destination unreacheable messages,
>>>>>
>>>>> Generally when a MTU is high and DF is set the intermediate router send
>>>>> a notification ICMP destination unreacheable messages,but in my case no
>>>>> feedback except success rate.
>>>>>
>>>>> On Tue, Oct 6, 2009 at 10:36 PM, Bryan Bartik
<bbartik_at_ipexpert.com>wrote:
>>>>>
>>>>>> Well make sure it raised on ALL the core links. Is the link between
>>>>>> the CE and PE straight Ethernet? 1500 should be fine there.
>>>>>>
>>>>>>
>>>>>> On Tue, Oct 6, 2009 at 12:32 PM, mike arnold <haynessmith70_at_gmail.com
>>>>>> > wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> The link on distribution PE facing to customer A is default to MTU
>>>>>>> 1500,on distribution to core it is mpls mtu 1524 and SVI on core
facing to
>>>>>>> ISP is default to 1500 on ethernet.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Tue, Oct 6, 2009 at 10:15 PM, Bryan Bartik
<bbartik_at_ipexpert.com>wrote:
>>>>>>>
>>>>>>>> Did you raise the MTU on the other links?
>>>>>>>>
>>>>>>>> On Tue, Oct 6, 2009 at 12:02 PM, mike arnold <
>>>>>>>> haynessmith70_at_gmail.com> wrote:
>>>>>>>>
>>>>>>>>> Hello Dear's
>>>>>>>>>
>>>>>>>>> Topology:
>>>>>>>>>
>>>>>>>>> Its a Carrier Supporting Carrier Scenario.
>>>>>>>>>
>>>>>>>>> A------DS/PE---CORE/P---ISP/PE--P---ISP/PE------B
>>>>>>>>>
>>>>>>>>> AS per above topology,am not able to transfer files with MTU frame
>>>>>>>>> size
>>>>>>>>> above 1492 though the MTU configured on interface is 1500.The
>>>>>>>>> interface on
>>>>>>>>> which customer A is connected MTU is default to 1500.The link
>>>>>>>>> between
>>>>>>>>> DS/PE---CR/PE is mpls enabled i have configured the MTU size to
>>>>>>>>> 1524 to
>>>>>>>>> carry labels of MPLS as recommended by cisco.
>>>>>>>>>
>>>>>>>>> When i do extnded ping from DS/PE above MTU size of 1492 with even
>>>>>>>>> DF bit
>>>>>>>>> not set am not able to ping customer end (B),
>>>>>>>>>
>>>>>>>>> AND also i tried with DF bit set am not able to recieve any
>>>>>>>>> notifications
>>>>>>>>> ICMP destination unreacheable messages.
>>>>>>>>>
>>>>>>>>> The file transfer from customer B with packet size above 1492 are
>>>>>>>>> received
>>>>>>>>> by customer A but when the connection are initiated through
>>>>>>>>> customer A they
>>>>>>>>> are not.
>>>>>>>>>
>>>>>>>>> Within an enterprise it works with larger MTU size when tried to
>>>>>>>>> ping from
>>>>>>>>> Distribution loopback to core loopback.How can i troubleshoot it is
>>>>>>>>> a
>>>>>>>>> problem through my end or from the ISP.
>>>>>>>>>
>>>>>>>>> The link between core and ISP is back to back VRF connected via a
>>>>>>>>> layer 2
>>>>>>>>> trunk.The ISP is creating a sub-interfaces on his 3800 router with
>>>>>>>>> encapsulation and vlan number.AND on Core am creating a SVI with a
>>>>>>>>> same vlan
>>>>>>>>> number and ip address within the subnet.
>>>>>>>>>
>>>>>>>>> Each customer VRF for exampl:(Customer A) is associated with SVI
>>>>>>>>> virtual
>>>>>>>>> interface on 6500(core)and traffic is passed to customer B.On SVI
>>>>>>>>> the MTU is
>>>>>>>>> default to 1500.
>>>>>>>>>
>>>>>>>>> BGP is routing protocol between CORE nd ISP.
>>>>>>>>>
>>>>>>>>> ON 3800 router (ISP End)
>>>>>>>>> int gig0/0
>>>>>>>>> no shut
>>>>>>>>> no ip add
>>>>>>>>>
>>>>>>>>> int gig0/0.1
>>>>>>>>> no shut
>>>>>>>>> encapsulation dot1q 50
>>>>>>>>> ip vrf forwarding XX
>>>>>>>>> ip add 10.10.10.1 255.255.255.254
>>>>>>>>>
>>>>>>>>> ON MY END (CORE)
>>>>>>>>> int gig5/1
>>>>>>>>> switchport trunk encapsulation dot1q
>>>>>>>>> switch mode trunk
>>>>>>>>>
>>>>>>>>> int vlan 50
>>>>>>>>> ip vrf forwarding customer A
>>>>>>>>> ip add 10.10.10.2 255.255.255.254.
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Blogs and organic groups at http://www.ccie.net
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
Received on Wed Oct 07 2009 - 00:16:09 ART