Re: MTU Issues

When a customer is trying to send larger MTU size the next hop to
core notifies him icmp: 10.X.X.X unreachable  need to frag (mtu 1492)but
not my core nor my DS notifies him.

On Wed, Oct 7, 2009 at 12:16 AM, mike arnold <haynessmith70_at_gmail.com>wrote:

> 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 - 00:26:00 ART