IS-IS is used by a large percentage of 1st tier internet backbone providers. From
what I understand, it was chosen not for technical superiority over OSPF, but becuase
cisco's IS-IS code was more stable at the time. At present, IS-IS maintains a couple
advantages over OSPF in the ISP world. Primarily, it is a simply methodology due to
its use of only two LSP types (Link state protocol data unit, the ISIS LSA) Within
these LSP's, a type/length/value (TLV) architecture is used to support additional
functionality. Hence, when the world needs IS-IS to carry some additional data (say
for Traffic Engineering), simply another TLV is incorporated. Routers that do not
support the new TLV simply ignore per the spec. However, in the OSPF world, new
functionality demands entirely new LSA types (see LSA type 10, Opaque LSA's for
Traffic engineering).
Operationally, despite a few variances, the protocols work in a similar fashion. The
down side of IS-IS and the reason more places do not use it is simply lack of
technical skills. Take 30 professionals running routed IP and you'll likely find 25
or so OSPF experts and 1 or 2 IS-IS experts. (probably 2 or 3 people running EIGRP)
Pete
*********** REPLY SEPARATOR ***********
On 11/16/2000 at 8:45 AM Chuck Church wrote:
>All,
>
> I'm just curious as to when and why you'd use IS-IS rather than OSPF or
>EIGRP? I've never seen IS-IS in any business I've worked with or for in the
>6 years I've been doing this. Do any other router manufacturers support it?
>Is it eventually going to go away?
>
>Thanks,
>
>Chuck Church
>CCNP, CCDP, MCNE, MCSE
>Sr. Network Engineer
>Magnacom Technologies
>140 N. Rt. 303
>Valley Cottage, NY 10989
>845-267-4000 x218
>
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