THE IS-IS PROTOCOL

Objectives

 Introducing IS-IS and Integrated IS-IS Routing

 Performing IS-IS Routing Operations
 Configuring Basic Integrated IS-IS
 Module Summary
Introducing IS-IS and Integrated IS-IS
Routing
Requirements for Internet Connection Uses for IS-IS Routing

Large ISPs
 Stable protocol Giao thức ổn định
 Originally deployed by ISPs because U.S. government
mandated Internet support of OSI and IP
IS-IS Routing

 IS = router.
 IS-IS was originally designed as the IGP for the
Connectionless Network Service (CLNS), part of
the OSI protocol suite.
 The OSI protocol suite layer 3 protocol is the
Connectionless Network Protocol (CLNP).
 IS-IS uses CLNS addresses to identify routers and
build the LSDB.
IS-IS Features

 Link-state routing protocol

 Supports VLSM
 Uses Dijkstra’s SPF algorithm; has fast convergence
 Uses hellos to establish adjacencies and LSPs to exchange
link-state information
 Efficient use of bandwidth, memory, and processor
 Supports two routing levels:
 Level 1: Builds common topology of system IDs in local area and
routes within area using lowest cost path.
 Level 2: Exchanges prefix information (area addresses) between areas.
Routes traffic to area using lowest-cost path.
IS-IS Link-State Operation
Integrated (or Dual) IS-IS Routing

 Integrated IS-IS is IS-IS for multiple protocols:

 For IP, CLNS, or both
 Uses its own PDUs to transport IP routing
information; updates not sent in IP packets
 Requires CLNS addresses, even if only routing for
IP
Integrated IS-IS Design Principles
Issues with Integrated IS-IS
End System-to-Intermediate System
Four OSI Routing Levels
Similarities Between IS-IS and OSPF

 Integrated IS-IS and OSPF are both open standard

link-state protocols with the following similar
features:
 Link-state representation, aging timers, and LSDB
synchronization
 SPF algorithms

 Update, decision, and flooding processes

 VLSM support

 Scalability of link-state protocols has been proven

(used in ISP backbones).
 They both converge quickly after changes.
Integrated IS-IS vs. OSPF: Area
Design
Integrated IS-IS vs. OSPF: Area Design (Cont.)
Advantages of Integrated IS-IS
Advantages of OSPF
 OSPF has more features, including:
 Has three area types: normal, stub, and NSSA
 Defaults to scaled metric (IS-IS always 10)

 OSPF is supported by many vendors.

 Information, examples, and experienced engineers
are easier to find.
Performing IS-IS Routing Operations
OSI Addresses
 OSI network layer addressing is implemented with
NSAP addresses.
 An NSAP address identifies a system in the OSI
network; an address represents an entire node,
Toàn bộ
not an interface.
 Various NSAP formats are used in various systems,
because different protocols may use different
representations of NSAP.
 NSAP addresses are a maximum of 20 bytes:
 Higher-orderbits identify the interarea structure.
 Lower-order bits identify systems within area.
Integrated IS-IS NSAP Address
Structure
Integrated IS-IS NSAP Address
Structure
 AFI = Authority and Format Identifier: The type of
network address
 IDI = Initial Domain Identifier: Identifies a
subdomain under the AFI
 IDP = Initial Domain Part
 DSP = Domain Specific Part
 HO-DSP: Subdivides the domain into areas
 System ID: Identifies an individual OSI device
 NSEL: Identifies a process on the device (like port
or socket in IP)
Typical NSAP Address Structure
The simplest NSAP format used by most companies
running IS-IS as their IGP is as follows:
 Area address (must be at least 1 byte)
 AFI set to 49
 Locally administered; thus, you can assign your own
addresses.
 Area ID
 The octets of the area address after the AFI.
 System ID
 Cisco routers require a 6-byte system ID.
 NSEL
 Always set to 0 for a router.
Identifying Systems in IS-IS: Area Address
Identifying Systems in IS-IS: System
ID
 OSI Addressing: NET Addresses

 NSAP address includes NSEL field (process or port

number)
 NET: NSAP with a NSEL field of 0
 Refers to the device itself (equivalent to the Layer 3 OSI
address of the device)
 Used in routers because they implement the network
layer only (base for SPF calculation)
Subnetwork Point of Attachment (SNPA) and Circuit
 Level 1, Level 2, and Level 1-2 Routers

Level 1 (like OSPF internal nonbackbone routers):

 Intra-area routing enables ESs to communicate.
 Level 1 area is a collection of Level 1 and Level 1-2 routers.
 Level 1 IS keeps copy of the Level 1 area LSDB.
Level 1-2 (like OSPF ABR):
 Intra-area and interarea routing.
 Level 1-2 IS keeps separate Level 1 and Level 2 LSDBs and advertises
default route to Level 1 routers.
Level 2 (like OSPF backbone routers):
 Interarea routing.
 Level 2 (backbone) area is a contiguous set of Level 1-2 and Level 2
routers.
 Level 2 IS keeps a copy of the Level 2 area LSDB.
Addressing and Routing
 Area address is used to route between areas; system
ID is not considered.
 System ID is used to route within an area; area
address is not considered.
OSI IS-IS Routing Logic
Level 1 router: For a destination address, compare
the area address to this area.
 If not equal, pass to nearest Level 1-2 router.
 If equal, use Level 1 database to route by system ID.

Level 1-2 router: For a destination address, compare

the area address to this area.
 If not equal, use Level 2 database to route by area
address.
 If equal, use Level 1 database to route by system ID.
Example: Identifying Systems: OSI Addressing in Networks
Example: OSI Area Routing
Route Leaking
 Available since Cisco IOS Sofware Release 12.0
 Helps reduce suboptimal routing by allowing Level 2
information to be leaked into Level 1
 Uses up/down bit in Type, Length, and Value (TLV)
field
OSI PDUs
IS-IS PDUs
 IS-IS PDUs are encapsulated directly into a data-link
frame. There is no CLNP or IP header in a PDU.
 IS-IS PDUs are as follows:
 Hello (ESH, ISH, IIH)
 LSP

 PSNP (partial sequence number PDU)

 CSNP (complete sequence number PDU)
A Link-State Packet Represents
Router
LSP Header
 LSPs are sequenced to prevent duplication of LSPs.
 Assists
with synchronization.
 Sequence numbers begin at 1.

 Sequence numbers are increased to indicate the newest

LSP.
 LSPs in LSDB have a remaining lifetime.
 Allowssynchronization.
 Decreasing timer.
LSP TLV Examples
IS-IS Network Representation
 Generally, physical links can be placed in the
following two groups:
 Broadcast: Multiaccess subnetworks that support
addressing of a group of attached systems
 Point-to-point: Permanent or dynamically established
links
 Only two link-state representations are available in
IS-IS:
 Broadcast for LANs and multipoint WANs
 Point-to-point for all other topologies

 IS-IS has no concept of NBMA networks.

Implementing Network Types in
NBMA
 When implementing IS-IS in NBMA (such as Frame
Relay or ATM):
 Broadcast mode assumes fully meshed connectivity.
 In broadcast mode, you must enable CLNS mapping and
include the broadcast keyword, in addition to creating
IP maps with the broadcast keyword.
 Point-to-point mode is highly recommended (using
subinterfaces).
Broadcast Mode
 Used for LAN and multipoint WAN interfaces.
 Adjacency is recognized through hellos; separate
adjacencies for Level 1 and Level 2.
 Designated IS (DIS) creates a pseudonode and
represents LAN.
 DIS for Level 1 and Level 2 may be different.
 DIS is elected based on these criteria:
 Only routers with adjacencies are eligible.
 Highest interface priority.
 Highest SNPA (MAC) breaks ties.

 There is no backup DIS.

LSP Representing Routers: LAN Representation
Level 1 and Level 2 LSPs and IIHs
 The two-level nature of IS-IS requires separate types of LSPs:
Level 1 and Level 2 LSPs.
 DIS is representative of LAN:
 DISsends pseudo-Level 1 and pseudo-Level 2 LSPs for
LAN.
 Separate DIS for Level 1 and Level 2.
 LSPs are sent as unicast on point-to-point networks.
 LSPs are sent as multicast on broadcast networks.
 LAN uses separate Level 1 and Level 2 IIHs; sent as multicast.
 Point-to-point uses a common IIH format; sent as unicast.
Comparing Broadcast and Point-to-Point
Topologies
LSP Flooding
 Single procedure for flooding, aging, and updating of
LSPs.
 Level 1 LSPs are flooded within an area.
 Level 2 LSPs are flooded throughout the Level 2
backbone.
 Large PDUs are divided into fragments that are
independently flooded.
 Each PDU is assigned an LSP fragment number, starting
at 0 and incrementing by 1.
 Separate LSDBs are maintained for Level 1 and Level
2 LSPs.
LSDB Synchronization
 SNP packets are used to ensure synchronization and
reliability.
 Contents are LSP descriptions
 PSNP is used for the following:
 For acknowledgment of LSPs on point-to-point links
 To request missing pieces of LSDB

 CSNP is used for the following:

 Periodicallyby DIS on LAN to ensure LSDB accuracy
 On point-to-point link when the link comes up
LSDB Synchronization: LAN
LSDB Synchronization: Point-to-Point
LAN Adjacencies
Example: WAN Adjacencies
Configuring Basic Integrated IS-IS
 Integrated IS-IS: Requires NET
Addresses
 Common CLNS parameters (NET) and area planning
are still required even in an IP environment.
 Even when Integrated IS-IS is used for IP routing only,
routers still establish CLNS adjacencies and use CLNS
packets.
OSI Area Routing: Building an OSI
Forwarding Database (Routing Table)

 When databases are synchronized, Dijkstra’s

algorithm (SPF) is run on the LSDB to calculate the SPF
tree.
 The shortest path to the destination is the lowest
total sum of metrics.
 Separate route calculations are made for Level 1 and
Level 2 routes in Level 1-2 routers.
 Best paths are placed in the OSI forwarding database
(CLNS routing table).
Building an IP Routing Table

 Partial route calculation (PRC) is run to calculate IP

reachability.
 Because IP and ES are represented as leaf objects, they do
not participate in SPF.
 Best paths are placed in the IP routing table following
IP preferential rules.
 They appear as Level 1 or Level 2 IP routes.
Integrated IS-IS Configuration Steps

1. Define areas, prepare addressing plan (NETs) for

routers, and determine interfaces.
2. Enable IS-IS on the router.
3. Configure the NET.
4. Enable Integrated IS-IS on the appropriate
interfaces. Do not forget interfaces to stub IP
networks, such as loopback interfaces (although
there are no CLNS neighbors there).
Step 1: Define Area and Addressing

 Area determined by NET prefix

 Assign to support two-level hierarchy.
 Addressing
 IP: Plan to support summarization.
 CLNS: Prefix denotes area. System ID must be unique.
Step 2: Enable IS-IS on the Router
Step 3: Configure the NET
Step 4: Enable Integrated IS-IS
Simple Integrated IS-IS Example
Change IS-IS Router Level
Change IS-IS Interface Level
Change IS-IS Metric
Example: Tuning IS-IS Configuration
IP Summarization
Example: Is Integrated IS-IS Running?
Example: Are There Any IP Routes?
Troubleshooting Commands: CLNS
Troubleshooting Commands: CLNS and IS-IS
Example: OSI Intra-Area and Interarea Routing
Level 1 and Level 2 Topology Table
Simple Troubleshooting: What About CLNS Protocol?
Are Adjacencies Established?
Module Summary
 Module Summary

 IS-IS is a proven and extensible IP routing protocol that

converges quickly and supports VLSM.
 Unlike IP addresses, CLNS addresses apply to entire nodes
and not to interfaces. IS-IS runs directly on the data-link
layer and does not use IP or CLNS as a network protocol.
 Even when IS-IS is installed to support IP exclusively,
network devices must also be configured with NET
addresses. Default settings for IS-IS may result in the
inefficient use of router and network resources and
suboptimal routing.