IS-IS basis of dynamic routing protocol for HCNP Routing & Switching


Previously, we learned about topics related to OSPF’s special areas. For review, please refer to; Today, let’s talk about another topic related to dynamic routing protocol IS-IS;

Introduction to IS-IS

IS-IS (intermediate system to intermediate system) dynamic routing protocol is the same as OSPF routing protocol. Both are internal gateway protocol (IGP) based on link state and using SPF algorithm to calculate routing; IS-IS was originally a dynamic routing protocol designed by International Standards Organization ISO for its connectionless network protocol CLNP (connection less network protocol); Due to the wide use of IP protocol, in order to provide routing support for IP protocol, IETF expanded and modified IS-IS in rfc1195, so that IS-IS protocol can be applied in TCP / IP and OSI environment at the same time. The revised IS-IS protocol is called integrated IS-IS (integrated IS-IS or dual IS-IS); Because of its simplicity and strong expansibility, IS-IS is is widely used in large ISP networks;

Tip: IS-IS works in the data link layer. In the OSI seven layer model, its upper layer protocol is the network layer CLNP. In the early stage, IS-IS only supported in the OSI seven layer model environment; Later, because the TCP / IP five-layer model was widely used, in order to provide support for the TCP / IP five-layer network model, the later IS-IS protocol was revised and improved to expand the support for the TCP / IP five-layer network model environment. Therefore, the IS-IS protocols we use now are integrated IS-IS protocols; It can not only support the CLNP network of OSI seven layer model, but also support the five layer IP network of TCP / IP; Compared with OSPF, OSPF supports IP network and does not support CLNP network; OSPF works in the IP layer, while IS-IS protocol works in the data link layer;

IS-IS scenario application

Tip: the characteristics of the park network are mainly application-oriented networks, mainly for enterprise network users; The number of routers is relatively small, the LSDB library capacity of dynamic routing is relatively small, and the three-tier routing domain is relatively small; It has the concept of exit routing and is sensitive to the division of internal and external routes; The regional span is small, the bandwidth is sufficient, and the ratio of link state protocol overhead to broadband occupation is relatively small; Routing strategies and policy routing applications are frequent and changeable, which requires careful and detailed routing operations; OSPF’s characteristics of multiple routing types (internal / external), multi area types (backbone / non backbone / special), excellent overhead rules (set according to bandwidth) and various network types (P2P / MA / p2mp / nBMA) have been brought into full play in the campus network. Therefore, OSPF is widely used in the campus network at present; For the backbone network, its main feature is the service network, which is established by ISP (Internet service provider) and provides interconnection services for end users; Routing scheduling is absolutely dominant, and the number of routers is huge; The architecture level is flat, requiring IGP as the basic route to serve the upper BGP protocol; LSDB has a large scale, is extremely sensitive to link convergence, and the line cost is expensive; Pursue simplicity, efficiency and high expansibility to meet the business needs of public customers; Some characteristics of IS-IS, such as fast algorithm (RPC) strengthened, simple message mechanism (TLV), fast neighbor relationship establishment, high-capacity routing (based on low layer 2 overhead) and so on, have natural advantages in the backbone network, so IS-IS has been widely used in the backbone network;

IS-IS topology

Tip: for IS-IS, unlike OSPF, which divides areas by interface, a router can belong to multiple areas (ABR) at the same time; In IS-IS, the boundary of the area is divided according to the type of router, which also means that a router in IS-IS can only belong to one area, not multiple areas at the same time; In order to adapt to medium and large-scale networks, IS-IS uses hierarchical structure in its topology; The hierarchical structure of backbone area and non backbone area is adopted; Generally speaking, 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 router;

Comparison between IS-IS and OSPF areas

Tip: for is is, the area division is based on the type of router, while the area division of OSPF is based on the interface; The is is backbone area is composed of L2 and L1 / 2 routers, while the OSPF backbone area is area 0; For the routing algorithm, the is is calculation route uses the SPF algorithm, while OSPF uses the SPF algorithm in the same area. The inter area route is not the SPF algorithm, but is transmitted according to three types of LSA (the packaged routing information is transmitted directly, and the SPF algorithm does not need to be used for calculation again);

IS-IS router classification

Tip: there are three types of is is routers: level-1 router (L1 for short); This type of router can only create LSDB of L1 type router; L2 type router can only create LSDB of L2 type router; L1-2 type router is the default type. It can create LSDB of L1 type router and L2 type router at the same time;

IS-IS neighbor relationship

Tip: for L1 type routers, it can only establish neighbors with L1 or L1-2 type routers in the same area; The same area refers to the same area ID; For L2 type router, it has no regional restrictions. It can establish neighbors with L2 or L1-2 in the same region or different regions; For L1-2 router, it can establish neighbors with L1 or L2 or L1 / 2 routes in the same area or different areas; It should be noted here that L1 type routers cannot establish adjacency relationship with L2, regardless of whether they are in the same area or not; For L1 type routes in different regions, the adjacency relationship cannot be established;

Conclusion: the condition for L1 type routers to establish neighbors is that the area ID must be the same. Secondly, they cannot establish neighbors with L2, regardless of whether the area ID is the same or not; For L2, there is no restriction on whether the area ID is the same or not, but it can only establish neighbors with L2 or L1-2 routers, and cannot establish neighbors with L1, regardless of whether the area is the same or not; For L1-2, there is no restriction on whether the area ID is the same or not, and there is no restriction on the type of router. It can establish neighbors in different areas or the same area with L1, L2 and L1-2 routers;

IS-IS network type

Tip: there are only two network types of IS-IS: broadcast network (LAN) and point-to-point network (P2P); On PPP and HDLC, the default network type of the interface is point-to-point (P2P); On Ethernet and token ring, the default network type of interface is broadcast network (LAN);

Adjacency in point-to-point networks

Tip: for L1 adjacency relationship, it can only be established in the same area; For different regions, L1 neighbors cannot be established; In the same region, L1 and L1 or L1-2 establish the adjacency relationship of L1; L1-2 and L1-2 establish the adjacency relationship between L1 and L2; For different regions, L1-2 and L1-2 can only establish the adjacency relationship of L2, not L1; L2 and L2 establish the adjacency relationship of L2;

Adjacency on broadcast networks

Tip: for broadcast network IS-IS and OSPF, Dr should be selected; It is not called Dr but DIS in IS-IS; The function is to create and more pseudo nodes to simplify topology and reduce resource consumption; Adjacent relationships will be formed between routers at the same level; In the figure above, L1, L1-2 and L1 dis establish the adjacency relationship of L1; L1-2 establishes L2 adjacency relationship with L2 and L2 dis;

Comparison of DIS and Dr

Tip: pseudo node is a virtual node automatically generated in the broadcast network. It is connected with other routes through virtual links to form an adjacency relationship; The main function is to send cSNP and synchronize LSDB database; CSNP can be understood as DD package in OSPF, which is used to compare database summaries; In OSPF, the main function of DR is to reduce LSA flooding; Secondly, for the adjacency relationship, DIS in IS-IS establishes the adjacency relationship with all routers, while drother in OSPF establishes the 2WAY relationship; Dis has no backup in IS-IS, and BDR in OSPF; At the same time, the DR in OSPF is not preemptive. Once selected, only restart the process can trigger Dr re-election; DIS in IS-IS is preemptive; In terms of hierarchy, OSPF has no hierarchy. There are L1 DIS and L2 DIS in IS-IS. Different types of adjacency relationships are established between different types of routes to generate different types of dis;

IS-IS address structure

Prompt: NSAP (network service access point); It is the address of network layer in OSI and provides the interface between network layer and upper application, which is equivalent to IP address and protocol number; It is mainly composed of dip and DSP; IDP (initial domain part) is equivalent to the primary network number in the IP address; DSP (domain specific part) is equivalent to the subnet number and host address in the IP address; AFI (authority and format identifier) identifies the address distribution organization and address format; IDI (initial domain identifier) identifies the domain; Split area by hodsp (high order DSP); AFI + IDI + hodsp constitute the area address; System ID distinguishes hosts, which is equivalent to router ID in OSPF; SEL is used to identify the service type; SEL is 00, indicating that it is a net service type. In IP networks, the value of sel is 00; That is, the NSAP service type in the IP network is net;

Comparison of address structure between TCP / IP protocol stack and OSI system

Tip: the CLNP protocol of OSI system is similar to the IP protocol in TCP / IP protocol stack; The NSAP address is similar to the IP address in the IP protocol; For IS-IS protocol in OSI system, its net identifier is similar to OSPF area ID and router ID in IP protocol; Net is a kind of special NSAP address (SEL = 00). When configuring IS-IS on the router, you only need to consider net;

Net address structure

Tip: the net address consists of three parts. The first part is the area ID, which is composed of hexadecimal characters, with a minimum of 1 byte and a maximum of 13 bytes; The area address is mainly used to identify the routing domain and represent the area in the routing domain, which is equivalent to the area number in OSPF; Two different routing domains are not allowed to have the same area address, that is, multiple regions are not allowed to have the same area number; Generally, all nodes in the same area need to be configured with the same address; In order to support the smooth merging, division and conversion of regions, a router can be configured with up to 3 region addresses; The second part is the system ID, which occupies 6 bytes and is composed of 12 hexadecimal characters. Every 4 bits is divided into three groups. The system ID is mainly used to uniquely identify the host or router in the area, similar to the router ID in OSPF; Finally, SEL, accounting for 1 byte, is composed of 2-bit hexadecimal characters; In the IP network environment, the value is fixed 00; Net is a special NSAP address, that is, the NSAP address with sel = 00, with a maximum of 20 bytes and a minimum of 8 bytes. A router can be configured with up to three net addresses. When configuring multiple net addresses, it is necessary to ensure that their system IDS are the same;

Net generation method

Tip: in order to facilitate management, the corresponding system ID is generally generated according to the router ID; This is only a way to generate net address, not mandatory. We can also manually customize the net address according to the network environment planning;

Net address planning considerations

Tip: the area ID of the same area must be the same; System ID must be unique;

Experiment: configure IS-IS with the following topology to realize interworking between LO1 interface of R1 and lo interface of R2

R1 configuration interface IP address

IS-IS basis of dynamic routing protocol for HCNP Routing & SwitchingIS-IS basis of dynamic routing protocol for HCNP Routing & Switching

sys R1
int g0/0/0
ip add 24
int lo 1
ip add 32

View Code

R2 configure interface IP address

IS-IS basis of dynamic routing protocol for HCNP Routing & SwitchingIS-IS basis of dynamic routing protocol for HCNP Routing & Switching

sys R2
int g0/0/0
ip add 24
int lo 1
ip add 32

View Code

The type of Isis router configured on R1 is L1

Tip: the number 77 after the Isis command indicates the process number;

The type of Isis router configured on R2 is L1

Tip: the default router type is L1-2;

Configure net address on R1

Configure net address on R2

Enable Isis process on G0 / 0 / 0 and LO1 interfaces of R1

Enable Isis process on G0 / 0 / 0 and LO1 interfaces of R2

Tip: the operation of enabling Isis on the interface can be understood as announcing the corresponding network in OSPF;

Capture the package and view the Isis Hello package structure

Tip: from the above packet capturing situation, we can see that Isis is encapsulated on the data link layer. Its layer 2 protocol is not Ethernet II protocol, but 802.3 Ethernet, which is the same as STP layer 2 protocol; For Isis packets, like OSPF, it is also divided into general header and special header; General packet header is mainly used to describe packet type, Isis protocol, packet length and other information; The special packet header is used to describe the relevant information of the corresponding type of packet;

Verify: View Isis neighbors on R1

Tip: you can see that Isis and system ID 0000.0000.0002 on R1 establish neighbors of type L1;

Verification: check the routing table of R1 to see if you have learned the LO1 interface routing of R2?

Tip: you can see the route from R1 to LO1 interface of R2 through Isis;

Verification: Ping the lo interface address of R2 with the LO1 interface of R1 to see if it can communicate normally?

Tip: it can be seen that the LO1 interface of R1 can communicate with the LO1 interface of R2 normally;