ISIS

ISIS protocol study summary

2010-04-08T04:24:32Z

- Link state protocol
- Large ISP used it
- Use Dijkstra's SPF alorithm to calculate OSI route.
- use Partia Route Calculation for IP routes (much effiecient than SPF)
- use "Default metric" (1 ~ 63), VS OSPF use bandwidth
- OSI addressing
- No need backbone area, but use Level
- It was developed by OSI model. and it was design for OSI and well fit to the model
- Integrated ISIS support TCP/IP model(carrying tcp/ip info).. and it was well fit to TCP/IP so.. Very litter modification requires for IPv6.

## OSI address ##

- Hexadecimal
- Reading right to left
- Three parts
    - Area ID (Veriable length)
    - System ID (6 Bytes), it need to be unique, it can be MAC address.
    - NSEL(1Byt)

example
    - 49.0003.1111.2222.3333.00
    00 = NSEL
    1111.2222.3333 = System ID
    49.0003 = Area ID

    - 49.0141.4176.1AB2.FE00.63c9.00
    - 49.0005.1111.1111.1111.00

** 49. is private range

## Routing table

IS-IS Routing maintains two IP routing databases
- Level 1 database : includes Intra-area routes
- Level 2 database : includes Backbone routes

Router can be L1, L2 or L1/L2

## Configuration

1. Enable IS-IS on the router
    * automatically create L1 and L2 database, unless create is-type.
    ** if you are not tagging, default is "0"

    Router(config)# router isis
    Router(config-router)# net 49.0001.1111.1111.1111.00

2. Configure single OSI address per each router

3. Enable ISIS on per interface basis

    ** sending both L1/L2 hello packet to interface which is enable ISIS
    *** Default metric 10, which is lower than Rip


    Router(config)# interface serial1/0
    Router(config-if)# ip router isis

4. Summary

summary-address 172.16.0.0 255.255.0.0 level 1 or level 2 or level 1/2 ; have to specify what database will be updated.

5. Redistribute
    - To redistrubte ISIS to other protocol, network that is directly connected couldn't be redistrubuted... so use redistrubte connected with route-map to block some of route that doesn't want to redistrubte.

How DIS is elected on ISIS protocol ?

2009-05-05T23:48:44Z

The default L1 and L2 priority is 64 in Cisco routers, with a value range of 0-127. A value of 0 means the router is ineligible to become the DIS. The router with the highest priority is chosen as the DIS, and in the case of a tie, the router with the numerically highest SNPA becomes the DIS.
DIS is the highest Subnetwork Points of Attachment (SNPA) address on the LAN segment. The SNPA address refers to the data link address, and in this case is the MAC address.
On a LAN, one of the routers elects itself the DIS, based on interface priority (the default is 64). If all interface priorities are the same, the router with the highest subnetwork point of attachment (SNPA) is selected. The SNPA is the MAC address on a LAN, and the local data link connection identifier (DLCI) on a Frame Relay network. If the SNPA is a DLCI and is the same at both sides of a link, the router with the higher system ID becomes the DIS. Every IS-IS router interface is assigned both a L1 priority and a L2
priority in the range of 0 to 127.
The DIS election is preemptive (unlike OSPF). If a new router boots on the LAN with a higher interface priority, the new router becomes the DIS. It purges the old pseudonode LSP and floods a new set of LSPs.

Reference:http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094b42.shtml

What is CNSP and PNSP ?

2009-05-05T23:46:03Z

PSNP(Partial Sequence Number PDU) are used to acknowledge the receipt or to request the retransmission of the latest version of an LSP while the CSNP(Complete Sequence Number PDU) are used for synchronizing the LS Database on adjacent neighbors.

Protocols ISIS VS. OSPF

2009-05-05T23:43:26Z

L2 routers are similar to OSPF backbone routers, and the set of L2 routers (including L1/L2 routers) and their interconnecting links make up the IS-IS backbone, similar to area 0 in an OSPF network.

IS-IS does not use level 3 routers.
Some routers, called L1/L2 routers, belong to both area types.
Unlike OSPF, IS-IS routers are not required to be connected to a contiguous backbone area. In fact, the backbone area can also be segmented in IS-IS.
IS-IS uses the concepts of router levels, which is similar to OSPF areas. L2 routers are similar to OSPF backbone routers, and L1/L2 routers are analogous to OSPF ABRs.
With IS-IS, there is no restriction that all backbone routers (level 2 routers) be contiguous
such as the backbone area of OSPF.
In OSPF all areas must be directly linked to area 0, and the backbone area must also not be segmented.
With IS-IS, the backbone area can be more easily extended since all L2 routers need not be linked directly together.
With regard to CPU use and the processing of routing updates, IS-IS is more efficient
than OSPF.
In IS-IS, one LSP is sent per IS-IS router in each area (including redistributed prefixes [routes]), compared to the many OSPF LSAs that would be sent.
Not only are there fewer LSPs to process, but the mechanism by which IS-IS installs and withdraws prefixes is less processor intensive.
In IS-IS, the entire SPF table is not refreshed periodically like OSPF, which does so every 30 minutes by default.

Reference: Jeff Doyle, “Routing TCP IP, Volume 1”