Table of contents
Principle and Configuration of IS-IS Protocol
DIS and the analogy between DIS and DR
Carrier of Link State Information
Exchange of Link State Information
Network Hierarchical Routing Domain
Differences between IS-IS and OSPF
IS-IS Routing Configuration Requirements
Implementation of IS-IS routing configuration (1)
Implementation of IS-IS routing configuration (3)
Principle and Configuration of IS-IS Protocol
IS-IS (Intermediate System-to-Intermediate System) intermediate system to intermediate system
ISIS Knowledge Points
ISIS basic configuration, ISIS 9 types of packets, 2 types of networks, 3 types of routers, enable and disable padding in hello packets, P2P link 3-way, interface authentication, modify router level, interface level, route overload, DIS election, the difference between DIS and DR, interface priority modification, interface cost value, interface cost type (narrow, wide), modify network type (p2p, broadcast), ISIS route aggregation, ISIS default route, route import, import Type of external route (internal, external), route filtering filter-policy, IS-IS route penetration
Introduction to IS-IS protocol:
B BYTE byte
b bit bit
IS-IS (Intermediate System-to-Intermediate System, intermediate system to intermediate system)
routing protocol was originally designed by ISO (the International Organization for Standardization, International Organization for Standardization) for CLNP (Connection Less Network Protocol, connectionless network protocol). A dynamic routing protocol.
Like the OSPF routing protocol, IS-IS is also a widely used IGP routing protocol. Many ISP networks, especially large ISP networks, have deployed the IS-IS routing protocol.
Many IGPs such as RIP OSPF are routing protocols developed for the network layer protocol IP, but IS-IS is a routing protocol developed for the network layer protocol CLNP (Connection-Less Network Protocol ) initially. Later, the extended IS-IS can support both CLNP and IP. Such IS-IS protocol is called Integrated IS-IS protocol. Currently, IS-IS is usually referred to as the integrated IS-IS protocol.
The IS-IS protocol was initially standardized by ISO, so there are many ISO special terms in the IS-IS protocol.
==========
When communicating between routers, IS-IS uses the Protocol Data Unit (PDU) defined by ISO.
The PDU types used in IS-IS mainly include:
Hello PDU (IIH PDU)
Link State PDU (LSP)
Complete Sequence Number Packet (CSNP)
Partial Sequence Number Packet (PSNP)
IIH PDU is similar to the hello message in the OSPF protocol. It is responsible for forming the neighbor relationship between routers, discovering new neighbors, and detecting whether any neighbors leave.
LSP is similar to LSA in OSPF protocol, and is used to describe all link state information in this router.
CSNP contains the summary information of each LSP in the network. When a router receives a CSNP, it will compare the CSNP with its link state database. If the router loses an LSP that exists in the CSNP, it will Send a multicast PSNP to ask other routers in the network for the LSPs they need.
PSNP is used in point-to-point links to confirm received LSPs; in point-to-point links and broadcast links, it is used to request the latest version or lost LSPs.
CSNP is similar to the DD message of OSPF, which conveys the summary of all link information in LSDB.
PSNP is similar to OSPF's LSR or LSAck message, which is used to request and confirm some link information.
The ISIS interaction process does not go through multiple stages like the OSPF protocol. It mainly uses CSNP and PSNP protocol messages to synchronize, request and confirm the link state information (carrying the summary of link state information), and the link state The detailed topology and routing information of the information is conveyed by LSP packets.
IS-IS protocol is very similar to OSPF protocol. They are all link-state-based routing protocols, both need to establish and maintain link-state databases, both use hello packets to establish and maintain neighbor/adjacency relationships, and both have regional and hierarchical structures.
On the other hand, there are many differences between IS-IS and OSPF. The OSPF area boundary is located on the router, while the IS-IS protocol area boundary is located on the link. The OSPF protocol supports point-to-point, point-to-multipoint, NBMA, and Broadcast. These 4 types of networks, while the IS-IS protocol only supports point-to-point There are two types of networks, point-to-point and broadcast.
The router running the IS-IS protocol must have a network address called NET (Network Entity Title), even in the IP environment. NET is also called network entity name, its length is 8 to 20 bytes, and its format can be varied. Usually, in the IP environment, the format of NET is: area ID (1 byte) + system ID (6 bytes) + SEL (1 byte), and SEL in NET is always 00.
The area ID in the IS-IS protocol refers to a hexadecimal number, such as area 20, which refers to 20 in hexadecimal, which is equivalent to 32 in decimal.
In the OSPF protocol, the message describing the link state and routing information is called LSA; in the IS-IS protocol, the message describing the link state and routing information is called LSP (Link State PDU, Link State Packet), LSP There are also two types of Level-1 LSP, Level-2 LSP
==========
Conditions for IS-IS to form a neighbor relationship
1. The same level
2. The same area
3. The same network segment
4. The same network type P2P broadcast
5. The same mtu value
6. The same authentication
7. In the P2P network, the length of the system-id must be the same, and the maximum number of area addresses must be the same
IS-IS Interface Cost Types at Both Ends Are Inconsistent, Neighborhoods Can Be Established, but Routes Cannot Be Learned
Generally, a route only needs to be configured with one area address, and all nodes in the same area must have the same area address. In order to support the smooth merging, division and conversion of regions, in the implementation of the device, a maximum of 3 region addresses can be configured under one ISIS process.
isis
is-level level-1 : level-1 and level-2 cannot establish neighbors
int g0/0/0
isis circuit-type p2p : The network types of the interfaces at both ends are different, and neighbors cannot be established
isis
cost-style wide : The cost types of the two devices are different, neighbors can be established, but packets cannot be sent and received, so there is no route
Routers of different levels cannot form neighbor relationships, that is, Level-2 routers cannot form neighbor relationships with Level-1 routers, but Level-1-2 routers can form Level-1 neighbor relationships with Level-1 routers in the same area. Form a Level-2 neighbor relationship with Level-2 routers in the same or different area.
A Level-1 router can only form a Level-1 neighbor relationship with a Level-1 router or a Level-1-2 router in the same area.
The addresses of the ISIS interfaces at both ends of the link must be on the same network segment.
isis
cost-style narrow , cost value range 1-63
cost-style widd , cost value range 1-16777215
Areas (Areas)
IS-IS allows the entire routing domain to be divided into multiple areas;
areas are connected through L2 (L1/L2) routers;
a router currently has up to 3 Area Ids belonging to 3 areas (IOS and VRP Realization)
A router must belong to a certain area, and different interfaces on the same router can belong to different areas like OSPF.
For Level-1 routers, neighbors can only be established if they belong to the same area. For Level-2 Routers do not have this same area restriction
Take two L2 routers establishing a neighbor relationship on a broadcast link as an example.
R1 multicasts a Level-2 LAN IIH (multicast MAC: 01-80-C2-00-00-15), and there is no neighbor identifier in this message.
After receiving this message, R2 marks the neighbor status of itself and R1 as Initial. Then, R2 multicasts a Level-2 LAN IIH reply to R1, in which R1 is identified as a neighbor of R2.
After receiving this message, R1 marks the state of the neighbor between itself and R2 as Up. Then R1 multicasts to R2 a Level-2 LAN IIH identifying R2 as a neighbor of R1.
After receiving this packet, R2 marks the state of its neighbor with R1 as Up. In this way, the two routers successfully establish a neighbor relationship.
Because it is a broadcast network, a DIS needs to be elected, so after the neighbor relationship is established, the router will wait for the interval between two Hello packets before electing a DIS. The Hello message contains the Priority field, and the one with the highest Priority value will be elected as the DIS of the broadcast network. If the priorities are the same, the interface with the higher MAC address is elected as the DIS. In IS-IS, DIS sends Hello intervals at 10/3 seconds, while other non-DIS routers send Hello intervals at 10 seconds.
IS-IS Hello packets can be subdivided into: L1 IIH, L2 IIH, and P-2-P IIH.
The multicast address of L1 IIH is: 0180-C200-0014;
the multicast address of L2 IIH is: 0180-C200-0015;
P-2-P IIH uses unicast address for communication.
The function of the Hello message is to discover, establish and maintain the neighbor relationship, and its function is similar to the Hello message in the OSPF protocol.
By default, the interval for sending Hello packets for ordinary routes is 10 seconds, and the
interval for sending Hello packets for DIS is 1/3 of that for ordinary routes, which is 3 seconds, which ensures that DIS failures can be quickly detected.
Differences between IS-IS and OSPF on the adjacency relationship
In IS-IS, as long as two neighboring routers exchange HELLO data packets, they consider that they have formed an adjacency relationship; while in OSPF, when two routers enter the 2-Way state, they consider that they have formed an adjacency relationship, but only when they enter the Full state can they be considered to have entered the full state. adjacency.
In IS-IS, a router with a priority of 0 can also participate in DIS election; while an OSPF summary priority of 0 means that it does not participate in the election.
In IS-IS, DIS is based on preemption; in OSPF, DR/BDR has been elected and cannot be preempted.
==========
The difference between DR in OSPF protocol and DIS in ISIS protocol:
DR election first looks at the priority, then compares the router-id, DIS first looks at the priority, and then compares the mac address.
DR defaults to 1, and the value range is 0-255. DIS defaults to 64, and the value range is 0-127.
DR The value of 0 means that the DR selection is abandoned. The value of DIS is 0, but the value is small, and the DIS election
DR is mainly to reduce LSA flooding. DIS is to periodically send CSNP and synchronize LSDB
DR. There is a backup device BDR, DIS The election of the DIS DR without backup
is done on the link. The election of the DIS is divided into Level-1 and Level-2. The election of the DR on the router does not enable the
preemption by default, and the DIS preempts
the OSPF election DR/BDR by default. Waiting time 40 seconds, the process is relatively complicated, and the ISIS election DIS can wait for two Hello message intervals, simple and fast
After the election is completed, all routers in the ISIS network link establish adjacency relationships. In OSPF, DRothers only form a full adjacency relationship with DR/BDR, and there is only a 2-way relationship between DRothers.
What is the difference between ISIS DIS and OSPF DR?
Answer:
DIS is preempt. No backup router. Pri >=0 (the bigger the priority and mac, the better); while OSPF DR has BDR, the higher the
priority, the better, or the higher the router id, the better. Good. It supports preemption, so DIS is predictable, but ospf's DR is unpredictable.
Electing DR requires a wait timer, and then generates BDR, DR, which is complicated, and ISIS, as long as the LAN hello is received, it can be compared with DIS, no backup, so it is simple and fast.
Full adjacency, DIS guarantee reliable flooding. (Synchronization method on MAlink Not the same)
DIS hello =3s ;
ISIS DIS uses LAN id to represent, namely DIS's system + circuit id identification
foreword
- Like OSPF, IS-IS is also an IGP protocol based on the link state and using the shortest path first algorithm for route calculation. IS-IS is originally a dynamic routing protocol designed by ISO for its connectionless network protocol CLNP.
- In order to provide routing support for IP, IETF has expanded and modified IS-IS in RFC1195 so that it can be applied in both TCP/IP and OSI environments. The revised IS-IS protocol is called integrated IS -IS. Due to its simplicity and strong scalability, IS-IS is currently widely deployed in the networks of large ISPs.
scene application
⦁ Campus network features:
⦁ Application-oriented network, mainly for enterprise network users.
⦁ The number of routers is relatively small, the capacity of the dynamic routing LSDB is relatively small, and the three-layer routing domain is relatively small.
⦁ Has the concept of egress routing, which is sensitive to the division of internal and external routes.
⦁ The regional span is not large, the bandwidth is sufficient, and the ratio of link state protocol overhead to bandwidth is relatively small.
⦁ Routing policies and policy routing applications are frequently and changeable, requiring fine-grained routing operations.
⦁ OSPF features multiple routing types (internal/external), multiple area types (backbone/common/special), excellent overhead rules (set according to bandwidth), and diverse network types (up to five types) in the campus network. big play.
⦁ Backbone network features:
⦁ Service-oriented network, established by ISP (Internet Service Provider) and providing interconnection services for end users.
⦁ Routing scheduling occupies an absolute dominant position, and the number of routers is huge.
⦁ The architecture level is flattened, requiring IGP as the basic route to serve the upper-layer BGP protocol.
⦁ LSDB has a large scale, is extremely sensitive to link convergence, and has high line costs.
⦁ Pursue simplicity, efficiency, high scalability, and meet various customer business needs (IPV6/IPX).
⦁ IS-IS's fast algorithm (PRC is strengthened), simple message structure (TLV), fast neighbor relationship establishment, large-capacity routing transfer (based on low Layer 2 overhead) and other features have natural advantages in the backbone network.
historical origin
⦁ IS-IS was originally a dynamic routing protocol designed by the International Organization for Standardization ISO (the International Organization for Standardization) for its connectionless network protocol CLNP (ConnectionLess Network Protocol).
⦁ In order to provide routing support for IP, IETF has expanded and modified IS-IS in RFC1195, enabling it to be applied in both TCP/IP and OSI environments, called Integrated IS-IS (Integrated IS-IS) , unless otherwise specified, the IS-IS mentioned refers to the integrated IS-IS.
⦁ IS-IS belongs to the interior gateway protocol and is used inside the autonomous system. IS-IS is a link state protocol that uses the shortest path first algorithm for route calculation.
route calculation process
⦁ Neighbor relationship establishment:
⦁ Neighbor relationship establishment is mainly through HELLO packet interaction and negotiation of various parameters, including circuit type (level-1/level-2), Hold time, network type, supported protocol, area code, system ID, PDU Length, interface IP, etc.
⦁ Link information exchange:
⦁ Unlike OSPF, the basic carrier of ISIS interactive link state is not LSA (link state advertisement), but LSP (link state PDU); the interaction process does not go through multiple stages like OSPF protocol, mainly It is synchronized through CSNP and PSNP two protocol messages, requesting and confirming the link state information (carrying the link state information summary), and the detailed topology and routing information of the link state information is transmitted by the LSP message.
⦁ Routing calculation:
⦁ SPF calculation is basically the same as OSPF, but the ISIS algorithm separates the topology and IP network segments, which speeds up network convergence.
address structure
⦁ NSAP address:
⦁ IDP is equivalent to the main network number in the IP address. It is stipulated by ISO and consists of two parts, AFI and IDI. AFI represents the address allocation authority and address format, and IDI is used to identify domains.
⦁ DSP is equivalent to the subnet number and host address in the IP address. It consists of three parts: High Order DSP, System ID and SEL. High Order DSP is used to divide areas, System ID is used to distinguish hosts, and SEL is used to indicate service types.
⦁ Area Address (Area ID) is composed of IDP and High Order DSP in DSP, which can identify both the routing domain and the area in the routing domain. Therefore, together they are called area addresses, equivalent to area numbers in OSPF.
⦁ System ID is used to uniquely identify a host or router within an area. In the implementation of the device, its length is fixed at 48bit (6 bytes).
⦁ The role of SEL is similar to the "protocol identifier" in IP, and different transmission protocols correspond to different SELs. SEL is 00 on IP.
⦁ NET:
⦁ The network entity name NET refers to the network layer information of the device itself, which can be regarded as a special type of NSAP (SEL=00). The length of NET is the same as that of NSAP, with a maximum of 20 bytes and a minimum of 8 bytes. When configuring IS-IS on a router, only NET needs to be considered, and NSAP does not need to be concerned.
⦁ In the process of configuring IS-IS, only 3 NETs can be configured at most. When configuring multiple NETs, you must ensure that their System IDs are the same.
Router Classification
⦁ Level-1 router:
⦁ Level-1 can only form a neighbor relationship with Level-1 and Level-1-2 routers belonging to the same area, and is only responsible for maintaining the link state database of Level-1. The LSDB contains the Routing information, packets destined for outside the area are forwarded to the nearest Level-1-2 router. Level-1 routers can only establish Level-1 adjacencies.
⦁ Level-2 routers:
⦁ Level-2 routers are responsible for inter-area routing, and they can form neighbor relationships with Level-2 routers in the same or different areas or Level-1-2 routers in different areas. A Level-2 router maintains a Level-2 LSDB, which contains inter-area routing information. Level-2 routers can only establish Level-2 adjacencies.
⦁ Level-1-2 router:
⦁ A router belonging to both Level-1 and Level-2 is called a Level-1-2 router. The Level-1-2 router maintains two LSDBs. The Level-1 LSDB is used for intra-area routing, and the Level-2 LSDB is used for inter-area routing.
⦁ A Level-1-2 router can form a Level-1 neighbor relationship with a Level-1 in the same area, and can also form a Level-2 neighbor relationship with Level-2 and Level-1-2 routers in other areas.
⦁ Between different areas, only Level-2 adjacency can be established:
⦁ Level-2 routers can establish adjacency with Level-2 routers.
⦁ Level-1-2 routers can establish adjacency with Level-2 routers.
⦁ Level-1-2 routers can establish adjacency with Level-1-2 routers.
Neighbor HELLO packet
⦁ HELLO PDU (Hello protocol data unit):
⦁ The function of the HELLO message is to discover neighbors, negotiate parameters and establish a neighbor relationship, and later serve as a keep-alive message.
⦁ Like OSPF, IS-IS establishes a neighbor relationship by exchanging hello packets. However, it will be divided into three types of hello packets according to the scenario.
⦁ Level-1 IS-IS in the broadcast network uses Level-1 LAN IIH (Level-1 LAN IS-IS Hello), and the destination multicast MAC is: 0180-c200-0014.
⦁ Level-2 IS-IS in the broadcast network uses Level-2 LAN IIH (Level-2 LAN IS-IS Hello), and the destination multicast MAC is: 0180-c200-0015.
⦁ P2P IIH (point to point IS-IS Hello) is used in non-broadcast networks. But it has no relevant field indicating DIS (dummy node).
⦁ The IIH message needs to use the padding field to negotiate the size of the sent message between the two neighbors.
⦁ Network types supported by IS-IS:
⦁ Point-to-point network type (P2P).
⦁ Broadcast Multiple Access network type (Broadcast Multiple Access).
⦁ In special environments such as frame relay, you can create sub-interfaces to support P2P network types.
Neighborhood Establishment
⦁ On the P2P link, there are two handshake mechanisms and three handshake mechanisms.
⦁ Two-way handshake As long as the router receives the Hello message from the peer, it will unilaterally announce that the neighbor is in the up state and establish a neighbor relationship, but there is a risk of one-way communication.
⦁ The neighbor relationship is finally established by sending the P2P IS-IS Hello PDU three times, which is the same as the establishment of the broadcast link neighbor relationship.
⦁ On the broadcast link, use the LAN IIH message to perform a three-way handshake to establish a neighbor relationship.
⦁ When receiving the Hello PDU message sent by the neighbor without its own system ID, the state machine enters initialized.
⦁ Only when the Hello PDU sent by the neighbor has its own system ID, it will be up, eliminating the risk of one-way link.
⦁ In the broadcast network, after the neighbor is up, the DIS (virtual node) will be elected. The function of DIS is similar to the DR (Designated Router) of OSPF.
DIS and the analogy between DIS and DR
⦁ DIS and Pseudo-Nodes:
⦁ DIS refers to Designated IS.
⦁ Pseudo-node refers to the virtual router created by DIS in the broadcast network.
⦁ DIS features:
⦁ In a broadcast network, DIS needs to be elected, so after the neighbor relationship is established, the router will wait for the interval between two Hello packets before electing DIS. The Hello message contains the Priority field, and the one with the highest Priority value will be elected as the DIS of the broadcast network. If the priorities are the same, the interface with the higher MAC address is elected as the DIS. In IS-IS, the default Hello interval for DIS is 10/3 seconds, while other non-DIS routers send Hello intervals for 10 seconds.
⦁ Analogy between DIS and DR:
⦁ Comparison of priorities during elections. DISs with a priority of 0 can also participate in elections. In OSPF, the priority is 0 and does not participate in DR election.
⦁ The election process takes a certain amount of time. OSPF election DR/BDR requires a waiting time of up to 40 seconds, and the process is relatively complicated. However, ISIS election DIS can only wait for the interval of two Hello packets, which is simple and fast.
Carrier of Link State Information
⦁ ISIS TLV:
⦁ The meaning of TLV is: type (TYPE), length (LENGTH), value (VALUE). It is actually a data structure that contains these three fields.
⦁ The advantage of using the TLV structure to construct messages is good flexibility and scalability. The use of TLV makes the overall structure of the message fixed, and adding new features only needs to add a new TLV. There is no need to change the overall structure of the entire message.
⦁ The network topology and routing information are represented by the TLV structure, which greatly maximizes the flexibility and scalability of the message.
⦁ LSP PDU (Link State Protocol PDU):
⦁ LSP is similar to OSPF LSA, carrying link state information, including topology and network number.
⦁ Level-1 LSP is transmitted by Level-1 router.
⦁ Level-2 LSP is transmitted by Level-2 routers.
⦁ Level-1-2 routers can transmit the above two LSPs.
⦁ The LSP message contains two important fields: ATT field and IS-Type field. The ATT field is used to identify that the route is sent by the L1/L2 router, and the IS-Type is used to indicate whether the IS-IS type that generates the LSP is Level-1 or Level-2 IS-IS.
⦁ The refresh interval of LSP is 15 minutes; the aging time is 20 minutes. However, in addition to waiting for 20 minutes for the aging of an LSP, there is also a 60-second zero aging delay; the LSP retransmission time is 5 seconds.
⦁ SNP PDU (Sequence Number PDU):
⦁ CSNP (Complete Sequence Number PDU) includes summary information of all LSPs in LSDB, so that LSDB can be kept in sync between adjacent routers.
Exchange of Link State Information
⦁ P2P network LSDB synchronization process:
⦁ After the neighbor relationship is established, RTA and RTB will first send CSNP to the peer device. If the LSDB of the opposite end is not synchronized with the CSNP, it sends a PSNP request to obtain the corresponding LSP.
⦁ Assume that RTB requests the corresponding LSP from RTA, and sends PSNP to RTA at this time. RTA starts the LSP retransmission timer while sending the LSP requested by RTB, and waits for RTB to send PSNP as an acknowledgment of receiving the LSP.
⦁ If after the interface LSP retransmission timer expires, RTA has not received the PSNP message sent by RTB as a response, then resend the LSP until it receives the PSNP message from RTB as a confirmation.
⦁ The newly added router in the MA network interacts with the LSDB of DIS synchronously:
⦁ Assume that the newly added router RTC has established a neighbor relationship with RTB (DIS) and RTA.
⦁ After the neighbor relationship is established, RTC sends its own LSP to the multicast address (Level-1: 01-80-C2-00-00-14; Level-2: 01-80-C2-00-00-15). In this way, all neighbors on the network will receive this LSP.
⦁ The DIS in this network segment will add the LSP that received the RTC to the LSDB, and wait for the CSNP message timer to expire (DIS sends a CSNP message every 10 seconds) and send the CSNP message to carry out the LSDB in the network Synchronize.
⦁ RTC receives the CSNP message from DIS, compares it with its own LSDB database, and then sends PSNP message to DIS to request the LSP it does not have (such as the LSP of RTA and RTB).
⦁ RTB as DIS receives the PSNP message request and sends the corresponding LSP to RTC for LSDB synchronization.
routing algorithm
⦁ IS-IS calculation characteristics:
⦁ When routers in this area start up for the first time, they execute the Full-SPF algorithm.
⦁ Subsequent received LSP update, if it is part of the topology change, perform iSPF calculation.
⦁ If only the routing information changes, the PRC calculation is performed.
⦁ Due to the algorithm of separating topology and network, the speed of routing convergence has been enhanced.
⦁ The cost method of ISIS route calculation:
⦁ Narrow mode (the default mode cost of the device is 10, and the value range of manual configuration interface cost is 1~63).
⦁ Wide mode (the default mode overhead of the device is 10, and the manual configuration interface overhead ranges from 1 to 16777215).
⦁ The auto-cost enable command is added to the process. Both Narrow mode and Wide mode will refer to the interface bandwidth to calculate the cost value, but the reference criteria are slightly different.
Network Hierarchical Routing Domain
⦁ IS-IS overall topology:
⦁ In order to support large-scale routing networks, IS-IS adopts a two-level hierarchical structure of backbone area and non-backbone area in the autonomous system. Generally, Level-1 routers are deployed in non-backbone areas, and Level-2 routers and Level-1-2 routers are deployed in backbone areas. Each non-backbone area is connected to the backbone area through Level-1-2 routers.
⦁ The topology is a network running the IS-IS protocol, which is very similar to the multi-area network topology of OSPF. The entire backbone area includes not only all Level-2 routers, but also Level-1-2 routers.
⦁ Level-1-2 routers can belong to different areas. In the Level-1 area, the Level-1 LSDB is maintained, and in the Level-2 area, the Level-2 LSDB is maintained.
⦁ Differences between IS-IS and OSPF reflected in the topology:
⦁ In OSPF, each link only belongs to one area; in IS-IS, each link can belong to different areas;
⦁ In IS-IS , a single area does not have the concept of physical backbone and non-backbone areas; in OSPF, Area0 is defined as the backbone area;
⦁ In IS-IS, Level-1 and Level-2 routers use the SPF algorithm to generate The shortest path tree SPT; in OSPF, the SPF algorithm is used only in the same area, and the routes between areas need to be forwarded through the backbone area.
Inter-area routing
⦁ Routing features of Level-1 routers:
⦁ Only have Level-1 link state database.
⦁ Only local router LSPs are included in its link state database.
⦁ There is no routing information of other areas in its routing table.
⦁ There is a default route in its routing table, and the next one points to the Level-1-2 router.
⦁ Routing features of Level-2 routers:
⦁ Level-2 routers only have Level-2 link state database.
⦁ There are LSPs of backbone area routers in its LSDB, but there are no LSPs generated by Level-1 routers.
⦁ The routing table contains the routing information of the entire network.
⦁ Routing features of Level-1-2 routers:
⦁ Level-1-2 routers have both Level-2 and Level-1 link state databases.
⦁ The Level-1 database contains LSPs in this area, and the Level-2 database contains LSPs in the backbone area.
⦁ Set the ATT bit to 1 in the Level-1 LSP generated by itself.
⦁ The routing table contains the routing information of the entire network.
Differences between IS-IS and OSPF
⦁ Network type and overhead mode:
⦁ IS-IS protocol only supports two network types, and the default overhead values of all bandwidths are the same, OSPF protocol supports four network types, and will set corresponding overhead values according to different bandwidths , has good support for network types such as frame relay and on-demand links.
⦁ Area type:
⦁ IS-IS protocol is divided into L1/L2 area, and L2 area is the backbone area with all detailed routes. There is only a default route from L1 to L2. The OSPF protocol is divided into backbone areas, common areas, and special areas. Cross-area access between common areas and special areas needs to pass through the backbone area.
⦁ Message type:
⦁ IS-IS protocol routing bearer message type is only LSP message, and the routing information in it does not distinguish between internal and external, simple and efficient, without recursive calculation. There are various types of LSAs for OSPF protocol routing bearer packets, including types 1/2/3/4/5/7. The routing level is strict and requires recursive calculations, which is suitable for fine-grained scheduling calculations.
⦁ Routing algorithm:
⦁ When the network segment of a certain node in the ISIS protocol area changes, the PRC algorithm is triggered, which converges faster and the message overhead for calculating the route is relatively small. In the OSPF protocol, the network address participates in the construction of the topology. When the address of the network segment changes in the area, the i-spf algorithm is triggered. The process is relatively cumbersome and complicated.
⦁ Scalability:
⦁ Any routing information of the ISIS protocol is transmitted using TLV, which has a simple structure and is easy to expand. For example, the support for IPv6 can only be solved by adding 2 TLVs. And ISIS itself supports protocols such as IPX. The OSPF protocol itself is developed specifically for IP, and the OSPF protocol supporting IPv4 and IPv6 is two independent versions (OSPFv2 and OSPFv3).
Glossary
IS-IS Routing Configuration Requirements
⦁ NET address number:
⦁ RTA: 49.0001.0000.0000.0001.00
⦁ RTB: 49.0001.0000.0000.0002.00
⦁ RTC: 49.0001.0000.0000.0003.00 ⦁
RTD: 49.0002.00
00.0000.0004.00 ⦁ RTE: 49.0002.0000.0000.0005.00
Implementation of IS-IS routing configuration (1)
⦁ Configuration ideas in the area:
⦁ Service configuration in area 49.0001:
⦁ Each router enters the IS-IS process 100 to configure the network entity name NET.
⦁ RTA configures the router level as level-1 under the ISIS process. RTB and RTC default to level-1-2 without modification.
⦁ RTA, RTB and RTC enable ISIS protocol under interface.
⦁ The link interface of RTA modifies the priority of its DIS to be the highest, making it a DIS.
Implementation of IS-IS routing configuration (2)
⦁ Configuration ideas in the area:
⦁ Service configuration in area 49.0002:
⦁ Each router enters process 100 to configure the network entity name NET.
⦁ RTD and RTE configure router level-2 under the ISIS process.
⦁ RTD and RTE enable ISIS protocol under interface.
⦁ RTD and RTE modify the network type to P2P on the interface.
Implementation of IS-IS routing configuration (3)
⦁ Inter-area configuration ideas:
⦁ Enter the ISIS process of level-1-2 router RTB and RTC and configure the network entity name NET.
⦁ Enter the link interface and enable the ISIS protocol.
⦁ Enter the router RTE to introduce a direct link.
⦁ Route penetration:
⦁ If there are more than two Level-1-2 routers in a level-1 area, the Level-1 router in the area will choose the nearest Level-1-2 router when accessing other areas, but the calculated cost value is only calculated In the local area, if the nearest Level-1-2 router reaches the destination network in the Level-2 area, the cost is relatively high, which will actually result in a suboptimal path for services. In this scenario, route penetration operations are required to import the detailed routes (including costs) of the Level-2 area to the Level-1 area, and the Level-1 routers will calculate and select the optimal path to access the inter-area network.
⦁ This example requires the optimal path to reach area 49.0002. Since the link bandwidth between RTB and RTD is relatively large, it is best to let the data flow through RTB. You can import level-2 routes to level-1 under the ISIS process of RTB and RTC respectively. By mastering all detailed routes of level-2 in RTA's LSDB, you can choose the optimal path to reach area 49.0002.
thinking questions
1. What are the types of IS-IS routers?
2. What role do PSNP packets play in neighbor interaction?
3. Compared with OSPF, what are the advantages of IS-IS?
⦁ Answer: IS-IS router types include Level-1 routers, Level-2 routers, and Level-1-2 routers.
⦁ Answer: PSNP messages are used for LSP request and confirmation.
⦁ Answer: IS-IS message structure is simple, the routing carrying capacity is stronger, the routing algorithm is better, and the scalability is stronger.