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IS-IS Packet Types

IS-IS Protocol uses L2 encapsulation Ethernet 802.3/802.2. It do not use Ethernet II. Ethernet II is used for IP traffic.

IS-IS Protocol exchanges protocol information by using Link-State PDUs (Protocol Data Units).

There are four types of PDU packets in IS-IS Protocol.

These IS-IS packets are :

– IS-IS Hello PDU (IIH)
– Link State PDU (LSP)
– Partial Sequence Number PDU (PSNP)
– Complete Sequence Number PDU (CSNP)

In IS-IS PDUs there is a TLV (Type, Length, Value) field. According to the need, these fields are added to the PDUs. This TLV field makes IS-IS extendable and it is the major advantage of IS-IS over OSPF. There are several TLVs in IS-IS packets. IS-IS Header always same, but according to the type of the packet, added TLV fields change.

TLVs are maximum 256 bytes and they contains sub-TLVs.

IS-IS Header Format, ISIS TLVs

IS-IS Header Format

IS-IS discovers the IS-IS neighbours and IS-IS adjacency via Hello PDUs. DIS election is also done with this PDUs.

There are three types Hello PDUs in IS-IS Protocol:

ESH -> ES sent to IS
ISH -> IS sent to ES
IIH -> Between two ISs

IS-IS Hellos, ESH, ISH, IIH

IS-IS Hellos, ESH, ISH, IIH

IS-IS Hello interval is 10 seconds and the dead interval is 30 seconds by default.

In IS-IS Hello packet, 6 types of data can be included with TLVs. These are:

– Area Addresses TLV
– Padding TLV
– IP Interface Address TLV
– Authentication Information TLV
– Protocols Supported TLV
– Intermediate System Neighbors TLV

For more information about IS-IS TLVs, you can check Intermediate System-to-Intermediate System (IS-IS) TLVs article on Cisco Website.

IS-IS LSPs are like LSAs in OSPF and contains many information about the neighbour ISIS Routers and links. They are flooded periodically in IS-IS network. L1 and L2 PDUs use different formats LSPs are stored in IS-IS LSDBs. Seperate datababses are used for L1 and L2 LSPs. Each LSP has sequence number that shows its version.

In LSPs, below TLVs are supported:

– Area Addresses TLV
– Originating LSP Buffer Size TLV
– Intermediate System Neighbors TLV
– End System Neighbor TLV
– Authentication Information TLV
– Prefix Neighbor TLV
– Protocols Supported TLV
– IP Interface Address TLV
– IP Internal Reachability Information TLV
– IP External Reachability Information TLV

PDU Type field shows the level of PDU. Level 1 PDU is showed with the value 18, Level 2 PDU is showed with the value 20.

ATT bit is the bit that set if the IS is connected to another area.

OL bit is set to show that the link-state database of IS is overloaded.

IS Type shows the level of the IS. For level 1, it is 1. For level 2 it is set as 3.

CSNP (Complete Sequence Number PDU) provides LSP to be sent reliably. CSNPs are like

Database Description (DD) packets in OSPF and they used to synronize the LSDBs.

CSNPs are exchange at the beginning, at the router initialization. And every 10 seconds after this, they are sent. In point to point links, both end routers send CSNPs. But for broadcast networks, only DIS sends CSNPs.

IS-IS CSNPs carry two types of TLVs . These are :

– Authentication TLVs
– Entries TLVs

Entries TLVs are identify the LSPs and CSNPs include the below TLVs for all LSPs in its database:

– The remaining life of LSP
– The ID of LSP
– The Sequence number of LSP
– Checksum Value

PSNPs (Partial Sequence Number PDU) are sent when there is a missing information. With PSNP, this specific missing information is requested. And it is also sent for acknowledgement of the receipt. So PSNPs are like LSAcks and LSRs ( Link State Request) in OSPF.

IS-IS CSNPs and PSNPs

IS-IS CSNPs and PSNPs

In IS-IS Protocol, PSNPs are used in point-to-point links. PSNPs also includes the TLVs like CLNPs. (The remaining life of LSP, the ID of LSP, the Sequence number of LSP, Checksum Value)

Flooding Rules



IS-IS Protocol compares the new coming LSP and the old one in LSDB (Link State Database). According to the comparement:
If the version of new coming LSP is newer, then;

1. It is installed to the LSDB,
2. PSNP PDU is send as an Ack,
3. This new version flooded to the other neighbors,
4. IS-IS check that if any SPF is needed or not.

New version LSP, LSDP Database Update, PSNPs

New version LSP, LSDP Database Update

If the version of new coming LSP is older, then;

1. PSNP PDU is send as an Ack,
2. Send the new version of that LSP
3. Wait for Ack PSNP

Old version LSP, Send New LSP, PSNPs

Old version LSP, Send New LSP

Lastly, if the version of new coming LSP is same, then;

1. PSNP PDU is send as an Ack

Same version LSP, Ack PSNP

Same version LSP, Ack PSNP

Sequence Number is used for checking the version of LSP. Higher sequence numbers are newer ones, means new versions.

The other articles on IS-IS Protocol are below:

Routing With IS-IS Protocol – Part 1
Routing With IS-IS Protocol – Part 2 (IS-IS Addresses)
Routing With IS-IS Protocol – Part 3 (IS-IS Packet Types)
Routing With IS-IS Protocol – Part 4 (IS-IS Adjacency)
Routing With IS-IS Protocol – Part 5 (IS-IS versus OSPF)

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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