IS-IS An introduction to IGP routing protocols Hagai Kahana

IS-IS An introduction to IGP routing protocols

Hagai Kahana

Compass-EOS Confidential

156.10.24.1 16.55.42.10

57.13.29.2

Interior Gateway Protocol (IGP) – intended to use within an AS


AS 11

AS 22 AS 33

iBGPeBGP

AS 11

IGP routing protocols

Distance-vector routing protocol• RIP - Routing Information protocol – used for small

networks • EIGRP- Enhanced Interior Gateway Routing Protocol –

Cisco proprietary routing protocol – used for medium size networks

Link-state routing protocol • OSPF - Open Shortest Path First – Design for IPv4

networks, a similar to IS-IS and more common in enterprise networks

• IS-IS


IS-IS introduction

Was first published as ISO 10589 in 1987

IETF republished the protocol in RFC 1142 for the Internet community and extended for IP routing as RFC 1195 in 1990

Runs natively on Layer 2 of the OSI model (Ethernet/PPP)


Link-state protocol – High-level description

Neighbor discovery – • How two link state router discover each other and agree

to exchange routing information



Flooding – • How routing information is forward reliably to all routers

in a network



Link state database – • How routing information is stored, synchronized and

kept up to date



SPF (short path first) calculation – • How we use the information to calculate the routes


IS-IS introduction

Neighbor discovery.

Flooding.

Link state database Synchronization.

SPF (short path first) calculation.


IS-IS Addressing

System ID – Unique identifier, can be set by using the IP loopback address

Area ID


TLV

Type-Length-Value

Why length if we have type?

Flexibility

Max of 256 bytes

Contains sub-TLV

Major advantage over OSPF


TYPE

LENGTH

VALUE...

Length in Octets

1

1

Length of value

TLV

TLV 10 – authentication TLV

Added to each PDU transmitted

Packet discarded when not matched

Key - password for the PDU type

Text - IS-IS PDU to be authenticated


Type (10 )

Length (Variable)

Authentication type

Authentication value

Authentication type code

Value

Reserved 0

Clear text password 1

HNAC-MD5 Authentication

54

Private authentication 255

IS-IS Common Header

Definition Values

network layer identifier assigned to IS-IS (ISO 9577)

0x83

Length of the packet header fields in octets

Variable

Version 0x1

Indicates length of the source ID (SysID) field

0x0 -> 6 byte

IS-IS packet type (3 bits reserved)

Hello, LSP or SNP

Version 0x1

Reserved 0

Number of area addresses permitted for this intermediate system’s area

0x0 = max 3 areas


Packet Data

…

Neighbor discovery

Neighbor discoveryOnce you enable IS-IS on an interface, the routing protocol will automatically find out if there are IS-IS router at the other end.If one found a verification of version and two way interaction on the link is been done – called handshaking.Handshaking is done with hello message called IIH (Intermediate System to Intermediate System Hello) message. All messages sent to 2 multicast MAC addresses

0180:c200:0014 and 0180:c200:0015 on Broadcast media no unicast messages.

IIH message

Neighbor discovery -cont

Two types of connection broadcast LAN and P2P.Different IIH for each connection type P2P IIH are for both levels (bandwidth consideration)

Value Name

15 / 16 IIH LAN

17 IIH P2P

Handshaking – 3 ways

Router 1 sends router 2 IIH.

Router 2 Gets IIH from 1 and immediately send an IIH back which says “I have seen 1”.

Route 1 receive IIH from route 2, set Adj as up and immediately send an IIH back which says “I have seen 2”.

Router 2 receive second IIH from 1 and set Adj to up


IIH Router 1

TLV s

IIH Router 1

“I see Router 2”+

TLVs

IIH Router 2

“I see Router 1”+

TLVs

Adjacency UP

Adjacency UP

Router 2Router 1

IIH – LAN Level 1 and Level 2

Field Definition

Circuit type Level 1, Level 1-2, or Level 2 only (6 bit reserved)

Source ID System ID of the router that generated packet

Holding time

Maximum interval between two consecutive hello packets before the router is considered no longer available

PDU length Length of the entire PDU, including header

Priority designates the DIS (Level 1 or Level 2) on the LAN

LAN ID SysID of the DIS plus an octet-long unique ID for this router assigned by the DIS.

TLV fields Type 1 - Area Addresses Type 6 - IS Neighbors Type 8 - Padding Type 10 - Authentication Type


IIH – IS-IS Hello Packet

Hello Interval – time between each IIH packets

Hold-time - maximum time lapse allowed between receipts of two consecutive hello packets received • If hold-time expire an adjacency is declared

“dead”• Can be modified on the fly (unlike OSPF)


IIH – IS-IS Hello Packet

IIH sent in these cases:• Expiration of the hello interval• Any change in network conditions• Election to or resignation from LAN DIS position


40

30

20

10

Hello Packet 30sec hold time



Flooding

LSP Link State Protocol data unit

Link-state routing distribute their topological view beyond their immediate neighbor (unlike vector distance).

A single router create a LSP (Link state PDU -> Link State Protocol Data Unit) that contain the reachability and immediate connected neighbored (imagine it as a single part of a puzzle).



When a router receive updated LSP from all the routers in the network it has a complete map view of the puzzle and can accurately compute paths in the network.

The process of sending and updating topology information is called flooding.

The LSP is stored in LSP Data base, a different DB exist for each level

LSP contain other information

such as IP reachability, check sum

and even the routers name


LSP Revision control


Sequence number – version of the LSP.

Every new LSP generated the router increase the sequence number by one.

The Sequence number is stored in a 32-bit identifier. If a new LSP is sent every 5 second we have 681 years till the end of the address space

Which LSP is the most recent one?Which LSP is the most recent one?

LSP Revision contolIMI#show isis database

Area ipi:

IS-IS Level-2 Link State Database:

LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL

0000.0000.0001.00-00* 0x0000035F 0x24D8 506 0/0/0


LSP Lifetime default is 20 minutes (a 16 bit field)

Periodic Refreshes

LSP-ID – a unique identifier of the router consist of the System ID, pseudo-node ID and fragment ID (8B total)

Sequence number – version of the LSP.

Checksum – check if info corrupt on the way


LSPs are acknowledged by the receiving router

A router must not originate more than one LSP every 5 seconds

Intradomain Routing Proto Descriminator

Lenth Indicator

Version/Protocol ID Extension

ID Length

PDU TypeR R R

Version

Reserved

Maximum Area Addresses

PDU Length

Remaining Lifetime

LSP ID

Sequence Number

Checksum

LSPDBOL IS TypeP ATT

TLVFIELDS


Purge LSP - negative LSP update• DIS election• Expiration of LSP• Duplicate System IDs


Link state database Synchronization.

Pseudo-nodes and Designated Routers

A LAN Topology is equivalent to full mash P2P connection.

Effect of an additional router attached to the LAN


Pseudo-nodes and Designated Routers

Pseudo-node (PSN) is a virtual node, whose role is played by an elected DIS for the LAN.


LSP 1111.1111.1111.01-00

LSP 5555.5555.5555.00-00 LSP 6666.6666.6666.00-00LSP 3333.3333.3333.00-00 LSP 4444.4444.4444.00-00

LSP 1111.1111.1111.00-00

Nbr 1111.1111.1111.01-00

Nbr 1111.1111.1111.00-00Nbr 2222.2222.2222.00-00Nbr 3333.3333.3333.00-00Nbr 4444.4444.444.00-00Nbr 5555.5555.5555.00-00Nbr 6666.6666.6666.00-00

Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00

LSP 2222.2222.2222.00-00

Nbr 1111.1111.1111.01-00

Pseudo-nodes and Designated RoutersDIS election is preemptive, based on interface priority with the highest SNPA (MAC address) breaking ties

Separate DISs are elected for Level 1 and 2 routing

DIS transmits IIH three times faster other routers, for quick detection of failure and immediate replacement.


Synchronizing Databases in LANs

DIS sends periodically a directory of its link-state database



CSNP

Router XXXX.00-00Life time 1678

Seq 0x05Checksum 0xABCD

Router ….….

Router 2Router 1

PSNP

Router XXXX.00-00Life time 0Seq 0x00

Checksum 0xABCD

LSP

Router XXXX.00-00Life time 1678

Seq 0x05Checksum 0xABCD

SPF (short path first) calculation.

SPF

There are three type of SPF calulation• Full SPF – Find the shortest path tree on the

topology – also called first pass.

• Partial SPF – Extract prefix and metric information from link state database – also called second pass.

• Incremental SPF – optimization on the full SPF calculation


SPF – Hold down

A trigger for SPF calculate does not invoke the

calculation immediately. A change in the network invoke several LSP flooding.

An SPF cause a burst of CPU usage without hold down the router will choke under the SPF calculation stress.


Thank you


Documents

IS-IS An introduction to IGP routing protocols Hagai Kahana