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RE: OT- MTU issues (with addition) posted 11/09/2003
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Thank you for reply
Here is a layout easier to see

my 3550 is configured like this 

ip routing
ip name-server
spanning-tree mode pvst
spanning-tree extend system-id
interface Loopback0
 ip address
interface FastEthernet0/1
 description TrunktoOA
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 1,122
 switchport mode trunk
 duplex full
 speed 100
interface FastEthernet0/3
 description 2600
 switchport mode access
 duplex full
 speed 100
interface FastEthernet0/4
 description FW1
 switchport mode access
 duplex full
 speed 100

interface Vlan1
 ip address
(my class C)

interface Vlan122
 ip address

router bgp 27484
 no synchronization
 bgp default local-preference 200
 bgp log-neighbor-changes
 network mask
 network mask
 neighbor remote-as 16631
peer A 2948
 neighbor soft-reconfiguration inbound
 neighbor route-map ALLOWLOOPIN in
 neighbor route-map SENDNETSOUT out
 neighbor remote-as 27484
 neighbor soft-reconfiguration inbound
 neighbor remote-as 16631
peer b 12000 
 neighbor ebgp-multihop 5
 neighbor update-source Loopback0
 neighbor soft-reconfiguration inbound
 neighbor route-map ALLOWDEFAULTIN in

so ISP A and ISP B are out of the picture. I am no longer peering and
the ser int's are shut down. They work fine when up. 

Now I am peering with ISP C. I have 2 peers to them peer A between my
int vlan 122 and their 2848 and peer b between my loopback 0 and their
They have required it to be done this way. So they are sending me a
default route and a route to the default route.  
Everything between my3550 and ISP C's 2948 is layer 2. The transport is
actually being provided by a third party who owns the ring.
Unfortunately I have little details on the info from that transport but
I have been trying to get more. 

>From inside I can ping to everywhere successfully and traceroute. The
path's are correct. From outside in, from every route server on the
internet I could find the trace back to my network is correct and
successful as is name resolution. So I am satisfied that DNS, BGP is
correct. So from the outside in you can telnet to
80 and you will hit my web server. My firewall sees the traffic and the
web server logs the connection. Once any larger traffic is sent it
doesn't work. 

I have done pings with different packet sizes and that's when problems
occur. With and without DF set. 

-----Original Message-----
From: Howard C. Berkowitz [mailto:hcb@xxxxxxxxxxxx] 
Sent: Sunday, November 09, 2003 9:11 AM
To: ccielab@xxxxxxxxxxxxxx
Subject: Re: OT- MTU issues (with addition)

At 10:54 PM -0500 11/8/03, Messina, John V wrote:
>We have a situation with an ISP that's a little strange.

In medicine, there's a distinction between symptoms and signs. A 
symptom is something subjective reported to the patient "I feel hot," 
where a sign is something objective "Temperature is 103 degrees F". 
In your writeup, you are talking about symptoms presented by 
applications. Drop back and use diagnostic tools to find the signs.

>We are
>multihomed with 2 T1's to 2 ISP's for a long time. We are trying to add
>a third ISP via an Ethernet handoff and are running into fragmentation
>issues. The layout for the new ISP is this
>So whenever we plug the firewall into the 3550 and eliminate the other
>ISP's from the equation and use only this Ethernet ISP

OK. Stop at this point. Don't worry, for now, if you can run Internet 
applications. Start with extended ping and traceroute.

[I don't have a router next to me, but I thought traceroute, as well 
as ping, had a length option. A couple of CCO notes says no. If there 
is a length option on traceroute, or if you have another non-Cisco 
traceroute, use that and forget what I see as pings]

While you initially may ping without DF set, you probably want to set 
it as you localize the problem.

Let's redraw your picture a little. I really need to know if some of 
the L2/L3 capable devices are using L3, and if they are on different 

MYFirewall [address] ====[802.3?]====>
[inboundAddress] my3550 [outboundAddress] ====[802.1q]=====>
[inboundAddress] ISP-1a-3550 [outboundAddress] ====Fiber ring*======>
[inboundAddress] ISP-__-3550 [outboundAddress] =======?=============>
[inboundAddress] ISP-__-2948 [outboundAddress] =======?=============>
[inboundAddress] ISP-__-12000 [outboundAddress] =======?=============>

Several notes/questions:

? What medium (including crossover cable)
* What kind of fiber ring? FDDI? POS?
__ what ISP, or router # within ISP?

Ping to some distant endpoint, starting with about 500 bytes, and see 
if you can reach that endpoint.  When you get around 1500, and get a 
failure, walk up on it slowly (e.g., 1484, 1492, 1500, 1502, 1518) so 
you can look for undocumented tunnels.

Once you find a length at which extended ping fails, ideally use an 
extended traceroute with that length through the system.  If you 
don't have such a traceroute, than start pinging with the critical 
length until you find a failure (I hope) or get to the end.

If you do get to the end, consider whether or not the fragmentation 
problem might be in the return path. If you can get to a machine on 
the internet that can send pings/traceroute to you, repeat the 
process in the reverse direction.

>  most users and
>servers cannot browse the internet

>And traffic inbound does not work.

Does that include inbound traffic you know is short, such as a normal 
ping response?  If so, I'd start thinking of an access list, or even 
a QoS statement that classifies by length, or a really crazy WFQ.

>You can telnet to my web servers on
>port 80 but you cannot view it in IE. Similarly you can telnet to the
>terminal servers on 3389 but cannot connect via an RDP client.

RDP? I'm not a Microsoft person.

>We have gone outside the firewall to eliminate it as a source. We have
>plugged in a 3640 to do all routing instead of the 3550 and enabled
>mtu discovery since the 3550 does not support it ( if it does let me
>know)  We have enableb jumbo packet support with the sys mtu 1546
>command on the 3550. We are using a dot1Q trunk between our 3550 and
>ISP for the purposes of video conferencing on their ring. We have
>clients on the ring so this ISP just allows certain vlans over certain
>trunks. To me at least this points to an MTU issue along the way
>somewhere because fragmentation is definitely occurring.
>We have asked the ISP to enable jumbo packet support on the switches in
>the path but they have not done this yet. We have tried all different
>combinations of MTU changes on servers and pmtud disabling.

Again, don't just make changes to options. Find out where the problem 
appears under the present conditions. You may be hitting a tunnel no 
one has told you about--I'd think of that before a fragmentation 
problem based on default MTU.

Only if you can traceroute/ping everywhere should you start changing

>I am curious if anyone has any theories on how to resolve this.
>Thanks for any suggestions
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