HP 5500 Ei 5500 Si Switch Series Configuration Guide

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Figure 50 IS-IS topology 1 
 
 
Figure 51 is another IS-IS topology. The Level-1-2 routers connect to the Level-1 and Level-2 routers, and 
form the IS-IS backbone together with the Level-2 rout ers. No area is defined as the backbone in this 
topology. The backbone comprises all contiguous Leve l-2 and Level-1-2 routers, which can reside in 
different areas. 
Figure 51  IS-IS topology 2 
 
 
 NOTE: 
The IS-IS backbone does not need to be a specific area. 
 
Both the Level-1 and Level-2 routers use...

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The routing information of a Level-1 area is sent to the Level-2 area through the Level-1-2 router; therefore, 
the Level-2 router knows the routing information of the entire IS-IS routing domain. But the Level-1-2 router 
does not share the information of other Level-1 areas and the Level-2 area with the Level-1 area by default.   
Because a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router, the 
best paths may not be selected. To resolve this prob lem,...

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 NOTE: 
On IS-IS broadcast networks, all routers are adjacent wi th each other. However, the DIS is responsible for
the synchronization of their LSDBs. 
 
IS-IS PDU format 
PDU header format 
IS-IS packets are encapsulated into link layer frames. The Protocol Data Unit (PDU) consists of two parts, 
the headers and the variable length fields. The headers comprise the PDU common header and the PDU 
specific header. All PDUs have the same PDU common header. The specific headers vary by PDU type....

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Type PDU Type Acronym 
18  Level-1 Link State PDU  L1 LSP 
20 Level-2 Link State PDU  L2 LSP 
24 Level-1 Complete Sequence Numbers PDU  L1 CSNP 
25 Level-2 Complete Sequence Numbers PDU  L2 CSNP 
26 Level-1 Partial Sequence Numbers PDU  L1 PSNP 
27 Level-2 Partial Sequence Numbers PDU  L2 PSNP 
 
Hello 
Hello packets are used by routers to establish and maintain neighbor relationships. A hello packet is also 
an IS-to-IS hello PDU (IIH). For broadcast networks, the Level-1 routers use the Level-1...

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•  LAN ID —Includes the system ID and a one-byte pseudonode ID.  
Figure 56  sho
 ws the hello packet format on the point-to-point networks. 
Figure 56  P2P IIH format 
 
 
Instead of the priority and LAN ID fields in the LAN IIH, the P2P IIH has a Local Circuit ID field. 
LSP packet format 
The Link State PDU (LSP) carries link state information. LSP involves two types: Level-1 LSP and Level-2 LSP. 
The Level-2 LSPs are sent by the Level-2 routers, and the Level-1 LSPs are sent by the Level-1...

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Figure 57 L1/L2 LSP format 
 
 
Major fields of the L1/L2 LSP are as follows: 
•  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 for L2 LSPs; it indicates whethe r the router supports partition repair. 
•   ATT...

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Figure 58 LSDB overload 
 
 
•  IS type —Type of the router generating the LSP. 
SNP format 
A sequence number PDU (SNP) acknowledges the latest  received LSPs. It is similar to an Acknowledge 
packet, but more efficient. 
SNP involves Complete SNP (CSNP) and Partial SNP (PSNP), which are further divided into Level-1 CSNP, 
Level-2 CSNP, Level-1 PSNP and Level-2 PSNP. 
CSNP covers the summar y of all LSPs in the LSDB to synchronize the LSDB bet ween neighboring routers. 
On broadcast networks, CSNP...

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Figure 60 L1/L2 PSNP format 
 
 
CLV 
The variable fields of PDU comprise multiple Code-Length-Value (CLV) triplets.  
Figure 61 CLV format 
 
 
Tabl e  5 shows that different PDUs contai n different CLVs. Code 1 to 10 of CLV are defined in ISO 10589 
(code 3 and 5 are not shown in the table), and others are defined in RFC 1 19 5 .  
Table 5  CLV name and the corresponding PDU type 
CLV Code  Name  PDU T
ype 
1  Area Addresses  IIH, LSP 
2 IS Neighbors (LSP)  LSP 
4 Partition Designated Level2 IS...

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CLV Code  Name  PDU Type 
132 IP Interface Address  IIH, LSP 
 
Supported IS-IS features 
Multiple instances and processes 
IS-IS supports multiple instances and processes. Multiple processes allow an IS-IS process to work in 
concert with a group of interfaces. A router can run multiple IS-IS processes, and each process 
corresponds to a unique group of interfaces. 
For routers supporting VPN, each IS-IS process is as sociated with a VPN instance. The VPN instance is 
also associated with...

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The LSP fragment extension feature allows an IS-IS router to generate more LSP fragments. Up to 50 
additional virtual systems can be configured on the router, and each virtual system is capable of 
generating 256 LSP fragments to enable the IS-IS router to generate up to 13056 LSP fragments. 
•   Terms 
{ Originating system —It is the router actually running IS -IS. After LSP fragment extension is 
enabled, additional virtual systems can be configured for the router. Originating system is the...