Is-Is Packets - Juniper ACX1000 Configuration Manual

IS-IS Packets

Copyright © 2017, Juniper Networks, Inc.
Each IS-IS PDU shares a common header. IS-IS uses the following PDUs to exchange
protocol information:
IS-IS hello (IIH) PDUs—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—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information in the CSNP then purges the out-of-date entry and updates the link-state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)—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
Chapter 18: Configuring Routing Protocols
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