2.2.0 ISIS 4 message types IIH, LSP, CSNP, PSNP, neighbor establishment process, interactive LSP process
ISIS's 3 levels, 4 major categories and 9 subcategories
ISIS has 3 levels of routers, namely level-1、level-2、level-1-2.
Messages interacting between different levels are also different. Regular ISIS messages are divided into four categories : IIH、LSP、CSNP、PSNP.
Messages based on the interaction between different levels can be subdivided into 9 subcategoriesPDU Type . The ISIS device identifies the message type through the general header :
| type | Abbreviation |
|---|---|
| 15 | level-1 IIH |
| 16 | level-2 IIH |
| 17 | P2P IIH |
| 18 | level-1 LSP |
| 20 | level-2 LSP |
| 24 | level-1 CSNP |
| 25 | level-2 CSNP |
| 26 | level-1 PSNP |
| 27 | level-2 PSNP |
IIH message introduction
Name : IIH (IS-IS Hello) is similar to the Hello message
Function : used to establish and maintain adjacency
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Regarding the adjacency relationship of ISIS: In OSPF, 2way is a neighbor and Full is an adjacency, but there is no such thing as establishing a neighbor relationship in ISIS, and it usually means establishing an adjacency.
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There are three subcategories about IIH:
L1 IIH、L2 IIH、P2P IIH, because ISIS needs to elect a DR device ( called DIS in ISIS ) when it works in the broadcast type, and another IIH type message is used for interaction in the P2P type network. -
Modify the ISIS link type to P2P:
isis circuit-type p2p, delete this command to restore the default broadcast type.
message format
Introduction to LSP packets
Name : LSP (Link State PDU) link state protocol unit
Function : Used to exchange link state information, add corresponding TLV content to LSP based on LSDB for transmission, similar to OSPF LSU.
message format
- Variable Length Fields is the TLV field, which contains TLV information.
CSNP message introduction
Name : CSNP (Complete Sequence Number PDU, complete timing message)
SNP time-series message, which synchronizes the information of each LSDB by describing the LSP link status of the database.
Function : It is used to send its own route summary information, which is similar to the DD message of OSPF.
message format
- On broadcast networks, the CSNP is sent periodically by the DIS ( the missing sending period is 10 seconds ).
- On point-to-point networks, CSNP is sent only when an adjacency is first established. Because there are too many routers in the broadcast and the LSDB is uneven, the DIS is responsible for periodically sending CSNP, so that devices lacking routes can send PSNP applications by themselves. In the P2P network, only one peer can update and send the LSP information, and the peer can directly reply and confirm through PSNP, without applying through CSNP.
Introduction to PSNP packets
Function : used to request and confirm routing information, similar to OSPF LSR and LSACK.
- When receiving CSNP and comparing with its own LSDB database and finding that some information is missing, request the missing LSP through PSNP.
- Under the P2P network type , when the received LSP is confirmed to be correct, use PSNP to confirm the received LSP【
经过实验，几乎不能在广播类型下看到PSNP报文】.
message format
adjacency establishment process
Sometimes it is called the neighbor establishment process, but it does not have much impact.
Adjacency Establishment Principles Regarding ISIS
1. Only adjacent routers at the same level can become an adjacency, because L1 can only be established with L1 and L1/2 in the same area.
2. The network types of the ISIS interfaces at both ends of the link must be consistent . It is known that the packets of different interface network types will be different, which will cause the peer end to fail to recognize and establish adjacency.
3. The addresses of the ISIS interfaces at both ends of the link must be on the same network segment (by default), because the interface addresses in the P2P network can communicate in different segments ( the NCP auto-negotiation address learned earlier ).
(By default) The addresses of the ISIS interfaces at both ends of the link must be in the same network segment
(P2P link) Adjacency can be established even if the addresses of the ISIS interfaces at both ends of the link are in different segments
Since ISIS is a protocol working on the data link layer and designed for CLNP, the formation of ISIS adjacency has nothing to do with IP addresses.
But why does it say that ISIS adjacency cannot be established by default?
》Because the link is a broadcast Ethernet network by default, when the device knows the IP address of the other party through the Type (132) of the TLV in the ISIS message, if the address is different from its own, it will not be able to communicate normally. The PPP link is different, it has the NCP negotiation function, which can realize the communication of different network segments.
1. Adjacency establishment
The adjacency relationship is established by exchanging IIH messages, and the IIH is sent in multicast mode, and the IIH multicast addresses sent by different levels of routers are different.
By default, the sending interval of IIH is 10 seconds.
Among broadcast network types :
IIH multicast address at L1 level:01:80:C2:00:00:14
IIH multicast address at L2 level:01:80:C2:00:00:15
Among the P2P network types :
The IIH multicast addresses of L1 and L2 are:09:00:2B:00:00:05
Broadcast network three-way handshake to establish adjacency
About the state machine in the process of adjacency establishment :
1. Down, it is in the Down state when no neighbor IIH packet is received.
2. Initiated, received the neighbor IIH packet, but did not find its own information in the Hello packet, also known as the one-way state.
3. Up, receive neighbor IIH, and find your own information.
About DIS in ISIS Broadcast Type
IDS (Designated Intermediate System) specifies an intermediate system for creating and updating pseudo nodes, generating LSPs of pseudo nodes, and describing which network devices are in this network .
1. DIS election : Level-1 and level-2 DISs are elected separately without affecting each other, similar to MSTP multi-instance spanning tree root bridge election.
2. DIS election rules : the DIS with the higher priority takes priority, and if the priority is the same, the one with the higher MAC becomes the DIS.
Command to modify the DIS priority of an interface:
isis dis-priority 120, the default priority is 64.
3. DIS sending IIH interval : It is one-third of the conventional IIH sending interval, that is, 2~3 seconds, which can ensure that when DIS fails, it can be found by other devices faster.
Differences between DIS and DR in OSPF
1. In the ISIS broadcast network, routers with a priority of 0 also participate in the DIS election. However, routers with a priority of 0 in OSPF will not participate in DR election.
2. In the ISIS broadcast network, DIS has the ability to preempt, and the newly added optimal router can immediately become a new DIS, and the original pseudo-node will be deleted, causing LSP flooding at the same time. The newly added routers in OSPF do not have the preemption capability by default.
3. In the ISIS broadcast network, routers of the same level on the same network segment can form an adjacency relationship, that is, non-DIS routers can also establish adjacency with non-DIS routers. In OSPF, DROther can only establish adjacency with DR and BDR.
Point-to-point network two-way handshake to establish adjacency
华为设备默认点到点网络使用三次握手建立的方式, There are obvious defects in the two-way handshake mechanism. When Huawei equipment uses IS-IS in a point-to-point network, it uses a three-way handshake to establish an adjacency relationship by default.
Defect: There may be a single point of failure, because as long as the peer IIH message is received, it will unilaterally change to the Up state, and then exchange messages. At this time, if the other end fails, the Up state will continue to send packets until the IIH times out.
1. In a point-to-point network, two handshakes are used to establish the adjacency relationship : as long as the router receives the IIH message from the peer end, it unilaterally declares the adjacency as Up and establishes the adjacency relationship.
2. In the point-to-point network, the method of using the three-way handshake is the same as that of the broadcast type.
3. In interface mode, modify the number of handshakes by command:
interface g0/0/0
# 修改为两次握手
isis ppp-negotiation 2-way
# 修改为三次握手（默认为三次握手）
isis ppp-negotiation 3-way
# 默认三次握手可兼容对端为两次握手，如果设置only则不兼容
isis ppp-negotiation 3-way only
2. Routing information interaction
Broadcast network for LSP synchronization
As shown in the figure below, the newly added device R3 sends its own LSP message by default, and then waits for the CSNP message periodically sent by DIS, and uses PSNP to request the LSP information it lacks through CSNP.
After the DIS receives the CSNP request, it replies with an LSP to R3. Since it is a broadcast network, R3 does not need to reply to the PSNP to confirm the received LSP.
On a broadcast network, PSNP only acts as a request without confirmation.
(Because the DIS in the broadcast network sends a CSNP message every 10 seconds)
Point-to-point network for LSP synchronization
After the two parties establish the adjacency relationship, one end will give priority to sending CSNP summary information.
As shown in the figure below, R1 sends CSNP summary information to R2, and after receiving CSNP, R2 sends PSNP to request the missing LSP information.
After receiving the PSNP request, R1 replies to the LSP. After receiving the LSP and checking it is correct, R2 replies to PSNP for confirmation.
On the PPP network, since there is no DIS to periodically send CSNP information, it is necessary to use PSNP for confirmation.
Judgment on PSNP Request vs. PSNP Confirmation
Judging by the serial number of PSNP
1. The seq sequence number in PSNP is 0, which means PSNP request
2. If the seq sequence number in PSNP is not 0, it means PSNP confirmation (confirm the LSP of the sequence number)
How to capture packets to see the phenomenon?
1. If you need to capture the PSNP request packets, you need to capture packets in the broadcast ISIS network.
2. If you need to capture the packets confirmed by PSNP, you need to capture packets in the point-to-point ISIS network.
