There are some design and operational advantages that IS-IS has over OSPF.
As you mentioned, IS-IS is a ISO protocol which was originally designed to
route CLNS networks. Because of this the CLNS topology is created using
CLNS, not IP. Even when using IS-IS strictly for IP IS-IS still uses a CLNS
based topology database with IP addresses listed as attributes to each CLNS
entry. Due to this IS-IS does not require a complete SPF calculation when
individual IP links flap. As long as the CLNS NET that the IP addresses are
associated with do not change (flap or renumbered) then individual IP
changes in the IS-IS database only require Partial Route Calculation and not
full SPF calculation. A Partial Route Calculation consists of updating the
appropriate IP information associated with a NET in the IS-IS database.
This is less CPU intensive than full Dijkstra SPF calculations so single
IS-IS areas have been known to scale much greater than OSPF areas.
Depending on the network design, OSPF areas have been known to grow over 100
nodes in a single area while IS-IS has been tested at over 1000 nodes in a
single area. This is a major reason why most service providers prefer IS-IS
over OSPF.
Also, IS-IS is not limited to a backbone area constraint as OSPF is.
Therefore, there is no requirement to tie all areas to a single backbone
area which allows more scalability in the network design as well as
flexibility in controlling routes. There doesn't have to be a single
backbone area with a route to all destinations.
A drawback that relates to the absence of a backbone area within IS-IS is
that all areas are treated as "stub" areas. This means that IS-IS routers
within a area install default routes to all level 2 routers within their
area with active level 2 adjacencies with other areas via the Attached bit.
This can lead to sub-optimal routing due to the absence of specific routes
to destinations however route leaking and other design methods can be used
to overcome this...
Cheers,
-Michael Cohen
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
NRF
Sent: Thursday, May 17, 2001 10:05 PM
To: [EMAIL PROTECTED]
Subject: ISIS vs. OSPF, redux [7:4945]
Hello all:
Thank you for everybody who answered my previous IS-IS question.
I have been compiling a preliminary list on the advantages and disadvantages
of IS-IS vs. OSPF, and where you might want to use one over the other. This
is what I have come up with
IS-IS:
- has a foothold in ISP's for historical reasons, as it was developed
earlier than OSPF. Therefore, for backwards compatibility, ISP's continue
to demand routers that can do IS-IS
- also is used for out-of-band SDH management by telcos
- Converges slightly faster than OSPF
- (Naturally) is multiprotocol, so can handle CLNS, and Decnet phase V
(which is CLNS)
- Has some features that OSPF does not that can be useful in special
situations, like the OL bit, etc.
OSPF
- Is better known, and documentation for it is more readily available
- Has an overall richer set of features than IS-IS (at least, until the
latest IS-IS revisions)
- Is the standard link-state routing protocol for enterprises, and is also
popular in ISP's.
Does anybody have anything to add?
Also, I would like to know what people think the future of IS-IS is,
particularly after the latest revisions. Does anybody think that IS-IS will
be able to maintain and expand its foothold in ISPs, and even move to the
enterprise, or is it forever doomed to its niche (and why do you think so)?
I am especially interested in the opinions of people like Ms. Oppenheimer
and Mr. Berkowitz.
Thanx in advance
FAQ, list archives, and subscription info:
http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=4976&t=4945
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
