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IS-IS An introduction to IGP routing
protocols
Hagai Kahana
57.13.29.2
156.10.24.1
16.55.42.10
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Interior Gateway Protocol (IGP) – intended to use
within an AS
AS 22
AS 33
AS 11
AS 11
iBGP
eBGP
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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
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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)
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Link-state protocol – High-level description
Neighbor discovery –
• How two link state router discover each other and agree
to exchange routing information
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Link-state protocol – High-level description
Flooding –
• How routing information is forward reliably to all routers
in a network
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Link-state protocol – High-level description
Link state database –
• How routing information is stored, synchronized and
kept up to date
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Link-state protocol – High-level description
SPF (short path first) calculation –
• How we use the information to calculate the routes
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IS-IS introduction
Neighbor discovery.
Flooding.
Link state database Synchronization.
SPF (short path first) calculation.
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IS-IS Addressing
System ID – Unique identifier, can be set by using
the IP loopback address
Area ID
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TLV
Type-Length-Value
Why length if we have type?
Flexibility
Max of 256 bytes
Contains sub-TLV
Major advantage over OSPF
Length in Octets
TYPE
1
LENGTH
1
VALUE
.
.
.
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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
Value
Authentication type
code
Type (10 )
Length (Variable)
Reserved
0
Clear text password
1
HNAC-MD5
Authentication
54
Private authentication
255
Authentication type
Authentication value
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IS-IS Common Header
ISIS Protocol Discriminator
Length Indicator
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
Version ID Extension
ID Length
PDU Type
Version
Reserved
Maximum Area Addresses
Packet Data
…
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Neighbor discovery
Neighbor discovery
Once 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)
ISIS Protocol Discriminator
Length Indicator
Version ID Extension
ID Length
Value
Name
15 / 16
IIH LAN
17
IIH P2P
PDU Type
Version
Reserved
Maximum Area Addresses
TLV Fields
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
Router 1
Router 2
IIH Router 1
TLV s
IIH Router 2
“I see Router 1”
+
TLVs
IIH Router 1
Adjacency UP
“I see Router 2”
+
TLVs
Adjacency UP
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IIH – LAN Level 1 and Level 2
ISIS Protocol Discriminator
Length Indicator
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
designates the DIS (Level 1 or Level 2)
on the LAN
Version ID Extension
ID Length
PDU Type
Version
Reserved
Maximum Area Addresses
TLV Fields
Reserved
circuit type
Source IS
Holding time
PDU Length
R
Priority
DIS
TLV
Priority
LAN ID
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TLV fields
SysID of the DIS plus an octet-long
unique ID for this router assigned by
the DIS.
Type 1 - Area Addresses
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)
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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
Hello Packet
40sec hold time
4
0
Hello Packet
30sec hold time
3
0
Hello Packet
30sec hold time
2
0
1
0
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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).
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LSP Link State Protocol data unit
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
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LSP Revision control
Which LSP is the
most recent one?
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
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LSP Revision contol
IMI#show isis database
Area ipi:
IS-IS Level-2 Link State Database:
LSPID
LSP Seq Num LSP Checksum LSP Holdtime
0000.0000.0001.00-00* 0x0000035F
0x24D8
506
ATT/P/OL
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, pseudonode ID and fragment ID (8B total)
Sequence number – version of the LSP.
Checksum – check if info corrupt on the way
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LSP Link State Protocol data unit
Intradomain Routing Proto Descriminator
Lenth Indicator
Version/Protocol ID Extension
ID Length
R
R
R
PDU Type
Version
Reserved
Maximum Area Addresses
PDU Length
Remaining Lifetime
LSP ID
Sequence Number
Checksum
P
ATT
TLVFIELDS
LSPDBOL
IS Type
LSPs are acknowledged
by the receiving router
A router must not
originate more than one
LSP every 5 seconds
LSP Link State Protocol data unit
Purge LSP - negative LSP update
• DIS election
• Expiration of LSP
• Duplicate System IDs
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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
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Pseudo-nodes and Designated Routers
LSP 1111.1111.1111.00-00
LSP 2222.2222.2222.00-00
Nbr 1111.1111.1111.01-00
Nbr 1111.1111.1111.01-00
LSP 1111.1111.1111.01-00
Nbr 1111.1111.1111.00-00
Nbr 2222.2222.2222.00-00
Nbr 3333.3333.3333.00-00
Nbr 4444.4444.444.00-00
Nbr 5555.5555.5555.00-00
Nbr 6666.6666.6666.00-00
LSP 3333.3333.3333.00-00
LSP 4444.4444.4444.00-00
LSP 5555.5555.5555.00-00
LSP 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
Pseudo-node (PSN) is a virtual node, whose role is
played by an elected DIS for the LAN.
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Pseudo-nodes and Designated Routers
DIS 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.
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Synchronizing Databases in LANs
Router 1
Router 2
CSNP
Router XXXX.00-00
Life time 1678
Seq 0x05
Checksum 0xABCD
Router ….
….
PSNP
Router XXXX.00-00
Life time 0
Seq 0x00
Checksum 0xABCD
LSP
Router XXXX.00-00
Life time 1678
Seq 0x05
Checksum 0xABCD
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DIS sends periodically a directory of its linkstate database
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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
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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.
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Thank you
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