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ISIS PDU – Types and Wireshark sample packets

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Intermediate systems send and receive packets and relay (forward) packets. ISO packets are called ISIS Protocol Data Unit packets, or in short ISIS PDU.

There are four PDU types and all share a common header. This common header has the same structure and information for all four PDU types, except the ‘PDU Type’ filed as highlighted in the sample bellow.

Sample ISIS common header: 
ISO 10589 ISIS InTRA Domain Routeing Information Exchange Protocol
    Intradomain Routing Protocol Discriminator: ISIS (0x83)
    Length Indicator: 20
    Version/Protocol ID Extension: 1
    ID Length: 0
    000. .... = Reserved: 0x0
    ...1 0001 = PDU Type: P2P HELLO (17)
    Version: 1
    Reserved: 0
    Maximum Area Addresses: 0

IS-IS uses the following 4 x PDUs to exchange protocol information:

  • IS-IS hello (IIH) PDUs – identify neighboring ISIS system and establish adjacencies
    • Broadcast to discover the identity of neighboring IS-IS systems and to determine whether the neighbors are Level 1 or Level 2 intermediate systems.
    • IS-IS hello PDUs establish adjacencies with other routers and have three different formats: one for point-to-point hello packets, one for Level 1 broadcast links, and one for Level 2 broadcast links. Level 1 routers must share the same area address to form an adjacency, while Level 2 routers do not have this limitation. The request for adjacency is encoded in the Circuit type field of the PDU.
    • Hello PDUs have a preset length assigned to them. The IS-IS router does not resize any PDU to match the maximum transmission unit (MTU) on a router interface. Each interface supports the maximum IS-IS PDU of 1492 bytes, and hello PDUs are padded to meet the maximum value. When the hello is sent to a neighboring router, the connecting interface supports the maximum PDU size.
  • Link-state PDUs (LSP) – used to advertise link-state information about themselves and their connected links to other routers in the network.
    • Contain information about the state of adjacencies to neighboring IS-IS systems. Link-state PDUs are flooded periodically throughout an area.
      Also included is metric and IS-IS neighbor information. Each link-state PDU must be refreshed periodically on the network and is acknowledged by information within a sequence number PDU.
    • On point-to-point links, each link-state PDU is acknowledged by a partial sequence number PDU (PSNP), but on broadcast links, a complete sequence number PDU (CSNP) is sent out over the network. Any router that finds newer link-state PDU information in the CSNP then purges the out-of-date entry and updates the link-state database.
    • Link-state PDUs support variable-length subnet mask addressing.
  • Complete sequence number PDUs (CSNPs) – Contain a complete list of all link-state PDUs in the IS-IS database.
    • CSNPs are sent periodically on all links, and the receiving systems use the information in the CSNP to update and synchronize their link-state PDU databases. The designated router multicasts CSNPs on broadcast links in place of sending explicit acknowledgments for each link-state PDU.
    • Contained within the CSNP is a link-state PDU identifier, a lifetime, a sequence number, and a checksum for each entry in the database. Periodically, a CSNP is sent on both broadcast and point-to-point links to maintain a correct database. Also, the advertisement of CSNPs occurs when an adjacency is formed with another router. Like IS-IS hello PDUs, CSNPs come in two types: Level 1 and Level 2.
    • When a device receives a CSNP, it checks the database entries against its own local link-state database. If it detects missing information, the device requests specific link-state PDU details using a partial sequence number PDU (PSNP).
  • Partial sequence number PDUs (PSNPs) – message used to acknowledge received LSPs or request missing LSPs during database synchronization.
    • Sent multicast by a receiver when it detects that it is missing a link-state PDU (when its link-state PDU database is out of date). The receiver sends a PSNP to the system that transmitted the CSNP, effectively requesting that the missing link-state PDU be transmitted. That routing device, in turn, forwards the missing link-state PDU to the requesting routing device.
    • A PSNP is used by an IS-IS router to request link-state PDU information from a neighboring router. A PSNP can also explicitly acknowledge the receipt of a link-state PDU on a point-to-point link. On a broadcast link, a CSNP is used as implicit knowledge. Like hello PDUs and CSNPs, the PSNP also has two types: Level 1 and Level 2.
    • When a device compares a CSNP to its local database and determines that a link-state PDU is missing, the router issues a PSNP for the missing link-state PDU, which is returned in a link-state PDU from the router sending the CSNP. The received link-state PDU is then stored in the local database, and an acknowledgment is sent back to the originating router.

For a more in depth understanding of ISIS PDUs, what can be better than actually seeing the sample packets with a Wireshark capture?

IIH PDU 
ISIS HELLO
    .... ..10 = Circuit type: Level 2 only (0x2)
    0000 00.. = Reserved: 0x00
    SystemID {Sender of PDU}: 0100.4922.7100
    Holding timer: 27
    PDU length: 108
    Local circuit ID: 1
    Point-to-point Adjacency State (t=240, l=15)
        Type: 240
        Length: 15
        Adjacency State: Up (0)
        Extended Local circuit ID: 0x00000165
        Neighbor SystemID: 0100.4923.5018
        Neighbor Extended Local circuit ID: 0x00000166
    Protocols Supported (t=129, l=2)
        Type: 129
        Length: 2
        NLPIDs: IP (0xcc), IPv6 (0x8e)
            NLPID: 0xcc
            NLPID: 0x8e
    IP Interface address(es) (t=132, l=4)
        Type: 132
        Length: 4
        IPv4 interface address: 17.8.1.2
    IPv6 Global Interface Address (t=233, l=16)
        Type: 233
        Length: 16
        IPv6 Global interface address: 2001:db88:8:1::2
    IPv6 Interface address(es) (t=232, l=16)
        Type: 232
        Length: 16
        IPv6 interface address: fe80::5604:dff:fe00:5c2a
    Area address(es) (t=1, l=18)
        Type: 1
        Length: 18
        Area address (13): 47.0005.80ff.f800.0000.0108.0001
        Area address (3): 49.0001
    Restart Signaling (t=211, l=3)
        Type: 211
        Length: 3
        Restart Signaling Flags: 0x00
            .... .0.. = Suppress Adjacency: False
            .... ..0. = Restart Acknowledgment: False
            .... ...0 = Restart Request: False
LSP PDU 
ISO 10589 ISIS Link State Protocol Data Unit
    PDU length: 370
    Remaining lifetime: 1196
    LSP-ID: 0100.4922.6166.00-00
    Sequence number: 0x0000000b
    Checksum: 0x9869 [correct]
    [Checksum Status: Good]
    Type block(0x03): Partition Repair:0, Attached bits:0, Overload bit:0, IS type:3
        0... .... = Partition Repair: Not supported
        .000 0... = Attachment: 0
            .0.. .... = Error metric: Not set
            ..0. .... = Expense metric: Not set
            ...0 .... = Delay metric: Not set
            0... .... = Default metric: Not set
        .... .0.. = Overload bit: Not set
        .... ..11 = Type of Intermediate System: Level 2 (3)
    Area address(es) (t=1, l=18)
        Type: 1
        Length: 18
        Area address (13): 47.0005.80ff.f800.0000.0108.0001
        Area address (3): 49.0001
    Originating neighbor buffer size (t=14, l=2)
        Type: 14
        Length: 2
        Neighbor originating buffer size: 1492
    Protocols supported (t=129, l=2)
        Type: 129
        Length: 2
        NLPIDs: IP (0xcc), IPv6 (0x8e)
            NLPID: 0xcc
            NLPID: 0x8e
    Traffic Engineering Router ID (t=134, l=4)
        Type: 134
        Length: 4
        Traffic Engineering Router ID: 17.0.0.4
    IP Interface address(es) (t=132, l=4)
        Type: 132
        Length: 4
        IPv4 interface address: 17.0.0.4
    IPv6 TE Router ID (t=140, l=16)
        Type: 140
        Length: 16
        IPv6 TE Router ID: abcd::128:49:226:166
    Hostname (t=137, l=2)
        Type: 137
        Length: 2
        Hostname: r4
    Extended IS reachability (t=22, l=138)
        Type: 22
        Length: 138
        IS Neighbor: 0100.4922.9056.00
            IS neighbor ID: 0100.4922.9056.00
            Metric: 1000
            SubCLV Length: 58
            subTLV: IPv4 interface address (c=6, l=4)
                Code: IPv4 interface address (6)
                Length: 4
                IPv4 interface address: 17.8.2.2
            subTLV: IPv6 Interface Address (c=12, l=16)
                Code: IPv6 Interface Address (12)
                Length: 16
                IPv6 interface address: 2001:db88:8:2::2
            subTLV: IPv4 neighbor address (c=8, l=4)
                Code: IPv4 neighbor address (8)
                Length: 4
                IPv4 neighbor address: 17.8.2.1
            subTLV: IPv6 Neighbor Address (c=13, l=16)
                Code: IPv6 Neighbor Address (13)
                Length: 16
                IPv6 neighbor address: 2001:db88:8:2::1
            subTLV: Link Local/Remote Identifiers (c=4, l=8)
                Code: Link Local/Remote Identifiers (4)
                Length: 8
                Link Local Identifier: 357
                Link Remote Identifier: 358
        IS Neighbor: 0100.4922.7100.00
            IS neighbor ID: 0100.4922.7100.00
            Metric: 10
            SubCLV Length: 58
            subTLV: IPv4 interface address (c=6, l=4)
                Code: IPv4 interface address (6)
                Length: 4
                IPv4 interface address: 17.8.3.2
            subTLV: IPv6 Interface Address (c=12, l=16)
                Code: IPv6 Interface Address (12)
                Length: 16
                IPv6 interface address: 2001:db88:8:3::2
            subTLV: IPv4 neighbor address (c=8, l=4)
                Code: IPv4 neighbor address (8)
                Length: 4
                IPv4 neighbor address: 17.8.3.1
            subTLV: IPv6 Neighbor Address (c=13, l=16)
                Code: IPv6 Neighbor Address (13)
                Length: 16
                IPv6 neighbor address: 2001:db88:8:3::1
            subTLV: Link Local/Remote Identifiers (c=4, l=8)
                Code: Link Local/Remote Identifiers (4)
                Length: 8
                Link Local Identifier: 358
                Link Remote Identifier: 358
    Extended IP Reachability (t=135, l=34)
        Type: 135
        Length: 34
        Ext. IP Reachability: 128.49.226.166/32
            Metric: 0
            0... .... = Distribution: Up
            .0.. .... = Sub-TLV: No
            ..10 0000 = Prefix Length: 32
            IPv4 prefix: 128.49.226.166
            no sub-TLVs present
        Ext. IP Reachability: 17.8.2.0/24
            Metric: 1000
            0... .... = Distribution: Up
            .0.. .... = Sub-TLV: No
            ..01 1000 = Prefix Length: 24
            IPv4 prefix: 17.8.2.0
            no sub-TLVs present
        Ext. IP Reachability: 17.0.0.4/32
            Metric: 0
            0... .... = Distribution: Up
            .0.. .... = Sub-TLV: No
            ..10 0000 = Prefix Length: 32
            IPv4 prefix: 17.0.0.4
            no sub-TLVs present
        Ext. IP Reachability: 17.8.3.0/24
            Metric: 10
            0... .... = Distribution: Up
            .0.. .... = Sub-TLV: No
            ..01 1000 = Prefix Length: 24
            IPv4 prefix: 17.8.3.0
            no sub-TLVs present
    IP Interface address(es) (t=132, l=4)
        Type: 132
        Length: 4
        IPv4 interface address: 128.49.226.166
    IPv6 reachability (t=236, l=72)
        Type: 236
        Length: 72
        IPv6 Reachability: abcd::128:49:226:166/128
            Metric: 0
            0... .... = Distribution: Up
            .0.. .... = Distribution: Internal
            ..0. .... = Sub-TLV: No
            Prefix Length: 128
            IPv6 prefix: abcd::128:49:226:166
            no sub-TLVs present
        IPv6 Reachability: 2001:db88::4/128
            Metric: 0
            0... .... = Distribution: Up
            .0.. .... = Distribution: Internal
            ..0. .... = Sub-TLV: No
            Prefix Length: 128
            IPv6 prefix: 2001:db88::4
            no sub-TLVs present
        IPv6 Reachability: 2001:db88:8:2::/64
            Metric: 1000
            0... .... = Distribution: Up
            .0.. .... = Distribution: Internal
            ..0. .... = Sub-TLV: No
            Prefix Length: 64
            IPv6 prefix: 2001:db88:8:2::
            no sub-TLVs present
        IPv6 Reachability: 2001:db88:8:3::/64
            Metric: 10
            0... .... = Distribution: Up
            .0.. .... = Distribution: Internal
            ..0. .... = Sub-TLV: No
            Prefix Length: 64
            IPv6 prefix: 2001:db88:8:3::
            no sub-TLVs present
    Router Capability (t=242, l=23)
        Type: 242
        Length: 23
        Router ID: 0x11000004
        .... ...0 = S bit: False
        .... ..0. = D bit: False
        IPv6 TE Router ID (t=12, l=16)
            IPv6 TE Router ID: abcd::128:49:226:166
CSNP PDU 
ISO 10589 ISIS Complete Sequence Numbers Protocol Data Unit
    PDU length: 115
    Source-ID: 0100.4922.7100
    Source-ID-Circuit: 00
    Start LSP-ID: 0000.0000.0000.00-00
    End LSP-ID: ffff.ffff.ffff.ff-ff
    LSP entries (t=9, l=80)
        Type: 9
        Length: 80
        LSP Entry
            LSP Sequence Number: 0x0000000a
            Remaining Lifetime: 421
            LSP checksum: 0x9a68
        LSP-ID: 0100.4922.6166.00-00
        LSP Entry
            LSP Sequence Number: 0x0000000c
            Remaining Lifetime: 623
            LSP checksum: 0x5111
        LSP-ID: 0100.4922.7100.00-00
        LSP Entry
            LSP Sequence Number: 0x0000000b
            Remaining Lifetime: 902
            LSP checksum: 0x99e7
        LSP-ID: 0100.4922.9056.00-00
        LSP Entry
            LSP Sequence Number: 0x00000009
            Remaining Lifetime: 632
            LSP checksum: 0x76be
        LSP-ID: 0100.4923.4086.00-00
        LSP Entry
            LSP Sequence Number: 0x0000000a
            Remaining Lifetime: 1057
            LSP checksum: 0xf074
        LSP-ID: 0100.4923.5018.00-00
PSNP PDU 
ISO 10589 ISIS Partial Sequence Numbers Protocol Data Unit
    PDU length: 35
    Source-ID: 0100.4923.5018
    Source-ID-Circuit: 00
    LSP entries (t=9, l=16)
        Type: 9
        Length: 16
        LSP Entry
            LSP Sequence Number: 0x0000000c
            Remaining Lifetime: 1196
            LSP checksum: 0x5111
        LSP-ID: 0100.4922.7100.00-00

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