IS-IS Network Types

1) Broadcast

    -assumes full conectivity (all IS can reach each other)

    - elects Designated IS (DIS) - same as DR/BDR

    - multicast

2) Point-to-point

    -p2p connectivity

    -no DIS

    -unicast

When dealing with NBMA, p2p mode with sub-if is preferred design

IS-IS packet types

packet = Protocol Data Unit (PDU)

network-layer packet = Network PDU

Data-link frame = Data-link PDU

PDU types:

1) Hello

2) Link-state packet (LSP) - аналог LSA в OSPF. Network advertisement

3) Partial sequence number PDU (PSNP) - выполняет роль ACK или request for missing network

4) Complete sequence number PDU (CSNP) - аналог DBD в OSPF, краткий summary DB. It is rather small, it sent once in 10 s in broadcast network; only once in p2p networks.

LSP header - тип пакета и служебная инфа

TLV - стандартный контейнер, может содержать различную инфу. Вся инфа об IP также передается в TLV. Таким образом можно сопоставить NSAP и соответствующих ему IP subnets.

IS-IS addressing

OSI uses connectionless network protocol addresses (CNLP)

When CLNP assigned to a router it becomes a Network Service Access Point address (NSAP)
or Net address

NSAP address can be up to 20 bytes

Original OSI implementation defines 5 fields in NSAP

Cisco implementation defines 3 fields:
1) The Area address
2)The system ID
3)The NSAP selector (NSEL)

NSEL - всегда 00, просто определяет, что это IS система.
System ID - всегда 6 байт. Идентификатор роутера. Должен быть уникален
Остальное - Area address

EX: 49.0003.4444.4444.4444.00

49.0003 - Area id

4444.4444.4444 - System ID

Best practice: System ID = Router MAC

IS-IS routing

IS-IS routing domains

routing
level 0 - взаимодействие End-System - IS
level 1 - взаимодействие IS-IS внутри area
level 2 - внутри AS
level 3 - с другой AS

IS-IS routing process

The Decision Process

After the link-state databases have been synchronized, it is necessary to decide which path to take 

to reach the destination. Because the routers and hosts may have multiple connections to each 

other, there may be many paths from which to choose.

To make the best path decision, link-state protocols employ the algorithm defined by Dijkstra. This 

algorithm creates a tree that shows the shortest paths to all destinations. The tree is used in turn 

to create the routing table.

If there is more than one path to a remote destination, the criteria by which the lowest cost paths 

are selected and placed in the forwarding database are as follows:

1. If there is more than one path with the lowest value metric, Cisco equipment places some or 

all paths into the table. Older versions of IOS support as many as six load-sharing paths, 

newer versions support more.

2. Internal paths are chosen before external paths. 

3. Level 1 paths within the area are more attractive than Level 2 paths.

4. The address with the most specific address in IP is the address with the longest IP subnet 

mask.

5. If there is no path, the forwarding database sends the packet to the nearest Level 2 router, 

which is the default router. 

The metric defines the cost of the path. Integrated IS-IS has four metrics, only one of which is 

required and supported. The metrics defined in ISO 10589 are as follows:

■ Default—Every Integrated IS-IS router must support this metric. Cisco set the default for all 

interfaces to 10.

■ Delay—Cisco does not support the transit delay metric.

■ Expense—Cisco does not support the expense metric.

■ Error—Cisco does not support the error metric.

By default, six-bit metrics are configured on the outgoing interface. A 10-bit field describes the 

total path cost. These default metrics are referred to as narrow. 

Because it considered these inadequate, Cisco increased the metric size to 24 bits. This larger 

metric field provides more granularity to distinguish between paths and is referred to as wide.

To determine shortest path, the lowest metric is chosen, internal paths are chosen over external 

paths, and Level 1 routes have precedence over Level 2 routes.

The default metric is the only metric supported by Cisco, because each metric used in Integrated 

IS-IS requires a different link-state database calculation for both the Level 1 and Level 2 routes.

IS-IS design

Нужно предусматривать резервные линки на случай падения одного из роутеров ядра.

IS-IS basics

Originally designed for OSI
Tuned for TCP/IP but still requires an OSI connectionless network services address (CLNS)

-Link-state - more tunable than OSPF
-Uses SPF - more efficient than OSPF
-Hello msg


L1 routers - internal routers, only knows about routes inside the area
L2 routers - backbone routers, only knows about backbone routes
L1/L2 routers - что-то вроде граничных роутеров. Поддерживают как базу internal area routes, так и backbone routes.

L1 - аналог клиента - внутри area он знает маршруты, но если надо попасть в другую area он обращается за помощью к L1/L2 ( аналог default gw)

Для IS-IS необязательно наличие area 0 - главное чтоб backbone был неразрывен.

По умолчанию, все линки имеют cost = 10. Необходимо вручную настроить все линки cost ( 0 - 63 )

Формирование relationship
L1 <----->L1
L2 <------>L2
L1 ////----/// L2 - не сформируются

L1/L2 <--------> L1/L2 - relationship формируется независимо для L1-to-L1 и для L2-to-L2 и формируются независимые DB

IS-IS NSAP addressing

- ISIS uses CLNP ( Connectionless Network Protocol) - аналог IP в стеке TCP/IP

- когда CLNP адрес назначается роутеру, он называется NSAP address (Network Service Access Point)

- 1 адрес на ноду, не на интерфейс

- NSAP address up to 20 bytes length

Адрес в hex и его лучше читать справа налево:

1) NSEL (1 байт справа - два символа в hex) - определяет тип системы. Для Inermediate System (IS) - роутера - всегда 00

2) System ID - 3 октета по 2 байта каждый. Всегда фиксировано 6 байт. Определяет ID роутера. Должен быть глобально уникальным.

3) Area address - Все остальное слева обозначает Area address. Должен быть одинаковым для роутеров в одной area