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IS-IS Packet Types
This document discusses the different packet types used in IS-IS routing protocol. There are four main packet types: Hello PDUs which discover neighbors, Link State PDUs which exchange routing information, Partial Sequence Number PDUs which request missing information, and Complete Sequence Number PDUs which synchronize link state databases. All packet types carry Type-Length-Value fields which allow for extensibility.
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Introduction to IS-IS packet types and L2 encapsulation using Ethernet. IS-IS uses Link-State PDUs for protocol information exchange.
Details on four IS-IS PDU types: Hello PDU, Link State PDU, PSNP, and CSNP. Discusses TLV fields for extensibility.
Explains the IS-IS Hello packet format, neighbor discovery, adjacency, and types of Hello PDUs with key timings.
Describes LSPs similar to LSAs in OSPF, including periodic flooding and supported TLVs for various functionalities.
Insight into PDU types, bits indicating IS connection and overload status, and CSNP's role in reliable LSP exchange.
Explains PSNP usage for requesting missing information and acknowledgement in point-to-point links.
Describes how IS-IS manages LSP versions in the LSDB, protocol actions for new, old, and same version LSPs.
Encouragement to follow Net Protocol Xpert's various social media channels for updates and information.
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IS-IS Packet Types
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- 2. • IS-IS Protocoluses 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).
- 3. • There arefour 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)
- 4. • In IS-ISPDUs 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.
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- 6. • IS-IS discoversthe 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
- 7. IS-IS Hellos, ESH, ISH,IIH IS-IS Hello interval is 10 seconds and the dead interval is 30 seconds by default.
- 8. • In IS-ISHello 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
- 9. • IS-IS LSPsare 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. • Separate databases are used for L1 and L2 LSPs. Each LSP has sequence number that shows its version
- 10. In LSPs, belowTLVs 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
- 11. • PDU Typefield 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
- 12. • Database Description(DD) packets in OSPF and they used to syntonizer 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
- 13. • PSNPs (PartialSequence 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.
- 14. IS-IS CSNPs andPSNPs • 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)
- 15. Flooding Rules • IS-ISProtocol compares the new coming LSP and the old one in LSDB (Link State Database). According to the compartment: 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.
- 16. New version LSP,LSDP Database Update
- 17. • If theversion 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
- 18. Old version LSP,Send New LSP
- 19. Same version LSP, AckPSNP • Lastly, if the version of new coming LSP is same, then; 1. PSNP PDU is send as an Ack • Sequence Number is used for checking the version of LSP. Higher sequence numbers are newer ones, means new versions.
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