CCNP BSCI 14

All protocols have some conditions before they can put the routes in their databases in their routing tables. Here is how IS-IS does it:1) L1 and L2 Intermediate Systems perform separate calculations
2) SPF Algorithm is run to determine best paths for NSAP routing
3) Best paths are added to the OSI routing table
4) PRC Algorithm is run to determine best path for IP routing
5) Best paths are added to the IP routing table

The only time when SPF has to run again is when a router is added or changed. PRC is run when IP networks are changed. Since PRC is very lightweight, it ends up more efficient than OSPF.

This is what we'll use to demonstrate basic IS-IS routing:


The Area ID would be in the format 49.00aa where "aa" stands for the Area ID
The System ID would be in the format 0000.0000.000s where "s" stands for the Router ID
Finally, the NSEL is always 00

We'll first configure R1. To do this, we'll go to global configuration and type:
router isis

You can actually enter a tag after the "isis" keyword, which is similar to an EIGRP's AS number. If you do not enter anything, the default "0" is used.

We'll now need to enter the NSAP address for R1. It would be:
net 49.0050.0000.0000.0001.00

By default, routers are L1/L2 routers, so we'll need to set it to L1:
is-type level-1

We'll then need to turn on IS-IS under the interface. In this case, I have Loopbacks to simulate networks:
in lo0
ip router isis
in lo1
ip router isis
in lo2
ip router isis
in lo3
ip router isis
in f0/0
ip router isis

Now we'll need to configure R2:
router isis
net 49.0050.0000.0000.0002.00

Then we'll configure R2's interfaces:
in f0/0
ip router isis
isis circuit-type level-1
in f0/1
ip router isis
isis circuit-type level-2

The circuit-type restricts L1 or L2 Hellos in the interfaces. At this point, we can do some verification:
show clns neighbors

To see the neighbor's IP, we can use:
show clns neighbors detail

To see the interface status, use:
show clns int

CLNS stands for Connectionless Network Service. It is connectionless and unreliable, requiring upper layer protocols for reliability. Sounds familiar? Yes, it's the equivalent of IP.

We'll enable R3 for IS-IS similarly:
router isis
net 49.0020.0000.0000.0003.00
is-type level-2

Here, we also configure the interfaces:
in f0/1
ip router isis
in f0/0
ip router isis

Finally, we'll configure R4:
router isis
net 49.0005.0000.0000.0004.00

And we'll configure its interfaces:
in lo0
ip router isis
in lo1
ip router isis
in lo2
ip router isis
in lo3
ip router isis
in f0/0
ip router isis
isis circuit-type level-2

At this point you should have full connectivity and routing across the domain. Another verification we can use is:
show clns

To do summarization, we'll hop on to R2, then type:
router isis
summary-address 192.168.0.0 255.255.252.0

What this does is that it summarizes that into the Level 2 database. You can also summarize into the L1 database from L2 by typing the "level-1" keyword behind the statement.

On R3, if we do a "show ip route" now, we would see IS-IS routes with a "i" marker. L2 routes will be marked with "L2" and L1 routes will be marked "L1". On R2, the summary route will be marked with "su" towards Null0.

We should also summarize on R4 by typing:
router isis
summary-address 10.0.0.0 255.255.252.0

To check the topology table, use:
show isis top

Finally, to change the metric, you go under the interface and type:
isis metric 1

Remember that the default is 10. They are treated as cost so lower is better.