What is IS-IS ?
- Intermediate System to Intermediate System intra-domain routing protocol,IS, ES
- „Is a Link State Protocol
- „Is a dynamic routing protocol based on SPF routing algorithm
- „IS is “OSI speak” for router (i.e., routing protocol for the ISO Connectionless Network Protocol)
- Is an IGP (Interior Gateway Protocol) scalable only for dynamic routing within a domain
- Supports a 2-level hierarchy : level-1 (areas) and level-2 (backbone)
- „Integrated IS-IS, also known as Dual IS-IS, was to provide a single routing protocol with the capabilities of routing both Connectionless Network Service (CLNS) and IP
ISIS Levels
Level-1 router
Has neighbours only on the same area
Has a level-1 LSDB with all routing information for the area
Level-2 router
May have neighbours in the same or other areas
Has a Level-2 LSDB with all routing information about inter-area
Level-1-2 router
May have neighbours on any area.
Has two separate LSDBs: level-1 LSDB & level-2 LSDB
NSAPs (Network Service Access Points) and Addressing
- ISIS does not have a backbone area as such (like OSPF)
- Instead the backbone is the contiguous collection of Level-2 capable routers
- ISIS area borders are on links, not routers
- Each router is identified with Network Entity Title (NET).NET is an NSAP where the n-selector is 0
NSAP: Network Service Access Point
NSAP is the network layer address for CLNS (Connectionless Network Service) packets
„An NSAP consists of 3 parts - area address,system ID and n-selector
„All routers in an area must use the same area address
„Each node in an area must have a unique systemID
„All L2 routers in a domain must each have a unique systemID
„Area address is like OSPF area number
„System ID is like an OSPF router ID
Total length between 8 and 20 bytes
Area Address: variable length field (up to 13 bytes) (used by Level 2 routing)
System ID: defines an ES or IS in an area. (used by level 1 routing; similar to OSPF router id)
NSEL: N-selector. identifies a network service user (transport entity or the IS
network entity itself)
- NET: the address of the network entity itself
A NET implies the routing layer of the IS itself (no transport layer)
ISs (routers) do not have any transport layer (selector=0)
ISPs typically choose NSAP addresses as:
First 8 bits – pick a number (usually 49)
First 8 bits – pick a number (usually 49)
Next 16 bits – area
Next 48 bits – router loopback address
Final 8 bits – zero
CLNS Addressing
NSAP: 49.0001.1921.6800.1001.00
Router: 192.168.1.1 (loopback) in Area 1
„ ESH (End System Hello, sent by end system and listen to ISH)
„ ISH (Intermediate System Hello, sent by IS for End Systems)
„ IIH (IS-IS Hello, sent by IS to form adjancency)
2. LSP (Link State PDU)—Advertises link-state information
Each router creates an LSP and flood it to neighbours
A level-1 router will create level-1 LSP(s)
A level-2 router will create level-2 LSP(s)
A level-1-2 router will create
level-1 LSP(s) and
level-2 LSP(s)
3. CSNP (Complete Sequence Number PDU)—An update containing the complete list of LSPs known to the router
4. PSNP (Partial Sequence Number PDU)—Used to acknowledge a routing update (LSP) on point-to-point links and to request missing information about a route after receiving a CSNP
Fast Hellos
Advantages
-Reduced link failure detection time
Disadvantages
-Increased BW/buffer/CPU usage can cause missed hellos, potential increased adjacency flapping can cause instability
Configuration:
Interface configuration mode:
Rtr-A(config)#int POS0/0
Rtr-A(config-if)#isis hello-interval minimal
Rtr-A(config-if)#isis hello-multiplier 4
Advertised hold time will now be 1 second, hello-interval will be 250 ms
Priority can be configured; the range is 1–127. In case of a tie, the router with the highest SPNA address for that interface is elected the DIS. No backup DIS exists.
DIS has two tasks
--Creating and updating the Pseudonode LSP
--Conducting the flooding over the LAN
A DIS is elected for each LAN
CLNS:
Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI Network Layer datagram service that does not require a circuit to be established before data is transmitted,
and routes messages to their destinations independently of any other messages.
Route Leaking:
The IS-IS routing protocol allows for a two-level hierarchy of routing information.There can be multiple Level 1 areas interconnected by a contiguous Level 2 backbone. A router can belong to Level 1, Level 2, or both. The Level 1 link-state database contains information about that area only.The Level 2 link-state database contains information about that level as well as each of the Level 1 areas. An L1/L2 router contains both Level 1 and Level 2 databases. It advertises information about the L1 area to which it belongs into L2.
Each L1 area is essentially a stub area. Packets destined for an address that is outside of the L1 area are routed to the closest L1/L2 router to be forwarded on to the destination area. Routing to the closest L1/L2 router can lead to sub-optimal routing when the shortest path to the destination is through a different L1/L2 router.Route leaking helps reduce sub-optimal routing by providing a mechanism for leaking, or redistributing, L2 information into L1 areas. By having more detail about interarea routes, an L1 router is able to make a better choice with regard to which L1/L2 router to forward the packet.
Both commands are entered within the router IS-IS configuration.
You must create an IP extended access list to define which routes will be leaked from Level 2 into Level 1.
If route leaking is configured without configuring wide style metrics, route leaking will not occur.
ReleaseCommand12.0S
advertise ip l2-into-l1 <100-199> metric-style wide
12.0T and 12.1
redistribute isis ip level-2 into level-1 distribute-list<100-199> metric-style wide
For More Info:
http://www.menog.org/presentations/menog-4/MENOG4-ISIS-Tutorial.pdf
http://map.twnic.net.tw/ip93/doc/k/is-is.pdf
Types of IS-IS Packets
1. IS-IS Hello Packets
Used for maintaining adjacencies
Sends hello packet every 10 sec, dead interval time is 30 sec. The DIS sends a hello every 3.3 seconds.„ ESH (End System Hello, sent by end system and listen to ISH)
„ ISH (Intermediate System Hello, sent by IS for End Systems)
„ IIH (IS-IS Hello, sent by IS to form adjancency)
2. LSP (Link State PDU)—Advertises link-state information
Each router creates an LSP and flood it to neighbours
A level-1 router will create level-1 LSP(s)
A level-2 router will create level-2 LSP(s)
A level-1-2 router will create
level-1 LSP(s) and
level-2 LSP(s)
3. CSNP (Complete Sequence Number PDU)—An update containing the complete list of LSPs known to the router
4. PSNP (Partial Sequence Number PDU)—Used to acknowledge a routing update (LSP) on point-to-point links and to request missing information about a route after receiving a CSNP
Fast Hellos
Advantages
-Reduced link failure detection time
Disadvantages
-Increased BW/buffer/CPU usage can cause missed hellos, potential increased adjacency flapping can cause instability
Configuration:
Interface configuration mode:
Rtr-A(config)#int POS0/0
Rtr-A(config-if)#isis hello-interval minimal
Rtr-A(config-if)#isis hello-multiplier 4
Advertised hold time will now be 1 second, hello-interval will be 250 ms
Electing the DIS
An IS-IS DIS is elected based on highest priority value, and then on highest SNPA address (typically the MAC address). The priority is assigned to each interface and has a default value of 64.Priority can be configured; the range is 1–127. In case of a tie, the router with the highest SPNA address for that interface is elected the DIS. No backup DIS exists.
DIS has two tasks
--Creating and updating the Pseudonode LSP
--Conducting the flooding over the LAN
A DIS is elected for each LAN
CLNS:
Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI Network Layer datagram service that does not require a circuit to be established before data is transmitted,
and routes messages to their destinations independently of any other messages.
Route Leaking:
The IS-IS routing protocol allows for a two-level hierarchy of routing information.There can be multiple Level 1 areas interconnected by a contiguous Level 2 backbone. A router can belong to Level 1, Level 2, or both. The Level 1 link-state database contains information about that area only.The Level 2 link-state database contains information about that level as well as each of the Level 1 areas. An L1/L2 router contains both Level 1 and Level 2 databases. It advertises information about the L1 area to which it belongs into L2.
Each L1 area is essentially a stub area. Packets destined for an address that is outside of the L1 area are routed to the closest L1/L2 router to be forwarded on to the destination area. Routing to the closest L1/L2 router can lead to sub-optimal routing when the shortest path to the destination is through a different L1/L2 router.Route leaking helps reduce sub-optimal routing by providing a mechanism for leaking, or redistributing, L2 information into L1 areas. By having more detail about interarea routes, an L1 router is able to make a better choice with regard to which L1/L2 router to forward the packet.
Both commands are entered within the router IS-IS configuration.
You must create an IP extended access list to define which routes will be leaked from Level 2 into Level 1.
If route leaking is configured without configuring wide style metrics, route leaking will not occur.
ReleaseCommand12.0S
advertise ip l2-into-l1 <100-199> metric-style wide
12.0T and 12.1
redistribute isis ip level-2 into level-1 distribute-list<100-199> metric-style wide
For More Info:
http://www.menog.org/presentations/menog-4/MENOG4-ISIS-Tutorial.pdf
http://map.twnic.net.tw/ip93/doc/k/is-is.pdf
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