> At 07:42 AM 10/12/00, [EMAIL PROTECTED] wrote:
>
> >Actually, EIGRP is a hybrid protocol which means it is
distance vector and link-state.
>
> EIGRP is not a link-state protocol. It's an "advanced"
distance-vector protocol.
>
> Regarding the main question, I wouldn't think OSPF would need
to deal with split horizon, but I can't answer that part for
sure.
>
> Priscilla
>
> >Split horizon kills routing loops which can occur in either
of the 2 so it is indeed available in OSPF. I know this for a
fact because there is a command to disable it in OSPF: no split-
horizon ospf, or something like that...
A couple of reasonable points can be made here. EIGRP is not
explicitly a link state protocol. At best, it is a subset of
link state functionality, but it does not have all of the
attributes that connote a link state protocol, nor was it meant
to be such. Here is Cisco's quote on what EIGRP represents
(from the ITH):
The Enhanced Interior Gateway Routing Protocol (IGRP)
represents an evolution from its predecessor IGRP (refer to
Chapter 38, "Interior Gateway Routing Protocol)." This
evolution resulted from changes in networking and the demands
of diverse, large-scale internetworks. Enhanced IGRP integrates
the *capabilities* of link-state protocols into distance-vector
protocols. It incorporates the Diffusing-Update Algorithm
(DUAL) developed at SRI International by Dr. J.J. Garcia-Luna-
Aceves.
A better explanation of this protocol would likely come from
its author, who did not call it the DUAL Algorithm. Here is
what Dr. J.J. Garcia-Luna-Aceves had to say on, "Loop-Free
Routing Using Diffusing Computations.":
"A family of distributed algorithms for the dynamic computation
of the shortest paths in a computer network or internet is
presented, validated, and analyzed. According to these
algorithms, each node maintains a vector with its distance to
every other node. Update messages from a node are sent only to
its neighbors; each such message contains a distance vector of
one or more entries, and each entry specifies the length of the
selected path to a network destination, as well as an
indication of whether the entry constitutes an update, a query,
or a reply to a previous query. The new algorithms treat the
problem of distributed shortest-path routing as one of
diffusing computations, which was first proposed by Dijkstra
and Scholten. They improve on algorithms introduced previously
by Chandy and Misra, Jaffe and Moss, Merlin and Segall, and the
author. The new algorithms are shown to converge in finite time
after an arbitrary sequence of link cost or topological
changes, to be loop-free at every instant, and to outperform
all other loop-free routing algorithms previously proposed from
the standpoint of the combined temporal, message, and storage
complexities."
Clearly from the explanation above, it does incorporate the
best of distance vector routing and link state routing. It is
however not one or the other. An adequate characterization
would be that supplied by Priscilla, which is that it is an
*advanced* distance vector protocol.
Regrading the issue of split horizon, that is just one of many
safety locks included in routing protocols to reduce or
eliminate the chance of routing loops. Split Horizon however,
seems to have limited functionality beyond distance vector
routing. Check out this link (watch wrap):
http://www.cisco.com/univercd/cc/td/doc/product/software/ios11/c
book/ciproute.htm
Specifically,
Enable or Disable Split Horizon
Normally, routers that are connected to broadcast-type IP
networks and that use *distance-vector* routing protocols
employ the split horizon mechanism to reduce the possibility of
routing loops. Split horizon blocks information about routes
from being advertised by a router out any interface from which
that information originated. This behavior usually optimizes
communications among multiple routers, particularly when links
are broken. However, with nonbroadcast networks, such as Frame
Relay and SMDS, situations can arise for which this behavior is
less than ideal. For these situations, you might want to
disable split horizon. This applies to IGRP and RIP.
I have found the ability to turn off/influence split horizon on
the following protocols: RIP, IGRP, EIGRP, CLNS, Vines, OSI
Routing, DECnet, IPX RIP, IPX SAPs, XNS. I did not find any
mention of the use of split horizon explicitly on an OSPF
network.
It is also interesting to note that RIP, IGRP, and EIGRP are
all sent out with a TTL of 2, while OSPF is sent via multicast
with a TTL of 1.
Maybe it is possible that the exact command could be listed to
disable split horizon on OSPF?
HTH,
Paul Werner
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