HP 5500 Ei 5500 Si Switch Series Configuration Guide
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132 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 the SPF algorithm to generate the shortest path tree (SPT). Route leaking An IS-IS routing domain is comprised of only one Level-2 area and multiple Level-1 areas. A Level-1 area consists of a group of Level-1 routers, and is connected with a Level-2 area rather than other Level-1 areas.
133 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, route leaking was introduced. A Level-2 router can advertise Level-2 routing information to a specified Level-1 area. By having the routing information of other areas, a Level-1 router in the area can make a better routing decision for a packet to another area. IS-IS network type Network type IS-IS supports the following network types: • Broadcast network, such as Ethernet and Token-Ring • Point-to-point network, such as PPP and HDLC DIS and pseudonodes On an IS-IS broadcast network, a router is electe d as the Designated Intermediate System (DIS). The Level-1 and Level-2 DISs are elected. You can assign different priorities to different level DIS elections. The higher a routers priority is, the more likely the router becomes the DIS. If multiple routers with the same highest DIS priority exist, the one with the highest SNPA (Subnetwork Point of Attachment) address (MAC address on a broadcast network) will be elected. A router can be the DIS for different levels. IS-IS DIS election differs from OSPF DIS election in the following ways: • A router with priority 0 can also participate in the DIS election. • When a router is added to the network and beco mes the new DIS, an LDP flooding process is triggered. As shown in Figure 52, the same le vel routers on a network, including non-DIS routers, establish adjacencies with each other. Figure 52 DIS in the IS-IS broadcast network The DIS creates and updates pseudonodes, as well as ge nerates their LSPs, to describe all routers on the network. A pseudonode represents a virtual node on the broadcast network. It is not a real router. In IS-IS, it is identified by the system ID of the DIS and a one-byte Circuit ID (a non zero value). Using pseudonodes can reduce the resources consumed by SPF and simplify network topology.
134 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. Figure 53 PDU format Common header format Figure 54 PDU common header format Major fields of the PDU common header are as follows: • Intradomain routing protocol discriminator —Set to 0x83. • Length indicator —Length of the PDU header in bytes, including both common and specific headers. • Version/Protocol ID extension —Set to 1(0x01). • ID length —Length of the NSAP address and NET ID. • R (Reserved)—Set to 0. • PDU type —See Tabl e 4 . • Ve rsion —Set to 1(0x01). • Maximum area address —Maximum number of area addresses supported. Table 4 PDU type T ype PDU Type Acronym 15 Level-1 LAN IS-IS hello PDU L1 LAN IIH 16 Level-2 LAN IS-IS hello PDU L2 LAN IIH 17 Point-to-Point IS-IS hello PDU P2P IIH Intradomain routing protocol discriminator Reserved Version R ID length Version/Protocol ID extension Length indicator Maximum area address RRPDU typeNo. of Octets 1 1 1 1 1 1 1 1
135 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 LAN IIHs; and the Level-2 routers use the Level-2 LAN IIHs. The P2P IIHs are used on point-to-point networks. Figure 55 illu strates the hello packet format in broadcast networks, where the blue fields are the common header. Figure 55 L1/L2 LAN IIH format Major fields of the L1/L2 LAN IIH are as follows: • Reserved/Circuit type — T h e fi r s t s ix b i t s a re re s e r ve d wi t h a v a l u e o f 0 . T h e l a s t t w o b i t s i n d i c a t e t h e router type. Here, 00 means reserved, 01 indicates L1, 10 indicates L2, and 1 1 indicates L1/2. • Source ID —System ID of the router advertising the hello packet. • Holding time —If no hello packets are received from the neighbor within the holding time, the neighbor is considered down. • PDU length —Total length of the PDU in bytes. • Priority —DIS priority.
136 • 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 routers. The Level-1-2 router can send both types of LSPs. The two types of LSPs have the same format.
137 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 (Attachment)—Generated by a L1/L1 router for L1 LSPs only; it indicates that the router generating the LSP is connected to multiple areas. • OL (LSDB Overload) —Indicates that the LSDB is not complete because the router has run out of memory. 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 58, R outer A forwards packets to Router C through Router B. Once other routers know the OL field of LSPs from 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.
138 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 is sent by the DIS period ically (10s by default). On point-to-point networks, CSNP is only sent during the adjacency establishment. The CSNP packet format is shown in Figure 59. Figure 59 L1/L 2 CSNP format PSNP only contains the sequence numbers of one or multiple latest received LSPs. It can acknowledge multiple LSPs at one time. When LSDBs are not synchronized, a PSNP is used to request new LSPs from neighbors.
139 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 L2 LSP 6 IS Neighbors (MAC Address) LAN IIH 7 IS Neighbors (SNPA Address) LAN IIH 8 Padding IIH 9 LSP Entries SNP 10 Authentication Information IIH, LSP, SNP 128 IP Internal Reachability Information LSP 129 Protocols Supported IIH, LSP 130 IP External Reachability Information L2 LSP 131 Inter-Domain Routing Protocol Information L2 LSP Intradomain routing protocol discriminator Reserved Version R ID length Version/Protocol ID extension Length indicator Maximum area address RRPDU typeNo. of Octets 1 1 1 1 1 1 1 1 PDU length Source ID Variable length fields 2 ID length+1
140 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 interfaces of the process. IS-IS Graceful Restart Graceful Restart (GR) ensures the continuity of packet forwarding when a routing protocol restarts or an active/standby switchover occurs: • GR Restarter —Graceful restarting router. It must be GR capable. • GR Helper —A neighbor of the GR Restarter. It helps the GR Restarter to complete the GR process. After an IS-IS GR Restarter restarts, it must complete the following tasks to synchronize the LSDB with its neighbors: • Obtain IS-IS neighbor information without changing adjacencies. • Obtain the LSDB. The GR Restarter sends an OSPF GR signal to GR He lpers so that the GR Helpers keep their adjacencies with the GR Restarter, and restores the neighbor ta ble after receiving responses from neighbors. The GR Restarter then synchronizes the LSDB with all GR capable neighbors, calculates routes, updates its routing table and forwarding table, and removes st ale routes. The IS-IS routing convergence is then complete. IS-IS NSR Nonstop routing (NSR) is a new feature that overcomes the application limit of GR. It backs up IS-IS link state information from the master device to the slave device. When a master/slave switchover occurs, NSR can complete link state recovery and route re-g eneration without requiring the cooperation of other devices. Management tag Management tag simplifies routing information mana gement by carrying the management information of the IP address prefixes (to control route redistribution from other routing protocols) and BGP community and extended community attributes. LSP fragment extension IS-IS advertises link state informat ion by flooding LSPs. Because one LSP carries a limited amount of link state information, IS-IS frag ments LSPs. Each LSP fragment is uniquely identified by a combination of the System ID, Pseudonode ID (0 for a common LSP or a non-zero value for a Pseudonode LSP), and LSP Number (LSP fragment number) of the node or pseu do node that generated the LSP. The one-byte LSP Number field, allowing a maximum of only 256 fragments to be generated by an IS-IS router, limits the amount of link information the IS-IS router can advertise.
141
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
actual IS-IS process that originally runs.
{ System ID —System ID of the originating system
{ Additional system ID —Additional virtual system IDs are configured for the IS-IS router after LSP
fragment extension is enabled. Each additional system ID can generate 256 LSP fragments.
Both the additional system ID and the system ID must be unique in the entire routing domain.
{ Virtual system —A virtual system is identified by an additional system ID and generates
extended LSP fragments.
{ Original LSP—The LSP generated by the originating system. The system ID in its LSP ID field is
the system ID of the originating system.
{ Extended LSP —Extended LSPs are generated by virtual systems. The system ID in its LSP ID field
is the virtual system ID.
After additional system IDs are configured, an IS-IS router can advertise more link state
information in extended LSP fragments. Each virt ual system can be considered a virtual router.
An extended LSP fragment is advertised by a virt ual system identified by an additional system
ID.
• Operation modes:
The LSP fragment extension feature operates in the following modes:
{ Mode-1 —Applicable to a network where some routers do not support LSP fragment extension.
In this mode, adjacencies are formed between the originating system and virtual systems, with
the l i nk c ost from the orig i nati ng system to e ach vi r tu al system as 0 . Each vi r tu al system acts as
a router connected to the originating system in the network, but the virtual systems are
reachable through the originating system only. The IS-IS routers not supporting LSP fragment
extension can operate properly without modify ing the extended LSP fragments received, but
some limitation is imposed on the link state information in the extended LSP fragments
advertised by the virtual systems.
{ Mode-2 —Applicable to a network where all the routers support LSP fragment extension. In this
mode, all the IS-IS routers know which virtual system belongs to which originating system; no
limitation is imposed on the link state information of the extended LSP fragments advertised by
the virtual systems.
The operation mode of LSP fragme nt extension is configured based on area and routing level.
Mode-1 allows the routers supporting and not suppo rting LSP fragment extension to interoperate
with each other, but it restricts the link state in formation in the extended fragments. Mode-2 does
not restrict the link state information in the exte nded fragments, and is recommended for an area
where all the routers are at the same routing level and support LSP fragment extension.
Dynamic host name mapping mechanism
The dynamic host name mapping mechanism provides the mappings between the host names and the
system IDs for the IS-IS routers. The dynamic host name information is announced in the dynamic host
name CLV of an LSP.
This mechanism also provides the mapping between a host name and the DIS of a broadcast network,
which is announced in the dynamic host name TLV of a pseudonode LSP.