3Com Switch 4800G 24-Port - Page 335

IS-IS Overview 335
Figure 107 L1/L2 LSP format
■ PDU Length: Total length of the PDU in bytes.
■ Remaining Lifetime: LSP remaining lifetime in seconds.
■ LSP ID: Consists of the system ID, the pseudonode ID (one byte) and the LSP
fragment number (one byte).
■ Sequence Number: LSP sequence number.
■ Checksum: LSP checksum.
■ P (Partition Repair): Only related with L2 LSP, indicates whether the router
supports partition repair.
■ ATT (Attachment): Generated by the L1/L1 router, only related with L1 LSP,
indicates that the router generating the LSP is connected with multiple areas.
■ OL (LSDB Overload): Indicates that the LSDB is not complete because the router
is running out of system resources. In this condition, other routers will not send
packets to the overloaded router, except packets destined to the networks
directly connected to the router. For example, in Figure 108, Router A uses
Router B to forward its packets to Router C in normal condition. Once other
routers know the OL field on Router B is set to 1, Router A will send packets to
Router C via Router D and Router E, but still send to Router B packets destined
to the network directly connected to Router B.
Intradomain routing protocol discriminator
Reserved
Version
R
ID length
Version/Protocol ID extension
Length indicator
Maximum area address
R R PDU type
No. of Octets
1
1
1
1
1
1
1
1
PDU length
Remaining lifetime
LSP ID
Sequence number
Checksum
Variable length fields
2
2
4
ID length+2
2
P
1
ATT OL IS type