IS-IS routing protocol

 IS-IS :-

IS-IS is an IGP, link-state routing protocol, similar to OSPF. It forms neighbor adjacencies, has areas, exchanges link-state packets, builds a link-state database and runs the Dijkstra SPF algorithm to find the best path to each destination, which is installed in the routing table.

intermediate System-to-Intermediate System (IS-IS) intra-domain routing information exchange protocol is designed by the International Organization for Standardization (ISO) for connectionless network protocol (CLNP). This protocol is a dynamic routing protocol. To enable this protocol to support IP routing, IETF expands and modifies IS-IS in RFC1195, applying the protocol to TCP/IP and OSI. The modified IS-IS is called IS-IS or Dual IS-IS.

 

l           Network Service Access Point (NSAP) is the network layer address of OSI. It identifies an abstract network service access point and describes the network address structure for the OSI model.

IS-IS routing protocol has two styles of route metric:

l           Narrow: The value of route metric ranges from 1 to 63.

l           Wide: The value of route metric ranges from 1 to 16,777,215.

A router can choose either or both of the styles

 

There are four types of PDU packets in IS-IS Protocol.

– IS-IS Hello PDU (IIH):- Used to establish/monitor neighbors
– Link State PDU (LSP) :- used to build a topology and share routes
– Partial Sequence Number PDU (PSNP) :- used to synchronize LSPs
– Complete Sequence Number PDU

 

In IS-IS PDUs there is a TLV (Type, Length, Value) field. According to the need, these fields are added to the PDUs. This TLV field makes IS-IS extendable and it is the major advantage of IS-IS over OSPF. There are several TLVs in IS-IS packets. IS-IS Header always same, but according to the type of the packet, added TLV fields change.

TLVs are maximum 256 bytes and they contains sub-TLVs.

IS-IS Hello interval is 10 seconds and the dead interval is 30 seconds by default.

IS-IS Adjacency States :-

There are only three adjacency states in IS-IS.

Down: This is the initial state. Its means that no hellos have been received from the neighbor.

Initializing: This state means that the local router has successfully received hellos from the neighboring router, however it’s not sure that the neighboring router has also successfully received local router’s hellos.

Up: Now it’s confirmed that neighboring router is receiving local router’s hellos.

IS-IS Adjacency Rules :-

 

Basic IS-IS configuration :

R1(config)#router isis

R1(config-router)#net 49.0001.0000.0000.0001.00

Where

49—AFI-- 8 bits

0012—AREA ID--- 16 bits

0000.0000.0001—SYSTEM ID-- 48 bits

00—DEVICE LIKE ROUTER-- 8 bits

R1(config-router)#is-type level-1

R1(config-router)#log-adjacency-changes

R1(config)#interface GigabitEthernet 0/1

R1(config-if)#ip router isis

R1(config)#interface Loopback 0

R1(config-if)#ip router isis

R2#clear isis *

 

 IS-IS Election of the Designated Intermediate System :-

If each IS advertised all of its adjacencies on a multiaccess circuit in its LSPs, the total number of advertisements required would be N 2—where N is the number of ISs that operate at a given level on the circuit. To address this scalability issue, IS-IS defines a pseudonode to represent the multiaccess circuit. All ISs that operate on the circuit at a given level elect one of the ISs to act as the Designated Intermediate System (DIS) on that circuit. A DIS is elected for each level that is active on the circuit.

The DIS is responsible for issuing pseudonode LSPs. The pseudonode LSPs include neighbor advertisements for all of the ISs that operate on that circuit. All ISs that operate on the circuit (including the DIS) provide a neighbor advertisement to the pseudonode in their non-pseudonode LSPs and do not advertise any of their neighbors on the multiaccess circuit. In this way the total number of advertisements required varies as a function of N—the number of ISs that operate on the circuit. A pseudonode LSP is uniquely classified by the following identifiers:

• System ID of the DIS that generated the LSP

 • pseudonode ID—ALWAYS NON-ZERO

 • LSP number (0 to 255)

• 32-bit sequence number

The DIS election process in IS-IS is based on the following criteria:

1.     Priority - Each router in the IS-IS domain is assigned a priority value, which determines its eligibility to become the DIS. The router with the highest priority value becomes the DIS. By default, the priority value is 64, but it can be manually configured on each router.

2.     Circuit ID - If two routers have the same priority value, the router with the highest circuit ID becomes the DIS. The circuit ID is a unique identifier assigned to each router interface.

3.     System ID - If two routers have the same priority value and circuit ID, the router with the highest system ID becomes the DIS. The system ID is a unique identifier assigned to each router in the IS-IS domain.

The DIS election process occurs automatically whenever a new router is added to a broadcast or NBMA network segment, or when

 IS-IS (Intermediate System to Intermediate System) routing protocol uses several tables to maintain and exchange routing information. These tables are as follows:

Level 1 Link State Database (LSDB) - This table contains information about the local IS-IS router's directly connected Level 1 networks and the reachable Level 1 routers. It also contains information about the router's own Level 1 interfaces.

Level 2 Link State Database (LSDB) - This table contains information about the local IS-IS router's directly connected Level 2 networks and the reachable Level 2 routers. It also contains information about the router's own Level 2 interfaces.

Forwarding Information Base (FIB) - This table is used to determine the next hop for a destination network. It is derived from the contents of the LSDBs.

Topology table - This table is derived from the contents of the LSDBs and is used to calculate the shortest path to a destination network. It contains information about the cost of each link and the list of routers that are reachable through each link.

Routing table - This table contains the final list of destinations and their next hop addresses. It is derived from the FIB.

IS-IS Troubleshooting :--

To view any CLNS neighbors, including the type of adjacency: Router# show clns neighbors

To view only IS neighbors: Router# show clns is-neighbors

To view specific IS-IS information about an interface: Router# show clns interface e0/0

 To view the IS-IS link-state topology table: Router# show isis database

To view a list of all known IS-IS routers in all areas: Router# show isis topology

 The IS-IS Metric :-

IS-IS utilizes an arbitrary cost for its metric (the optional metrics of delay, expense, and error are not supported by Cisco). By default, interfaces of all types (regardless of speed) are assigned a metric of 10.

To adjust the metric on an interface:

Router(config)# interface e0/0

 Router(config-if)# isis metric 30

 

The IS-IS process builds and maintains three separate tables:

 • A neighbor table – contains a list of all neighboring routers.

• A topology table – contains a list of all possible routes to all known networks within an area.

 • A routing table – contains the best route for each known network