[afnog] afnog Digest, Vol 54, Issue 23

[Peter Nyamukusa]

Hi Randy,

I see this topic has taken a slight twist but never the less below are some
comparison between ISIS and OSPF this is from Wikipedia so I don't know if
it is really true when they say ISIS is favoured in ISP environments because
it can support more routers in an area. I sure that would be true if you
have may routers in your backbone area right now I have over 20 and that
number should increase to over 100 within the next few months.

NB. I not trying to influence your choice OSPF is great I been using it for
many years

ISIS Vs OSPF

Both IS-IS and OSPF are link state protocols, and both use the same Dijkstra
algorithm for computing the best path through the network. As a result, they
are conceptually similar. Both support variable length subnet masks, can use
multicast to discover neighboring routers using hello packets, and can
support authentication of routing updates.

While OSPF is natively built to route IP and is itself a Layer 3 protocol
that runs on top of IP, IS-IS is natively an ISO network layer protocol (it
is at the same layer as CLNS), a fact that may have allowed OSPF to be more
widely used. IS-IS does not use IP to carry routing information messages.

IS-IS routers build a topological representation of the network. This map
indicates the IP subnets which each IS-IS router can reach, and the lowest
cost (shortest) path to an IP subnet is used to forward IP traffic.

IS-IS also differs from OSPF in the methods by which it reliably floods
topology and topology change information through the network. However, the
basic concepts are similar.

Since OSPF is more popular, this protocol has a richer set of extensions and
added features. However IS-IS is less "chatty" and can scale to support
larger networks. Given the same set of resources, IS-IS can support more
routers in an area than OSPF. This makes IS-IS favoured in ISP environments.
Additionally, IS-IS is neutral regarding the type of network addresses for
which it can route. OSPF, on the other hand, was designed for IPv4. Thus
IS-IS was easily adapted to support IPv6, while the OSPF protocol needed a
major overhaul (OSPF v3).

The TCP/IP implementation, known as "Integrated IS-IS" or "Dual IS-IS", is
described in RFC 1195.

IS-IS differs from OSPF in the way that "areas" are defined and routed
between. IS-IS routers are designated as being: Level 1 (intra-area); Level
2 (inter area); or Level 1-2 (both). Level 2 routers are inter area routers
that can only form relationships with other Level 2 routers. Routing
information is exchanged between Level 1 routers and other Level 1 routers,
and Level 2 routers only exchange information with other Level 2 routers.
Level 1-2 routers exchange information with both levels and are used to
connect the inter area routers with the intra area routers. In OSPF, areas
are delineated on the interface such that an area border router (ABR) is
actually in two or more areas at once, effectively creating the borders
between areas inside the ABR, whereas in IS-IS area borders are in between
routers, designated as Level 2 or Level 1-2. The result is that an IS-IS
router is only ever a part of a single area. IS-IS also does not require
Area 0 (Area Zero) to be the backbone area through which all inter-area
traffic must pass. The logical view is that OSPF creates something of a
spider web or star topology of many areas all attached directly to Area Zero
and IS-IS by contrast creates a logical topology of a backbone of Level 2
routers with branches of Level 1-2 and Level 1 routers forming the
individual areas.

Cheers

Peter

-----Original Message-----
From: Randy Bush [mailto:randy at psg.com] 
Sent: Wednesday, September 24, 2008 1:30 PM
To: Peter Nyamukusa
Cc: mtinka at globaltransit.net; 'Frank A. Kuse'; afnog at afnog.org
Subject: [!! SPAM] Re: [afnog] afnog Digest, Vol 54, Issue 23

the way we old folk say it is

  is-is is used by very few isps.  just the old stable ones.

:)

randy