IS-IS-Doyle

Ch. 7 IS-IS

CCNP 1 version 3.0

Rick Graziani

Cabrillo College

Source: Routing TCP/IP by Jeff Doyle, Cisco Press Rick Graziani [email protected] 2

Note from Rick

• In creating this PowerPoint presentation for IS-IS I examined various sources on IS-IS including:– Cisco Online curriculum, CCNP 1 version 3.0– IS-IS Network Design Solutions, Cisco Press– CCNP BSCI Exam Certification Guide (CCNP Self-

Study), Cisco Press

• The sources that I decided to use are…


Routing TCP/IP Volume I

• My old favorite, Routing TCP/IP Volume I by Jeff Doyle, ISBN: 1578700418

• Most of the information in this presentation is from this book.

• And a Networkers presentation given by Gerry Redwine of Cisco Systems.

Link State Routing Protocol Fundamentals

Gerry Redwine at Cisco Systems


About link-state protocols

• In a link-state protocol, the network can be viewed as a jigsaw puzzle• Each jigsaw piece holds one router• Each router creates a packet which represents its own jigsaw piece

– This packet is called a Link State PDU (LSP)

LSP for router-A

to B

to C to E


About link-state protocols

• These packets are flooded everywhere

• Therefore each router receives all pieces of the jigsaw puzzle

• Each routers compute SPF algorithm to put the pieces together– Input: all jigsaw puzzle pieces (LSPs)– Output: Area or network topology tree

Shortest Path Tree


The jigsaw puzzle

LSP for router-A

LSP for router-B

LSP for router-CLSP for router-D

to B

to Eto D

to C

to A

to D to C

to BLSP for routerE

to A to B

to A

to E


All routers have same view

• All routers exchange all LSPs• via a reliable flooding mechanism

• All routers store all LSPs in a so-called link-state database (LSPDB)

• separate from the routing table• all routers should have exactly the same LSPDB, but

different routing tables


What to do with LSPs ?

• Each router ‘composes the jigsaw puzzle’ by executing Dijkstra’s Shortest Path First algorithm (SPF)– the topology is calculated as a Shortest Path Tree

(SPT), with itself as root– each router computes a different SPT

• From the SPT the routing table is calculated


All routers have same LSPDB

RouterA’s LSPDB

RouterB’s LSPDB

RouterC’s LSPDB

RouterE’s LSPDB

RouterD’s LSPDB

lspA lspB

lspC lspDlspE

lspA lspB

lspC lspDlspE

lspA lspB

lspC lspDlspE

lspA lspB

lspC lspDlspE

lspA lspB

lspC lspDlspE


Routing TCP/IP Volume I


Introduction

• IGPs:– RIP– IGRP/EIGRP– OSPF

• “Oh, yeah, there’s also IS-IS, but I Idunnomuchaboutit.”

• IS-IS Intermediate System to Intermediate System– Routing protocol of ISO’s Connectionless Network Protocol

(CLNP)– ISO 10589– Developed by Digital Equipment Corporation for its DECnet Phase

V.


GOSIP and EPHOS

• ISO working on IS-IS about the same time IAB (Internet Architecture Board) was working on OSPF for TCP/IP.

• Late 1980’s, early 1990’s, ISO thought TCP/IP would become an interim solution and eventually replaced by OSI suite.

• United States Government Open Systems Interconnection Profile (GOSIP) and European Procurement Handbook for Open Systems (EPHOS) also added impetus to this movement.


IS-IS versus OSPF

• To support this transition:– IS-IS for CLNS– IS-IS for IP– Dual IS-IS or Integrated IS-IS: for both CLNS and IP

• Pro-ISO versus Pro-OSPF– For interesting information on this topic see the following books:

• Routing in the Internet, Christian Huitema (past chairman of IAB)

• Interconnections, Radia Perlman (chief designer of IS-IS)• OSPF, John Moy (chief designer of OSPF)

• TCP/IP became the protocol suite of the Internet, and OSPF became the IGP chosen by most network administrators.


Similarities between IS-IS and OSPF

Also:

• Authentication capabilities

But also, many differences…


Operation of IS-IS

• IETF (Internet Engineering Task Force) and ISO use different terms to describe similar entities.

• Router = Intermediate System (IS)

• Host = End System (ES)

• ES-IS: Protocol that provides communication between a host and a router. (not discussed)– Similar to Proxy ARP, IRDP, or a default gateway used in IP.

• IS-IS: Routing protocol routers use to communicate with each other. (is discussed)


Operation of IS-IS

SNPA (Subnetwork Point of Attachment)

• Somewhat conceptual.

• Defines the point at which the subnetwork services are provided, rather than an actual physical interface.

• Discussed more later…

NSAPNSAP

MAC (SNPA) address SNPA address

IP address IP address

IS-IS PDUs


IS-IS operation

• Routers send Hello packets out all IS-IS enabled interfaces to discover neighbors and establish adjacencies.

• Routers sharing a common data link will become IS-IS neighbors, forming an adjacency. – The criteria depending point-to-point or broadcast. – The main criteria are matching:

• Authentication• IS-type• MTU size

• Routers may build a LSP based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.

• Routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. – There are different forms of flooding and also a number of scenarios

in which the flooding operation may differ. • All routers will construct their link-state database from these LSPs. • A shortest-path tree (SPT) is calculated by each IS, and from this SPT

the routing table is built.


OSI PDUs

• The OSI stack defines a unit of data as a PDU. • A frame is therefore regarded by OSI as a data-link PDU. • There are three types of PDUs with 802.2 Logical Link Control

encapsulation. – The IS-IS and ES-IS PDUs are encapsulated directly in a data-link

PDU - There is no CLNP header and no IP header. – CLNP data packets contain a full CLNP header between the data-

link header and any higher layer CLNS information. • The IS-IS and ES-IS PDUs contain variable-length fields, depending

on the function of the PDU, each field contains a TLV.


OSI PDUs

• The TLV fields contain the following information: – The neighbor ISs for the router are used to build the

map of the network – The neighbor ESs for the router – Authentication information, used to secure routing

updates – Attached IP subnets, if running Integrated IS-IS.


OSI PDUs

There are four categories of IS-IS PDUs:

• Hello PDU (ESH, ISH, IS-IS Hello [IIH]) – Used to establish and maintain adjacencies. ESHs are sent from ESs to ISs. ISHs are sent from ISs to ESs. IIHs are sent between ISs. Note that ESH and ISH PDUs are ES-IS PDUs, not IS-IS PDUs.

• LSP – Used by IS-IS to distribute link-state information. There are independent pseudonode and non-pseudonode LSPs for both Level 1 and Level 2.

• Complete Sequence Number PDU (CSNP) – Used to distribute a complete link-state database on the router. CSNPs are used to inform other routers of LSPs that may be outdated or missing from their own database. This ensures that all routers have the same information and are synchronized. The packets are similar to an OSPF database description packet.

• Partial Sequence Number PDU (PSNP) – Used to acknowledge and request link-state information.


Hello PDUs

• Hello PDU (ESH, ISH, IS-IS Hello [IIH]) – Used to establish and maintain adjacencies. – ESHs are sent from ESs to ISs.– ISHs are sent from ISs to ESs. – IIHs are sent between ISs. (More later on these!)

• Note that ESH and ISH PDUs are ES-IS PDUs, not IS-IS PDUs.


LSP and CSNP PDUs

• LSP – Used by IS-IS to distribute link-state information.

• Complete Sequence Number PDU (CSNP) – Used to distribute a complete link-state database on the router. – CSNPs are used to inform other routers of LSPs that may be

outdated or missing from their own database. – This ensures that all routers have the same information and are

synchronized. The packets are similar to an OSPF database description packet.

• Partial Sequence Number PDU (PSNP) – Used to acknowledge and request link-state information.

(More later on these!)


IS-IS link-state PDU (LSP) formats


IS-IS link-state PDU (LSP) formats


Code Values for TLVs

IS-IS Areas


IS-IS Areas

• Two level hierarchy like OSPF• Fundamental difference is how they define their areas:

– OSPF: ABRs with some interfaces in one area and some interfaces in another area.

– IS-IS: All the routers are completely within an area• Area borders are on the links, not on the routers.• L2 routers connect areas• L1 routers have no connectivity to another area


IS-IS Areas

IS-IS Routers:

• Level 1 router (L1)– Analogous to OSPF Internal router

• Level 2 router (L2)– Analogous to OSPF Backbone router

• Both Level 1 and Level 2 router (L1L2)– Analogous to OSPF ABR router


IS-IS Areas

• L1L2 routers must maintain both a level 1 LSDB and a level 2 LSDB, similar to OSPF ABR must do it separate areas.

• L2 router (and L1L2 routers) and their interconnecting links is the IS-IS backbone.

• All inter-area traffic must traverse the backbone.

• All L1 routers within an area (including L1L2 routers) have same LSDB.

If the L1 routers in the backbone did not exist, the L1L2 routers could just be L2 routers.

X

X


IS-IS Areas

L1L2 routers• Unlike OSPF, L1L2 routers do not advertise L2 routes to L1 routers.• They only advertise a default route.• Similar to an OSPF totally stubby area• L1 routers have no knowledge of routes outside its own area.• To route packets in another area, L1 routers must forward packets to

an L1L2 router.• L1L2 router sends a level 1 LSP into an area with the Attached (ATT)

bit set in the LSP, which tells other routers that it can reach another area (later)

• L1L2 routers will calculate separate SPF trees for level 1 and level 2

Source: Routing TCP/IP by Jeff Doyle, Cisco Press Rick Graziani [email protected]

Level-1, Level-2 & Level-1-2 Routers

• Backbone MUST BE L2 contiguous

L1-only

L1-only

L1-only

L1-only

L1-L2

L1-L2

L2-only

L1-only

L1-L2

L1-L2

L1-only

This router has to behave as level-2 as well in order to guarantee backbone continuity


Level-1, Level-2 & Level-1-2 Routers

• Backbone MUST BE L2 contiguous

L1-only

L1-only

L1-only

L1-L2

L1-L2

L2-only

L1-L2

L1-L2

L1-only

This router has to behave as level-2 as well in order to guarantee backbone continuity

L1-L2


Level-2 Routing

• Transit traffic requires routers inside the area to know about other areas– routers in transit paths must be L1L2 routers to have the

full L2 LSDB

• L2 routers must be contiguous


The Backbone

• A router can’t tell whether it is a transit node– Therefore the cisco default is to be L1L2– This will make the backbone larger then necessary– So always configure L1-only or L2-only when possible (IP

routing)• L1L2 in one area is less scalable

– Especially with ISIS for IP


ISIS router cannot determine if they need to be L1 or L1L2Therefore By default all cisco routers will behave as L1L2

Area 4

Can an IS determine its level ?

Area 3

Area 2

Area 1“I’m in area 2 and ALL my neighbors are in thesame area. I must be a L1-only router ?”

!! NO !!Rtr C must have a full L2 LSDB to route between areas 1, 3 and 4. Remember, the backbone must be contiguous.


Another Example


Suboptimal IS-IS routing


Suboptimal IS-IS routing


IS-IS Areas

Area ID and System ID• Area ID: Because the entire router and not an interface is within a

single area, the Area ID (or area address) is associated with the entire router.

• System ID (IS-IS) analogous to Router ID (OSPF)• IS-IS Network Entity Title (NET) address contains both:

– Area ID – System ID


NET – Network Entity Title

• Even when IS-IS routes only IP, IS-IS still uses an ISO CLNP protocol.

• Even in an IP only environment IS-IS routers must have an ISO address.

• CLNS PDUs are used to communicate between IS peers.

• ISO Address (NET), described in ISO 8348

• From 8 to 20 octets

• Includes both Area ID and System ID

• Can be very flexible or cumbersome, depending upon your viewpoint.

• ISO designed NET to be many things to many systems….

Area ID Sys ID NSEL

20 bytes

Variable length 6 bytes 1 byte


NETs

There are three NET (or NSAP) formats. a. Simple 8-byte area ID and system ID format (Simplified NET Format). b. OSI NSAP formatc. Government OSI Profile (GOSIP) NSAP format.

• Cisco supports all NSAP formats that are defined by ISO 8348/Ad2, which are described in this section.

• See RFC 1237 for more information.


NETs

• Although the fields preceding the System ID differ, the System ID itself is the same.

• ISO 10589: System ID can be from 1 to 8 octets, but must be the same length with all routers with in the routing domain.

• Most common is to use a length of 6 octets (required on Cisco routers).

• Common to use a MAC address or convert an IP address.


NETs

• The preceding six bytes form the system ID. – The IOS fixes this length at six bytes. – It is customary to code either into the system ID:

• Media Access Control (MAC) address from the router • IP address, such as a loopback address,.

– With Integrated IS-IS, a loopback IP address is commonly used for this purpose.

– In this case, the system ID is obtained by converting a loopback address: 192.168.111.3 -> 192.168.111.003 -> 1921.6811.1003.


• NSEL Identifies a process on the device. – It is roughly equivalent to a port or socket in TCP/IP. The NSEL is

not used in routing decisions. • All router NETs have an n-selector of zero, implying the network layer

of the IS itself (0 means no transport layer). • For this reason, the NSAP of a router is always referred to as a NET. • If an NSEL is set to something other than 0x00, the address is

considered a NSAP address. (later)

NET: NSEL or SEL


NET

• Regardless of the format, NET addresses must comply with 3 rules:

1. NET must begin with a single octet.• 47.xxx

2. NET must end with a single octet.• …xxxx.00

3. Cisco routers: System ID of NET must be 6 octets.

Area ID Sys ID NSEL

20 bytes

Variable length 6 bytes 1 byte


NETs

Example 1: NSAP 47.0001.aaaa.bbbb.cccc.00

• Area ID is 47.0001

• System ID is aaaa.bbbb.cccc

• NSAP selector byte is 00

Example 2: NSAP 39.0f01.0002.0000.0c00.1111.00

• Area ID is 39.0f01.0002

• System ID is 0000.0c00.1111

• NSAP selector byte is 00


IS-IS Functional Organization

Network Layer - Subnetwork Layers:• Subnetwork-Independent sublayer

– Provides uniform network services to the transport layer

• Subnetwork-Dependent sublayer– Accesses the data link layer on behalf of the

Subnetwork-Independent sublayer– Hide characteristics of different kinds of data links from

the functions of independent layer.– Exchanges IS-IS Hello PDUs to discover neighbors

and establish adjancencies.– Uses one of two network types (OSPF uses four):

1. Broadcast: same as OSPF2. Point-to-Point: PVCs and SVCs, T1s, etc.


Neighbors and Adjacencies

• IS-IS discover neighbors and forms adjacencies using IS-IS Hello PDUs.

• Transmitted every 10 seconds• Can be changed using the interface command, is hello-interval

• Hold time defaults to 3 times the Hello time, before declaring a neighbor dead.– changed using the interface command is hello-multiplier


Neighbors and Adjacencies

• L1 routers form L1 adjacencies with L1 and L1L2 routers• L2 routers form L2 adjacencies with L2 and L1L2 routers• L1L2 routers form L1 and L2 adjacencies with each other• L1 router does not form an adjacency with an L2 router


Adjacency levels

L1-Adjacency L2-Adjacency

L2-AdjacencyL2-Adjacency

L1L2Adjacency

L1L2Adjacency

Router with adjacencies within the same area.

However, needs to have a L2 database as well since it is a transit node

Therefore L1L2 adjacency is required


IS-IS Hello PDU

• Allows IS-IS router to discover neighbors on a link.

• Used as a keepalive to maintain adjacency.

• Two kinds of IS-IS Hellos:

1. LAN Hellos– L1 LAN Hellos– L2 LAN Hellos

2. Point-to-Point Hellos


IS-IS Hello PDU

• Circuit ID (last two bits)– 01: L1 Router– 10: L2 Router– 11: L1L2 Router– 00: Ignore PDU

• Source ID – System ID of originating router

• Holdtime – Default 3 times hello

• Priority – Used for DR election (0 –127)

• LAN ID – System ID of DIS (DR)

• Note: Point-to-Point Hello PDUs are identical to LAN Hellos except:– No priority field– Local Circuit ID field instead of LAN ID field

IS-IS and Broadcast Networks


Pseudonodes and network LSAs

• For SPF, the whole network must look like a collection of nodes and point-to-point links– Multi-access networks are different

• LAN uses a virtual node called pseudonode. • It is not a real router, but just an extra LSP in the LSPDB

Logical view

LAN

Physical view

DIS

Pseudonode

DIS


Who creates the pseudonode

• Created by Designated IS (DIS), ie. Designate Router

• No Backup Designated Router in IS-IS

• All LAN routers report connectivity to the pseudonode in their LSPs

Logical view

LAN

Physical view

DIS

Pseudonode

DIS


DIS – Designated IS

• IS-IS elects a Designated IS (DIS), ie. Designated Router on broadcast multi-access networks

• Each router, including the DIS advertises a single link to the pseudonode.

• As the representative of the pseudonode, the DIS also advertises a link to all the attached routers.


DIS – Designated IS

• Only a DIS, no backup DIS.

• Highest priority becomes DIS– Priority: 0 to 127– 0 = ineligible to be DIS– Tie: Router with highest System ID (Router ID)– Default = 64

• New DR election if: DR fails or another router enters network with a higher priority (or System ID)

IS-IS Operations


Update Process

• Update process responsible for constructing L1 and L2 LSDBs.

• L1 LSPs are flooded throughout an area.

• L2 LSPs are flooded over all L2 adjacencies.

• Each LSP contains:– Remaining Lifetime– Sequence Number– Checksum

IS-IS link-state PDU (LSP)

Building the LSDB


Update Process

Remaining Lifetime is an age (ie. MaxAge in OSPF)

• 1200 seconds (20 minutes)

• Remaining Lifetime begins at Maximum age of 1200 seconds and counts down (OSPF MaxAge starts at 0 and counts up to 3600 seconds)

• Originator must periodically refresh LSPs– IS-IS Refresh Interval = 15 minutes (minus random jitter up to 25%)

• If the Remaining Lifetime reaches zero, the expired LSP will remain in LSDB for another 60 seconds (ZeroAgeLifetime).



Update Process

Checksum

• If any router receives an LSP with an incorrect Checksum, the router will purge the LSP by setting the LSP’s Remaining Lifetime to zero and flooding it.

• This purge causes the originator of the LSP to send a new instance of the LSP.



Update Process

Sequence Number

• Set to one when router first generates LSP.

• Each subsequent instance of LSP is incremented by one.

• If sequence number reaches 0xFFFFFFFF:– IS-IS process shuts down for 21 minutes (Remaining Lifetime +

ZeroAgeLifetime) to allow old LSPs to age out of all LSDBs.



Update Process

• Point to Point networks: L1 and L2 LSPs sent directly to neighbor.

• Broadcast networks: LSPs multicast to all neighbors– L1 LSPs AllL1ISs– L2 LSPs AllL2ISs


Update Process

• SNPs are used to acknowledge the receipt of LSPs and to maintain LSDB synchronization– Partial SNPs (PSNPs) on point-to-point networks– Complete SNPs (CSNPs) on broadcast networks

• P-2-P networks:– Once an LSP is sent, router sets a timer

(minimumLSPTransmissionInterval) of 5 seconds– If PSNP not received, resends LSP.


Update Process

• Broadcast networks:– LSPs are not acknowledged by each receiving router.– DIS periodically multicasts a CSNP that describes every LSP in

LSDB.• Default is 10 seconds

– L1 CNSPs are multicast to AllL1ISs– L2 CNSPs are multicast to AllL2ISs


Decision Process

• Once the update process has built the LSDB, the Decision Process uses the LSDB to calculate the SPF.

• Separate SPF for L1 routes and L2 routes.• Four types of metrics:

1. Default – Cisco only supports this metric.2. Delay3. Expense4. Error

• Each metric expressed as an integer between 0 and 63.• Separate route is calculated for each metric.• SPF must be run for each metric, for both L1 and L2 routes.• Because of these and other reasons, Cisco only supports the Default.


Metric

• Cisco assigns a default metric of 10 to every interface regardless of interface type.

• Left to the default, IS-IS metric becomes a simple measure of hop count.

• Interface command isis metric changes the default value.

• The total cost of any route is a sum of the individual metrics of the outgoing interfaces.

• The maximum metric value is 1023.

• Extended Metric– Cisco IOS software addresses this issue with the support of a 24-

bit metric field called the wide metric. – Using the new metric style, link metrics now have a maximum

value of 16777215 (224 - 1) with a total path metric of 4261412864 (232 - 225).


Metrics

Odds and Ends

• Cisco’s IS-IS implementation will perform equal cost load balancing up to six paths.

• Supports VLSM

• L1 routers calculate path to the nearest L2 router for inter-area routing.– When an L2 or L1L2 router is attached to another area,

the router will advertise this fact by setting the ATT bit in is LSP to one.

– The Decision Process in L1 routers will choose the metrically closest L1L2 router as the default router.

– An L1 0.0.0.0/0 route will be entered into the routing table.

Configuring IS-IS


Basic configuration


Basic configuration of Integrated IS-IS






Multiarea Integrated IS-IS configuration



Documents

IS-IS-Doyle