IS-IS Protocol

(Intermediate System -to- Intermediate System)

IS-IS (Intermediate System -to- Intermediate System) is a standardized link-state protocol that was developed to be the definitive routing protocol for the OSI (Open Systems Interconnect) Model, which was developed by ISO (International Standards Organization).

IS-IS shares many similarities to OSPF. Though it was designed as an interior gateway protocol (IGP), IS-IS is also used by ISPs, due to its scalability.

IS-IS was originally developed by DEC for CLNS, not IP and this is why it’s called IS-IS.

ISO also created something similar to IP and UDP called CLNP (Connectionless-mode Network Protocol) & CLNS (Connectionless-mode Network Service). ISO also uses some different terminology, for example:

- Router = Intermediate system (IS)

- Host = End system (ES)

IS-IS is a highly scalable routing protocol, which is why it is used often on large service provider network backbones.

Ethernet Header IS-IS Header IS-IS Data FCS

IS-IS also relies directly on top of an Ethernet header, using its own header format. It’s not encapsulated in an IP packet like other IGP routing protocols.

Link-State characteristics of IS-IS

IS-IS uses the Dijkstra Shortest Path First algorithm to determine the shortest path. IS-IS is a classless protocol, and supports VLSM. IS-IS also uses the concept of areas. IS-IS advertises the status of directly connected “links” in the form of Link-State Packets (LSPs). In IS-IS the topology information is flooded throughout the AS, so that every router within the AS has a complete picture of the topology of the AS. It provides support for multiple paths of equal cost. All protocol exchanges can be authenticated so that only trusted routers can join in the routing exchanges for the AS. In a link-state routing protocol, the next hop address to which data is forwarded is determined by choosing the best end-to-end path to the eventual destination.

IS-IS uses an arbitrary cost for its metric. IS-IS additionally has three optional metrics: delay, expense, and error.

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

Neighbor table – contains a list of all neighboring routers.

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

Routing table – contains the best route for each known network.

IS-IS Protocols and Addressing --

IS-IS consists of three sub-protocols that work in to achieve end to end routing which ISO defined as Connectionless Network Service (CLNS):

CLNP (Connectionless Network Protocol) –serves as the Layer-3 protocol for IS-IS (and was developed by ISO).

ES-IS (End System -to- Intermediate System) –used to route between hosts and routers. IS-IS (Intermediate System -to- Intermediate System) – used to route between routers.

IS-IS was modified to additionally support IP routing, and became Integrated (or Dual) IS-IS.

The CLNP address in IS-IS identifies the “Area” in which a device is located, the actual host “ID,” and the destination application on that host, in the form of the “SEL” field.

IDP (Initial Domain Part) - is portion of the address identifies the AS of the device. DSP (Domain Specific Part) - is portion of the address is used to route within the autonomous system.

THE IDP SECTION DETAILSAFI (Authority and Format Identifier)specifies the organization authorized assign addresses. & The AFI is always 8 bits. IDI (Initial Domain Identifier) – identifies the “suborganization” under the parent AFI organization.

THE DSP SECTION DETAILS(High Order DSP) – identifies the area within an autonomous system. System ID – identifies the specific host. NSEL – identifies the destination upper layer protocol of the host (always 8 bits)

IS-IS routers will refresh the full database

IS-IS Neighbors

IS-IS routers form neighbor relationships,which are known as adjacencies and they form neighborship by exchanging Hello packets which are sent out in every 10 seconds. As we have already disucussed that IS-IS supports three type pf hello packets.

One for point-to-point links, and two for broadcast (or LAN) links (Level-1 and Level-2 broadcast Hellos).

In IS-IS Neighbors also elects a DIS (Designated Intermediate System) on broadcast links. Which is more equivalent of an OSPF DR (Designated Router). Whichever IS-IS router has the highest priority will be elected the DIS Default priority value of DIS is 64).

R2#

! interface Loopback1

ip address 2.2.2.2 255.255.255.255 ip router isis

! interface FastEthernet0/0 ip address 10.10.10.2 255.255.255.0 ip router isis ! router isis net 49.0001.0000.0000.0002.00 is-type level-1

! R3# ! interface FastEthernet0/0 ip address 10.10.10.3 255.255.255.0 ip router isis ! router isis net 49.0001.0000.0000.0003.00 is-type level-1

IS-IS Metric

Unlike OSPF, in which the link metric is calculated automatically based on bandwidth, there is no automatic calculation for IS-IS. All IS-IS links use a metric of 10 by default.

IS-IS metrics can have values up to 63. The total cost to a destination is the sum of the metrics on all outgoing interfaces along a particular path from the source to the destination. By default, the total path metric is limited to 1023.

IS-IS utilizes an arbitrary cost for its metric (the optional metrics of delay, expense, and error are not supported by Cisco).

To manipulate metric on an interface: - Router(config)# interface < int number > - Router(config-if)# isis metric 30

To

IS-IS Vs OSPF

OSPF uses router-id to indentify router on a network whereas, IS-IS uses system-id. IS-IS elects a DIS on broadcast networks & OSPF elects DR. Both use the Dijkstra algorithm to determine the shortest path.

There is no Area 0 backbone area for IS-IS. IS-IS uses an arbitrary cost metric. OSPF’s cost metric is based on the bandwidth of the link. In OSPF, interfaces belong to areas. In IS-IS, the entire router belongs to an area.