GroupStudy.com GroupStudy.com - A virtual community of network engineers
 Home  BookStore  StudyNotes  Links  Archives  StudyRooms  HelpWanted  Discounts  Login
RE: OSPF vs EIGRP [7:41613] posted 04/27/2002
[Chronological Index] [Thread Index] [Top] [Date Prev][Date Next] [Thread Prev][Thread Next]


I remembered this email and ran into a problem with OSPF MTU on a tunnel
interface about an hour ago.  The tunnel was to connect a non backbone area
through an nssa area to the backbone.  Adjusting the mtu of the physical
interface that was the source of the tunnel on the router with the larger
mtu fixed it, and I found an interesting interface command, "ip ospf
mtu-ignore" which makes the router with the smaller mtu ignore the mismatch
and allow an adjacency to form.  I set the mtu's back to the defaults and
allowed the router to complain about the mismatch and then put in the
command above on the tunnel interface, works like a charm.  Just thought it
was interesting so I figured I would send this.

~-----Original Message-----
~From: Priscilla Oppenheimer [mailto:cilla@xxxxxxxxxxxxx]
~Sent: Thursday, April 18, 2002 10:18 PM
~To: cisco@xxxxxxxxxxxxxx
~Subject: Re: OSPF vs EIGRP [7:41613]
~
~
~He didn't say that BGP negotiates the MTU in any of its PDUs. 
~He just says 
~that mismatched MTUs can be a problem, which is all I mentioned in my 
~message about OSPF also (although OSPF does in fact also 
~include the MTU in 
~database description packets and refuse to become adjacent 
~with a router 
~that doesn't agree on the MTU). Did that have enough TLAs for you? ;-)
~
~Priscilla
~
~At 09:53 PM 4/18/02, nrf wrote:
~>Really?  I had never heard of this problem.  I'm not aware that BGP
~>negotiates MTU in any of its PDU's.  Can you provide the RFC 
~that discusses
~>this problem?
~>
~>
~>""suaveguru""  wrote in message
~>news:200204180648.CAA31134@xxxxxxxxxxxxxxxxx
~> > If I am not wrong this problem also occurs for BGP
~> > peers with unmatched MTU sizes which causes BGP to
~> > flap when they exchange routing tables , especially if
~> > one neighbour is configured with full-routes
~> >
~> >
~> > regards,
~> >
~> > suaveguru
~> > --- Priscilla Oppenheimer  wrote:
~> > > The problem happens when the routers try to exchange
~> > > database description
~> > > packets. One side can send packets that are too
~> > > large for the other side to
~> > > receive. Then the routers never achieve adjacency.
~> > > It's an infamous
~> > > problem. I was glad that Kevin brought it up. I was
~> > > thinking we should have
~> > > mentioned it in that other thread about OSPF Hellos
~> > > (although this problem
~> > > happens after the initial hellos).
~> > >
~> > > More here:
~> > >
~> > > http://www.cisco.com/warp/public/104/12.html
~> > >
~> > > Priscilla
~> > >
~> > > At 11:33 AM 4/17/02, Kane, Christopher A. wrote:
~> > > > > The most frequently mismatched parameters
~> > > relevant for OSPF
~> > > > > configuration
~> > > > > seem to be dead intervals & mtu sizes.
~> > > >
~> > > >OSPF doesn't care about MTU size.
~> > > ________________________
~> > >
~> > > Priscilla Oppenheimer
~> > > http://www.priscilla.com
~> > abuse@xxxxxxxxxxxxxx
~> >
~> >
~> > __________________________________________________
~> > Do You Yahoo!?
~> > Yahoo! Tax Center - online filing with TurboTax
~> > http://taxes.yahoo.com/
~________________________
~
~Priscilla Oppenheimer
~http://www.priscilla.com
~
~
~
~
~Report misconduct 
~and Nondisclosure violations to abuse@xxxxxxxxxxxxxx
~




Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=42755&t=41613
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to abuse@xxxxxxxxxxxxxx