RE: OT- MTU issues (with addition) posted 11/09/2003
- Subject: RE: OT- MTU issues (with addition)
- From: "Messina, John V" <john@xxxxxxxxxxxxxx>
- Date: Sun, 9 Nov 2003 11:15:37 -0500
- Content-class: urn:content-classes:message
- Thread-index: AcOmy42QIZNqmIxKQNKrlu7J75/0wwADcvHg
- Thread-topic: OT- MTU issues (with addition)
Thank you for reply
Here is a layout easier to see
my 3550 is configured like this
ip name-server 18.104.22.168
spanning-tree mode pvst
spanning-tree extend system-id
ip address 22.214.171.124 255.255.255.255
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,122
switchport mode trunk
switchport mode access
switchport mode access
ip address 126.96.36.199 255.255.255.0
(my class C)
ip address 188.8.131.52 255.255.255.252
router bgp 27484
bgp default local-preference 200
network 184.108.40.206 mask 255.255.255.0
network 220.127.116.11 mask 255.255.255.255
neighbor 18.104.22.168 remote-as 16631
peer A 2948
neighbor 22.214.171.124 soft-reconfiguration inbound
neighbor 126.96.36.199 route-map ALLOWLOOPIN in
neighbor 188.8.131.52 route-map SENDNETSOUT out
neighbor 184.108.40.206 remote-as 27484
neighbor 220.127.116.11 soft-reconfiguration inbound
neighbor 18.104.22.168 remote-as 16631
peer b 12000
neighbor 22.214.171.124 ebgp-multihop 5
neighbor 126.96.36.199 update-source Loopback0
neighbor 188.8.131.52 soft-reconfiguration inbound
neighbor 184.108.40.206 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 www.mydomainname.com
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
I have done pings with different packet sizes and that's when problems
occur. With and without DF set.
From: Howard C. Berkowitz [mailto:hcb@xxxxxxxxxxxx]
Sent: Sunday, November 09, 2003 9:11 AM
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.
>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] =======?=============>
? 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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