Subject: Re: MPLS and IS-IS

From: "J. Noel Chiappa" <[email-masked]>
Date: Wed, 17 May 2000 16:01:29 -0400

> From: Philip Chen <[email-masked]

    > Both IS-IS and OSPF follows two level of hierarchy, but they differ in
    > the amount of information flooded into each area. ... an internal OSPF
    > router can choose an optimal ABR (if there are more than one ABRs) for
    > traffic destinated to other areas. .. IS-IS has no information about
    > other level 1 area. So an internal IS-IS router will always send the
    > traffic destinated to other level 1 areas to the nearest level 2 router
    > ...
    > OSPF fixes some of the scalability problem by introducing stub area
    > .. One important difference is stub area cannot be a transit area.

In fact, it's fundamentally impossible for an area which does not have
complete information about the state of things outside the area to serve as a
transit area (at least in a pure datagram mode of operation - obviously, you
can tunnel stuff across).

The reason is fairly obvious - if an internal router simply routes packets
destined to external destinations to the nearest border router, and the
packet is a transit packet, it's quite likely that that router would be the
router the packet came in through!

It's because IS-IS doesn't import data that it has (unless there's been some
recent upgrade I'm not familiar with) the restriction that the level 2
routers have to form a directly connected graph (i.e. no virtual links across
level 1 areas, as supported by OSPF).

	Noel

---

From: Dave Katz <[email-masked]>
Date: Wed, 17 May 2000 13:30:42 -0700 (PDT)

There has been an upgrade...

Virtual links aren't really something to get too excited about.  The
urge to use them usually means that you should think about replumbing
your topology a bit instead, and implementations tend to be buggy in
this area because most people are smart enough to not use them (see
previous message about bugs and code use).  They're fundamentally
brittle because the routing information is tunneled but the data is
not, so it doesn't take much to mess them up.

---

From: Alex Zinin <[email-masked]>
Date: Thu, 18 May 2000 00:24:52 -0700

> can tunnel stuff across).

As we can see with NSSAs, this pretty much depends on the direction.

If the area has info how to reach the prefixes announced from it to
the backbone, it's all right, since the reverse traffic will be
forwarded to a more knowledgeable router [ABR] along the default.

> router the packet came in through!

We're talking about IGP routes, right? I mean if we have BGP on top of
this, we're gonna have much more fun :)

If we're talking about the IGP case, then things are not so bad.
If that border router received that packet, it means that it either:

- originated a route covering packet's destination address, or
- the best path to such a route is going through it [external route
  most probably]

In the first case we're supposed to know the destination (since that
guy forwarded the packet to us), but it's not interesting, since
we're not the transit area...

In the second case we need to have an explicit route to the external
destination, while having the default to the border. This is the case
with NSSA.

Alex.