IS-IS uses three types of adjacencies - L1, L2, and L1/2. Adjacencies are established through a three-way handshake and can be in one of three states: down, initializing, or up. Key IS-IS operations include routers discovering neighbors through hello messages, establishing adjacencies, building LSPs, flooding LSPs to neighbors, running SPF to build routing tables. DIS is elected to reduce the mesh in broadcast networks and is chosen based on priority and MAC address.

1. IS-IS PROTOCOL ADJACENCY
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2. • There are three types adjacency in IS-IS Protocol.These are:
• – Level 1 routers form L1 adjacency with L1 and L1/2 Routers
– Level 2 routers form L2 adjacency with L2 and L1/2 Routers
– Level 1/2 routers form L1/2 adjacency with L1/2 Routers

3. • There is no neighbour ship between L1 and L2 routers.
• In IS-IS Protocol, adjacency is established with 3-way handshake mechanism.
There are three states in IS-IS adjacency. Let’s check the adjacency states of IS-IS
Protocol.
• In the first place the router adjacency is in Down State. One of the routers, here
Router A sends a Hello PDU. When the router B receive this Hello including the
MAC address of the sending node Router A, it goes to Initialize State and sends
Hello to Router A too. Again, when Router A receives Hello including the MAC
address of Router B, bidirectional communication established. And the name of
this state is Up State.

4. • As a summary:
• No IS-IS neighbour (Down State)
• RTRA Sends Hello (Initializing
State)
• RTRB records MAC address f
RTRA and Sends Hello too. RTRA
sees MAC address of RTRB.
Bidirectional communication
established (Up State)

5. IS-IS Protocol Operation
• Beginning with IS-IS adjacency establishment, to
the RoutingTable establishment, IS-IS Operation
has some steps.

6. We can summarize IS-IS operation in 6 steps.
These IS-IS operation steps are given below:
1. IS-IS Routers are send Hello PDUs to discover the neighbours and establish the
adjacency.
2. IS-IS adjacency is established ( mainly authentication, IS-type, MTU must
match)
3. LSPs are build by routers about their self and the learned adjacent routers
4. Routers send the LSPs to the adjacent routers.
5. All routers build their LSDB according to these LSPsBy SPF algorithm best
paths are calculated and routing table is build.

7. DIS (Designated IS)
• In ISIS network, LSDB databases must be syntonized. To do this, full mesh or
another solution is required. So, in IS-IS Protocol, one router is selected as DIS
(Designated IS)and DIS is used to reduce the adjacency in areas. It is like DR in
OSPF.
• Router with the highest priority and then the highest MAC address is elected as
DIS. The default DIS priority is 64. There is no BDR like OSPF. And like OSPF, if
you want to prevent the router to be DIS, you can set the priority 0. If a new router
added to the ISIS network, the election occurs again.This is not like this in OSPF.

8. IS-IS Protocol DIS Election
• DIS is used on multipoint-
to-multipoint topologies.
It is not used in point-to-
point topologies.
Separate DISs are
selected for Level 1 and
Level 2.

9. IS-IS Protocol NetworkTypes
• IS-IS Protocol supports two types of networks.These network are :
– Broadcast networks
– Point-to-Point networks
The other network types that is used in OSPF is not used in IS-IS Protocol.
• Different IIH Hellos are used for different network types while establishing
neighbour ship.
• In Broadcast networks IS-IS Protocol requires a full mesh topology. By selecting
DIS, full mesh is eliminated. DIS election is used in IS-IS broadcast networks.

10. IS-IS Cost
• Default IS-IS Protocol cost is 10 for all IS-IS Protocol links. But these costs can be
set to a different value per interface. For an interface, normally maximum 63 is
accepted as cost. But you an set this as 16,777,215 with the wide
metrics command.
Cost is advertised by the TLV fields in PDU.
Two routers can set different cost values to reach each other.

11. IS-IS Protocol Administrative Distance
• The administrative distance of all IS-IS routes are 115 for Cisco . But for Alcatel-
Lucent and Juniper, these administrative distance (preference) value is different.
Both vendor use four different value for both Level 1 and Level 2 routers and for
both internal and external routes.These are:
– IS-IS Level 1 internal route = 15
– IS-IS Level 2 internal route = 18
– IS-IS Level 1 external route = 160
– IS-IS Level 2 external route = 165
• If you want one route not to be used in the route calculation, you can set the the
administrative distance manually to 255.

12. IS-IS Protocol Authentication
• In IS-IS Protocol, by default there is no authentication. But you can set two types
of authentication.These authentications types are:
– Simple
– MD5
• With authentication in Hello packets, you can secure Hello Packets and it
determines whether a adjacency is established or not between these routers.
• With authentication in LSPs, it allows the remote router to read theTLV fields and
according to this information it does the SFP calculation.

13. IS-IS ProtocolTimers
• IS-IS Protocol has some timers like other routing protocols. In IS-IS Protocol,
modifying default timer settings is recommended because modifying these timers
make your IS-IS Protocol better.
These timers are given below:
– Lsp-refresh timer
– Max-lsp-lifetime
– Prc-interval
– Spf-interval
– Hello-interval
– Hello-multiplier
– Isis-retransmit-interval
– Csnp-interval

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