Sunday, November 7, 2010


There are only two Link-State routing protocols in the world that are considered mainstream and popular: OSPF and IS-IS. IS-IS was never designed to support TCP/IP, but it is widely employed by service providers to route TCP/IP traffic!
In the past, the OSI model was never meant to be just a model. OSI was actually a full-blown routing suite for the Internet. It was a competing standard to TCP/IP and in the end OSI lost to TCP/IP because it is simpler.

In the OSI protocol, the a router is known as an Intermediate System (IS). IS-IS stands for Intermediate System To Intermediate System! IS-IS was the routing protocol created for the OSI protocol.

The OSI was originally deployed by ISPs due to government regulations because it was a good contender. When the Internet first appeared, ISPs had to support both the TCP/IP protocol AND the OSI protocol.

After OSI protocol died out, some people saw the potential in the IS-IS protocol and so they modified it to carry TCP/IP protocol. IS-IS was not completely written, but rather it was tuned. This tuning led to a new protocol known as Integrated IS-IS!

For IS-IS to work properly, a CLNP (Connectionless Network Protocol) address still needs to be assigned to the router. IS-IS is very similar to OSPF in many ways. Here's a brief rundown of IS-IS:
-Uses SPF
-Uses Hello Messages
-More Tunable than OSPF
-More Efficient that OSPF
-More Flexible than OSPF
-Much more difficult to understand than OSPF!

Once you've understood the basics of OSPF, you pretty much understand IS-IS. The only reason why IS-IS is more difficult in some ways is because of the CLNS address still required to be assigned to the router.

In IS-IS, there are no ABRs. All routers belong fully in one area. There are two types of routers, L1, L2 and L1/L2 routers.

L1 routers are designed to maintain the topology tables of an entire area. This is similar to an Internal router.

The L2 routers are considered the backbone routers. The L2 routers are those that know about the backbone routers.

L1/L2 routers are those that interface between L1 and L2 routers.

Analogically, L1 are Internal routers, L2 are Internal Backbone routers, and L1/L2 are ABRs.

The L1 (Level One) databases are kept completely separate from L2 databases. L1/L2 routers have to keep two databases, but it can summarize the L1 database before putting it in the L2 database.

When a L1 router needs to go to another area, it sends a message to the closest L1/L2 router. The L1/L2 router sort of serves as a default gateway for an area.

The Metric of IS-IS is not based on bandwidth. It is a value between 0 to 63 with 10 being the default. By default, IS-IS metric behaves like RIP. IS-IS metric is typically written in an organizationally-unique policy and implemented by Network Administrators.

In OSI, the End Systems (ES) would participate in routing by finding their closest IS. This is known as ES-IS. In the OSI world, you wouldn't have a default-gateway. The ES would be smart enough to discover default-gateways. (This is a "new" feature implemented in IPv6 as well).

L1 and L2 routers form like relationships and separate databases. L1 and L2 have separate Hello messages, so L1/L2 would have to send two types of different Hello messages. Two L1/L2 routers can form 2 relationships between them.

A typical IS-IS network design looks like this:

In the backbone area, L2 routers only know the summary of the different areas. These summaries are sent in by L1/L2 routers. Notice that in the end the concept is still roughly the same as OSPF in terms of design.

Here are the different Level of Routing in IS-IS:
-Level 0 routing refers to ES-IS routing.
-Level 1 routing refers to IS-IS routing.
-Level 2 routing refers to IS-IS inter-area routing.
-Level 3 routing refers to routing to other autonomous systems.

Here's some OSPF vs IS-IS breakdown:
-Both are Link-State protocols
-Topology, Neighbor and Routing Tables
-SPF Algorithm

OSPF: Interfaces belong to areas
IS-IS: Router belong to an area

OSPF: All areas tie to the backbone area (Area 0)
IS-IS: The backbone runs to all areas

OSPF supports more area types, supports a metric scaled by default, is supported by more vendors and is well understood by the majority of the industry.

IS-IS handles updates more efficiently, rarely runs the SPF algorithm, prefers using partial route calculation (PRC), is much faster (by default) to detect failures and converge, has less design constraints and is very easy to adapt to IPv6.

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