ISIS routing protocol datagram image/diagram?

[MicJameson1]

Hello.

Despite my googling, I can't find an adequate explanation or diagram on how the ISIS datagram looks. I'm looking for the image to show me the outside ethernet frame, then what comes next (which i expect is not an IP packet, but routing information.)

Can anyone find this seemingly basic diagram?

Thank you!

[M02@rt37 · Accepted solution]

Hello @MicJameson1 

IS-IS is a link-state routing protocol that operates directly over the data link layer, without relying on IP encapsulation. In an Ethernet environment, an IS-IS packet is encapsulated within an IEEE 802.3 frame, followed by an 802.2 LLC header. The LLC header includes Destination Service Access Point (DSAP) and Source Service Access Point (SSAP) fields, both set to 0xFE to indicate the IS-IS protocol.

Following the LLC header, the IS-IS PDU comprises a common header and specific fields depending on the PDU type, such as Hello PDUs, Link State PDUs, or Sequence Number PDUs.

This structure allows IS-IS to function independently of the network layer, facilitating efficient routing within an autonomous system.

https://support.huawei.com/enterprise/en/doc/EDOC1100174721/f220bba9/common-format-of-is-is-packets

[vishalbhandari]

@MicJameson1 

The ISIS datagram is encapsulated within an Ethernet frame, as ISIS operates directly on Layer 2 (Data Link Layer) rather than Layer 3 (Network Layer). The outer Ethernet frame includes the source and destination MAC addresses and an Ethertype field identifying the protocol as ISO CLNS (usually 0xFEFE). Inside, the payload contains the ISIS Protocol Data Unit (PDU), which carries routing information. This PDU includes headers like the ISIS version, PDU type, length, and details about routing topology or link states, depending on the type of PDU (e.g., LSP, Hello, or SNP). There's no IP header, as ISIS doesn’t rely on IP for transport.