Table of contents
The distinction between the backbone area and non-backbone area of ISIS:
Type of ISIS router: (Level-1/2 by default)
ISIS supports two network types: broadcast network and P2P
Neighbor relationship establishment under ISIS broadcast network: three-way handshake
DIS: Designated Intermediate System
The difference between OSPF "DR" and ISIS "DIS":
Functions and sending time of LSP packets, CSNP packets, and PSMP packets in ISIS:
The synchronization process of LSDB in the broadcast network:
The synchronization process of synchronizing LSDB in a point-to-point environment:
Triggered updates and periodic updates for OSPF and ISIS:
ISIS concept:
IS-IS: Intermediate system to intermediate system , originally running in the OSI environment, belongs to the network layer protocol of CLNP. In order to adapt to the current TCP/IP, ISIS has made some improvements. The improved ISIS can run on the CLNP network. Can run under TCP/IP, called integrated ISIS.
ISIS belongs to the network layer protocol , but runs on the data link layer, and is an IGP interior gateway protocol, a link state routing protocol.
NET: Network Entity Name
ISIS must set at least one NET address to run in the IP environment, and the NET address is a special NSAP address (SEL is fixed to 00)
Structure: Area ID + System ID + SEL
*The area ID indicates the area to which the device belongs, and the system ID identifies the device.
The distinction between the backbone area and non-backbone area of ISIS:
The backbone area in ISIS consists of Level-2 devices and Level-2 adjacencies established by Level-1/2 devices, regardless of the area ID. The remaining Level-1 adjacencies are non-backbone areas.
Type of ISIS router: (Level-1/2 by default)
Level-1 router: It can establish a Level-1 neighbor relationship with the same Level-1 device or Level-1/2 device, and synchronize the Level-1 LSDB.
Level-2 router: It can establish a Level-2 adjacency relationship with Level-2 devices or Level-1/2 devices in the same area or different areas, and synchronize the Level-2 LSDB.
Level-1/2 router: It can establish a Level-1 adjacency relationship with Level-1 or Level-1/2 devices in the same area, and synchronize the Level-1 LSDB. You can also establish a Level-2 adjacency relationship with Level-2 or Level-1/2 devices in the same area or a different area, and synchronize the Level-2 LSDB.
Note: The Level-1 LSDB only has Level-1 routing information in the same area, and the Level-2 LSDB has routing system information for the entire network (Level-1/2 devices will pass Level-1 routing information to Level-2 by default. But the routes of the Level-2 area will not be passed to Level-)
ISIS supports two network types: broadcast network and P2P
ISIS message type:
Hello message: used to discover, establish and maintain neighbor relationship, including L-1 Hello message, L-2 Hello message and P2P Hello message (the default sending time is 10s)
HSP message: carries complete link state information (equivalent to LSU message in OSPF), which are L-1 LSP and L-2 LSP respectively
SNP message (there are two types):
CSNP : Full sequence message, carrying the summary information of LSDB, used to judge whether LSDB is synchronized (equivalent to DD message), divided into L-1 CSNP and L-2 CSNP
PSNP: Partial sequence message, carrying part of the summary information, has the function of request and confirmation (equivalent to LSR and LSAC message) divided into L-1 PSNP and L-2PSNP
The structure of ISIS:
ISIS uses the TLV structure , which is highly scalable (T-Type, L-Length length/size, V-value value/data), and ISIS only needs to add new TLVs if it wants to support new protocols or features
Neighbor relationship establishment under ISIS broadcast network: three-way handshake
R1---R2
1. R1/R2 starts to send Hello packets, carrying its own system ID and area ID and other parameters. At this time, the neighbor list is empty, and it is in the Down state.
2. After receiving the Hello message, R1/R2 adds the system ID carried in the message to its neighbor list, enters the Init state, and carries the peer system ID when replying the Hello message
3. R1/R2 receives the Hello packet from the opposite end and finds that the neighbor list carries its own system ID. At this time, it enters the UP state, indicating that the neighbor relationship is established.
Note: After establishing the adjacency relationship in the broadcast network, wait for two Hello times to elect DIS
DIS: Designated Intermediate System
effect:
1. Create a pseudo-node to describe the network topology in a broadcast environment
2. Synchronize LSDB
DIS election process:
1. By priority, the range is 0-127. The default is 64, all priorities participate in the election , the bigger the better
2. Compare MAC addresses with the same priority, the bigger the better
Note: DIS supports preemption
The difference between OSPF "DR" and ISIS "DIS":
1. Priority:
DR is 0-255, the default value is 1, 0 does not participate in the election
DIS is 0-127, the default is 64, all participate in the election
2. Election parameters:
DR is priority + RID
DIS is priority + MAC
3. Backup:
DR has BDR backup
DIS is not backed up
4. Function:
DR in order to reduce the number of adjacencies
DIS describes network topology and synchronizes LSDB
5. Hello time:
DR is the same as a normal router
DIS is 1/3 of ordinary router
6. Preemption:
DR does not support preemption
DIS supports preemption
The ISIS point-to-point network is also a three-way handshake by default, and also supports two handshakes to establish an adjacency relationship
R1---R2
1. R1 sends a Hello packet, and after R2 receives it, the adjacency relationship goes up.
2. R2 sends a Hello message, and R1 goes UP after receiving it.
Note: This method has low reliability and is prone to one-way risk. Only P2P supports two handshakes
Functions and sending time of LSP packets, CSNP packets, and PSMP packets in ISIS:
LSP survival time: 1200s->0s change, 0s means aging
LSP ID=System ID + Pseudo-Node ID + Fragment ID
The system ID indicates which router the LSP is generated by, the pseudo-node identifier indicates whether this LSP is a pseudo-node LSP, if it is not 0, it is a pseudo-node LSP, and the fragmentation identifier indicates whether this LSP is fragmented , if it is not 0, it means that fragmentation is done.
CSNP message: DIR is periodically sent every 10s in the broadcast network to determine whether the LSDB is synchronized, and the point-to-point network will send each other once after the adjacency relationship is established
PSNP message: It has the function of request in the broadcast network, and the function of request and confirmation in the point-to-point network
The synchronization process of LSDB in the broadcast network:
*After the adjacency relationship is established, DIS is elected, and all devices send their own LSPs to the multicast address. DIS will have the LSPs of all devices, build LSDB, and then send the header and tail of the CSNP message automatically.
1. Add a new device and send its own LSP to the multicast address
2. DIS will add it to LSDB after receiving it, and periodically send CSNP message
3. After receiving the CSNP message, the newly added device can determine which LSPs it lacks, and send PSNP to request
4. After receiving the request, DIS will send the complete LSP to the newly added device
*The broadcast network does not need to confirm the message, and periodically sends the CSNP message through the DIS for implicit confirmation
ISIS multicast address:
Level-1：0180-c200-0014
Level-2：0180-c200-0015
The synchronization process of synchronizing LSDB in a point-to-point environment:
R1---R2
1. After the adjacency relationship is established, the two send CSNP messages to each other to determine whether the LSDB is synchronized.
2. If there is no synchronization, send a PSNP message to the other party to request the missing LSP
3. The party receiving the PSNP request will reply with a complete PSNP for confirmation
4. After receiving the complete LSP router will reply PSNP for confirmation
Note: If the retransmission timer expires and the LSP is not received, it will retransmit the LSP until the confirmation is received
The process of LSP:
When multiple instances of the same LSP are received:
1. First compare the serial number of the LSP, the bigger the better
2. The serial numbers are the same, check whether the lifetime of the LSP is 0s, and 0s is the best (aging)
3. The first two are the same and compare the checksum, the bigger the better
Note: The three parameters are consistent, indicating the same optimal
Triggered updates and periodic updates for OSPF and ISIS:
Both OSPF and ISIS support "triggered update (triggered by network changes)" and "periodical update (updated at regular intervals)". OSPF updates periodically in 1800s, and ISIS updates periodically in 900s.
