【HCIP】ISIS

ISIS route calculation process

(1) Establish neighbor relationship

(2) Synchronize LSDB database

(3) Point to SPF routing calculation

Backbone area: L2 neighbor relationship is established by continuous L2 and L1-2

Non-backbone area: L1 neighbor relationship is established by continuous L1 and L1-2

L1: Only L1 neighbors can be established, but the area IDs at both ends are required to be the same

L2: Only L2 neighbors can be established, and the area IDs at both ends are not required to be the same

L1-2: Both L1 and L2 neighbors can be established, but L1 requires the same area ID, and L2 does not.

Two types of ISIS networks

Broadcast type under Ethernet; P2P type under HDLC or PPP; P2MP and NBMA are not supported by ISIS.

DIS

ISIS will elect DIS in MA network, no backup DIS; preemptible

The default priority of DIS is 64, the highest is 127, and it still participates in the election when it is 0

DIS election rules: the higher the priority, the better. If the priority is the same, the MAC address will be compared, and the bigger the better.

No DIS in P2P network

Send CSNP periodically; Hello time is 3 seconds, expiration time is 9 seconds

ISIS message type

(1) Hello message

Used to discover, establish and maintain neighbor relationships; once every 10s; failure time is 30s

L1's Hello message: used to establish L1's neighbor relationship on the broadcast link

L2 Hello message: used to establish L2 neighbor relationship on the broadcast link

P2P Hello message: used to establish L1 or L2 neighbor relationship on a point-to-point link

The router of L1 only listens to the multicast address of L1 and receives the Hello message of L1; the router of L2 only listens to the multicast address of L2 and receives the Hello message of L2.

(2) LSP packet

Used to describe link status information

L1 LSP: used to describe the link state information of L1

L2 LSP: used to describe the link state information of L2

(3) SNP message

Serial number message

Divided into: Complete Sequence Number Packet (CSNP), Partial Sequence Number Packet (PNSP)

L1 CSNP: LSDB summary for describing L1

L2 CSNP: LSDB summary for describing L2

L1 PNSP: used to request or confirm LSP

L2 PNSP: used to request or confirm LSP

ISIS Neighbor Status

(1) Down: initial state

(2) Init: Status after receiving the neighbor's Hello message

(3) UP: I found myself in my neighbor's Hello message

Neighbor establishment condition: the interface MTU is consistent

LSP

LSP-ID identifies a unique LSP; 00 means real node LSP; non-00 means pseudo node LSP

Update mechanism: trigger update; periodic update (once in 900s)

Aging mechanism: 1200s, go to 0 from 1200s

Determine the old and new: compare the serial number, select the larger HoldTime (0 means delete), and select the larger checksum

P2P: Send CSNP first, then build a routing table based on the CSNP sent by the other party, then send PNSP for maintenance, and send the data missing from yourself or the other party

MA: Information requested by other devices sent by DIS

SOURCE: used to identify the router that generated the LSP

NLPID: Protocol Support

AREA ADDR: The area that generated this LSP router

INTF ADDR: Generates the IP address of the ISIS-enabled interface of this LSP router

NBR ID: Neighbor ID

IP-Internal: used to describe its own directly connected network number, mask, and cost to reach the destination network

ISIS Multi-Region

The L1-2 router will put the L1 route into the L2 LSP generated by itself in the form of a leaf

The L1-2 router will set the ATT BIT when the L1 LSP is generated, and a default route will be generated after the L1 receives it

attached-bit advertise always: always set att bitattached-bit advertise never: do not set the att bitattached-bit advertise always: do not count att bit

ISIS Anti-Ring Principles

(1) L1-2 ignores the calculation of ATT bit

(2) Regardless of cost, L1 routing is always better than L2 routing

(3) Regardless of cost, L2 routing is always better than L1* routing