Re: OSPF LSA type 3 filtering

I ran into that ISIS MTU issue at work this week on a production link. I
needed to increase the MTU on a link that was running OSPF and ISIS. When I
changed the MTU on one side, ISIS got a little angry with me until I
increased the MTU on the other side. The reason is that ISIS has a Padding
TLV wherein it pads its hello packets to the full MTU size. That's how it
insures that there is no mismatch. If one side has a larger MTU, that
packet will be dropped when it reaches the other side. Once I fixed both
ends, ISIS was happy again.

Interestingly, OSPF also bounced, which I thought was odd. In IOS, if you
already have an adjacency and routing is stable, you can change the MTU
without causing the adjacency to bounce. However, I was doing this in IOS
XR and it clearly bounced OSPF when I changed the MTU. I asked around on
the cisco-nsp list, but so far no one knows why OSPF would bounce like that
in IOS XR. One person thought that perhaps since it is a hardware
forwarding platform, interface buffer memory had to be reallocated after
the MTU change, but that was just a guess. If anyone knows, feel free to
end the mystery. lol I know it won't bounce the adjacency in IOS, at
least on the hardware and images I tested it on. And that makes sense to
me. The MTU is not in the hello packets in OSPF; it's in the DBD packets.

John

On Fri, Jan 4, 2013 at 7:14 PM, Paul Negron <negron.paul_at_gmail.com> wrote:

> I do like that feature.
>
> I also love the way ISIS handles MTU mismatch which is another big problem
> we
> have in real networks.
> You do not just simply ignore it as you do with OSPF.
>
> Paul
> Paul Negron
> CCIE# 14856
> negron.paul_at_gmail.com
>
>
> On Jan 4, 2013, at 1:17 PM, Yuri Bank <yuribank_at_gmail.com> wrote:
>
> > Except that with the IS-IS the hostname is carried in each routers LSP
> > (hostname TLV), so there is no dependency on DNS, or manual configuration
> > on the routers required (Which is very nice imho).
> >
> > -Yuri
> >
> >
> > On Fri, Jan 4, 2013 at 12:10 PM, Marko Milivojevic
> <markom_at_ipexpert.com>wrote:
> >
> >> You mean the output you'd get if you used "ip ospf name-lookup" ;-)
> >>
> >> --
> >> Marko Milivojevic - CCIE #18427 (SP R&S)
> >> Senior CCIE Instructor - IPexpert
> >>
> >> On Fri, Jan 4, 2013 at 12:07 PM, rakesh madupu <raaki.88_at_gmail.com>
> wrote:
> >>> For me, I love isis in our customer deployments because it shows
> >>> neighboring devices names which is peers with, specially with RR's
> names
> >> ,
> >>> life get so much simpler instead of reading an Ip address and
> >> associating it
> >>> again :)
> >>>
> >>>
> >>> On Sat, Jan 5, 2013 at 12:34 AM, Marko Milivojevic <
> markom_at_ipexpert.com>
> >>> wrote:
> >>>>
> >>>> IS-IS supports multiple routed protocols, i.e. IPv4 and IPv6, whereas
> >>>> OSPF doesn't.
> >>>>
> >>>> Also, in the time when MPLS-TE was emerging as a technology, IS-IS
> >>>> behavior to flood unknown TLVs instead of resetting adjacencies when
> >>>> it receives them (OSPF does that when it receives an unknown LSA).
> >>>> meant a very controlled deployment of new technologies. The fact it's
> >>>> not IP, also has some security benefits (cannot be remotely attacked).
> >>>> Etc.
> >>>>
> >>>> What Joseph said is... not quite the reason, since IS-IS also has a
> >>>> requirement for a contiguous L2 area.
> >>>>
> >>>> --
> >>>> Marko Milivojevic - CCIE #18427 (SP R&S)
> >>>> Senior CCIE Instructor - IPexpert
> >>>>
> >>>> On Fri, Jan 4, 2013 at 10:34 AM, Imran Ali <immrccie_at_gmail.com>
> wrote:
> >>>>> marko i need to know why they use is-is over ospf
> >>>>>
> >>>>> On Fri, Jan 4, 2013 at 8:53 PM, Marko Milivojevic <
> >> markom_at_ipexpert.com>
> >>>>> wrote:
> >>>>>>
> >>>>>> In reality, for this purpose, IS-IS and OSPF are pretty much the
> same
> >>>>>> (Type 2 vs Pseudonode LSP). They both use a very similar approach to
> >>>>>> solve the same calculation problem.
> >>>>>>
> >>>>>> Carriers tend to use IS-IS for one other reason (to some extent
> >>>>>> remedied by OSPFv3). This is a separate discussion though.
> >>>>>>
> >>>>>> --
> >>>>>> Marko Milivojevic - CCIE #18427 (SP R&S)
> >>>>>> Senior CCIE Instructor - IPexpert
> >>>>>>
> >>>>>>
> >>>>>> Blogs and organic groups at http://www.ccie.net
> >>>>>>
> >>>>>>
> >> _______________________________________________________________________
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> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>> Any Fool can Know The Point is to Understand - Einstein
> >>>
> >>> www.cciematrix.com
> >>
> >>
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Received on Sat Jan 05 2013 - 12:17:58 ART