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.
NOTE - Cisco does not support these optional
metrics.
AD Value for IS-IS is 115
IS-IS is only available on enterprise versions of the
Cisco IOS.
IS-IS is also used as the control plane for IEEE
802.1aq Shortest Path Bridging (SPB). SPB allows for
shortest-path forwarding in an Ethernet mesh
network context utilizing multiple equal cost paths.
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 DETAILS -
AFI (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 Packet Types
IS-IS additionally defines three categories of
packet types:
1. Hello packets are exchanged for neighbor
discovery. Hello packets have the following
subcategories -
LAN Level 1 hello packets (PDU Type 15)
LAN Level 2 hello packets (PDU Type 16)
Point-to-point hello packets (PDU Type 17)
2. LSP (Link State Packet) is used to share
topology information between routers. LSP
packets have the following subcategories -
Level 1 link-state packets (PDU Type 18)
Level-2 link-state packets (PDU Type 20)
3. SNP (Sequence Number Packet) - is an
update containing the full link-state database.
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).
IS-IS Lab
Task 1: Configure ISIS in multi-access area type between R1,R2 and R3
Task 2: Configure ISIS over point to point network between R1 and R4
Task 3: Configure R2 and R3 as the L1 ISIS router
Task 4: Verify the routing tables of R2 and R3
R1#
interface FastEthernet0/0
ip address 10.10.10.1 255.255.255.0
ip router isis
!
interface FastEthernet1/0
ip address 20.20.20.2 255.255.255.252
ip router isis
!
router isis
net 49.0001.0000.0000.0001.00
!
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
R4 #
interface Loopback1
ip address 4.4.4.4 255.255.255.255
ip router isis
!
interface FastEthernet0/0
ip address 20.20.20.1 255.255.255.252
ip router isis
!
router isis
net 49.0002.0000.0000.0004.00
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
IS-IS Troubleshooting
To view any CLNS neighbors -
#show clns neighbors
To view IS-IS neighbors -
#show clns is-neighbors
To view specific IS-IS interface -
#show clns interface number
To view the IS-IS link-state topology table-
#show isis database
To view a list of all known IS-IS routers in all
areas-
#show isis topology
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.
Telegram Group for Discussions
Telegram Channel for Jobs
