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Intermediate System to Intermediate System (IS-IS) Routing Protocol

When comparing the routing protocols, Intermediate System to Intermediate System (IS-IS) to Open Shortest Path First (OSPF), you see some similarities. Both are link-state protocols and both use the Dijkstra algorithm to calculate the best route through a network.

One major difference between the protocols relates to how they operate in the OSI model. IS-IS is a native Layer 3 (network layer) protocol, so it is capable of passing routing information for any routable protocol, and it is not restricted to IP like OSPF and many other routing protocols are.

Most other routing protocols required modification in order to support IPv6, whereas because IS-IS is network-protocol neutral, it can support IPv6 right out of the gate.

Regarding the way it supports areas, IS-IS also differs from OSPF in that routers route as Level 1 or intra-area within an area, as Level 2 or inter-area between areas, or as Levels 1–2 when performing both types of routing.

The basic command to enable IS-IS routing is router isis [area tag], where the area tag is either 1, 2, or 1–2. By default, if the area tag is omitted, the first instance of IS-IS uses Level 1–2 and Level 1 for later instances.

After specifying the router command, you specify network entity titles on the net statements, which is like specifying the RouterID, the only requirement is that this value be unique across your network. In this case, you use 49.0001.0000.000a.00 as a base for the IDs and specify unique values for each router using 0a.00 and incrementing upward.

Unlike other protocols where you use the network command to identify networks that will be included in routing, with IS-IS, the ip router isis command is issued on each interface for which you want to route for FastEthernet 0/0 and FastEthernet 0/1, as shown here:

Router2>enable
Password:
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#router isis
Router2(config-router)#net ?
  XX.XXXX. ... .XXX.XX  Network entity title (NET)
Router2(config-router)#net 49.0001.0000.0000.000a.00
Router2(config-router)#exit
Router2(config)#interface FastEthernet 0/0
Router2(config-if)#ip router isis
Router2(config-if)#exit
Router2(config)#interface FastEthernet 0/1
Router2(config-if)#ip router isis
Router2(config-if)#exit
Router2(config)#exit

When viewing the routing table, you see all the IS-IS routes listed with i and another identifier to show whether they are Level 1, Level 2, or inter-area (Level 1–2). You can see in the listing that the network of 192.168.6.0/24 is an IS-IS Level 1 network.

Router2>enable
Password:
Router2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C    192.168.5.0/24 is directly connected, FastEthernet0/1
i L1 192.168.6.0/24 [115/20] via 192.168.1.2, FastEthernet0/0
C    192.168.255.0/24 is directly connected, Loopback0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
S    192.168.3.0/24 [1/0] via 192.168.1.1
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