Addresses

CLNS

• CLNS is the service provided by CLNP
• CLNS addresses are required even if routing only IP.
  • Because IS-IS was originally designed for CLNS, IS-IS requires CLNS node addresses even if router is used for routing only IP
• CLNS addresses apply to entire nodes and not to interfaces.
• NSAP – CLNS addresses that are used by routers are called network service access points (NSAP).
  • NSEL – a part of the NSAP address is called NSAP Selector (NSEL).
  • NET – When an NSAP is specified with an NSEL 0, the NSAP is called the network entity title (NET).
  • NSAP Addresses is equivalent to the IP address and upper-layer protocol (IP protocol number) in the IP header.
  • NSAP addresses have a maximum size of 20 bytes.

NSAP Address

Figure 1: NSAP Address Structure

• In the figure above, the high-order bits identify the inter-area (Level 2) structure, and the low order bits identify unique systems within an area (intra-area -  Level 1)
• The Cisco implementation of Integrated IS-IS divides the NSAP address into three fields:
  1. Area Address
  2. System ID
  3. NSEL
• Example address: 49.0001.aaaa.bbbb.cccc.00
  • Area = 49.0001
  • System ID = aaaa.bbbb.cccc
  • NSEL = 00

IS-IS Area Address

• The area address is used in L2 routing.
• The first part of the NSAP associated with the routing process.
• An IS-IS router can be a member of only one area
• All routers in an area must use the same area address.
• ESs recognize only ISs and other ESs on the same subnetwork that share the same area address.

IS-IS System ID

• The system ID is used for intra-area (L1) routing.
• Cisco enforces that the System Id is fixed to a length of 6 bytes.
• The system ID must be unique in each area.
• By custom, the routers MAC address, which is conveniently 6-bytes in length, is used as the System ID.

NSAP Selector

• A one octet (1 byte) field at the end of an NSAP address is called the NSAP Selector, or NSEL.
• When the NSEL’s value is set to 00, the NSAP is called a NET address -  the address of the node’s network layer itself.
• The NSEL field identifies a process on the device, which corresponds roughly to a port number in IP.

Addressing and Routing

• The area address portion of the NSAP address can range from 1 to 13 bytes in length.
• The area address is the same for devices within the same area and unique for different areas.
• Routing within an area (intra-area) involves knowing all the system IDs and adjacencies for all devices (ISs and ESs) in the same area and choosing the best paths between these devices by using the Dijkstra algorithm.
  • The system ID is used to route within an area; the area address is not considered.
• When routing between areas (inter-area), L2 (or L1/L2 routers in different areas exchange area address information and compute the best paths between areas using the DIjkstra algorithm.
  • The area address is used to route between area; the system ID is not considered.
• Sending packets from an ES to another ES requires the packets to be sent to an IS that the destination ES is attached to.
  • If the destination ES is in the same area, the IS knows where that ES is based on the ESH it receives from it. The IS proceeds to forward the packet to that ES using the best path.
  • If destination ES is in another area, the L1 IS sends the packet to the nearest L1/L2 router.

Route Leaking

• Because L1 and L2 routing are separate, there is a chance that packets sent to one direction might take a different direction coming back. This is called asymetric routing.
• Asymetric routing does not bring down the network. However, it can prove difficult to troubleshoot a network with asymetric routing.
• Route Leaking is a feature introduced in Cisco IOS 12.0 which allows L2 routes to be redistributed, or leaked, into L1 routers.
  • 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.
• Route leaking is defined in RFC 2966, Domain-wide Prefix Distribution with  Two-Level IS-IS, for use with the narrow metric TLV types 128 and 130.
• For use with wide metric, the IETF defined route leaking using TLV type 135.
• To implement, an up/down bit in the TLV is used to indicate whether or not the route indentified n the TLV has been leaked.
  • If the up/down bit is set to 1, the route has been redistributed into the area from L2
  • If the up/down bit is set to 0, the route was originated within that L1 area.
  • The up/down bit is used to prevent routing loops: An L1/L2 router does not re-advertise into L2 and L1 routes that have the up/down bit set.

IS-IS PDU

1. Hello PDUs – Used to establish and maintain adjacencies.
   • ESH – End System Hellos
   • ISH – Intermediate System Hellos
   • IIH – IS-IS Hellos
2. LSP – Link State PDUs. Used to distribute link-state information
3. Partial Sequence Number PDU (PSNP) – Acknowledges and requests missing link-state information.
4. Complete Sequence Number PDU (CSNP) – Describes the complete list of LSPs in a router’s link-state database.

LSP Link-State PDUs

Figure 2: An LSP PDU

Some of the notable features of an LSP header are:

• PDU type and length
• LSP ID
• The LSP sequence number
  • Used to identify duplicate  LSPs and to ensure that the latest LSP information is stored in the topology table.
  • Allows receiving routers to do the following:
    • Ensure that they use the latest LSPs in their route calculations
    • Avoid entering duplicate LSPs in the topology tables
• Remaining Lifetime
  • Used to age out LSPs.
  • 1200 seconds = 20min is the default start value.

TLVs

• TLV stands for Time, Length and Value
  • It is also sometimes called Code, Length, and Value (CLV)
  • Type (or Code) is a number specifying the information content of the value field.
  • Length is the maximum size of the Value field (255 octets)
  • Value is the information itself.
• The TLV structure is a flexible way to add data to teh LSP and an easy mechanism for adding new data fields that might be required in the future.

Example of LSP TLV

IS-IS Network Types

Two general types of IS-IS network topologies are:

1. Point-to-point Networks
   • Links that are permanently estblished (leased line, PVCs)
   • or dynamically established (ISDN, switched virtual circuit [SVCs])
2. Broadcast Networks
   • Multipoint WAN links or LAN links such as Ethernet, Token Ring, or FDDI

Implementing IS-IS in NBMA Networks

• IS-IS has no concept of NBMA Networks. It is recommended to use point-to-point links, such as point-to-point subinterfaces, over NBMA networks, such as ATM or Frame Relay.
• Cisco IOS automatically uses broadcast mode for LAN links and multipoint WAN links.
  • It uses point-to-point mode for point-to-point links, such as point-tp-point subinterfaes and dialer interfaces.
• In NBMA networks, Cisco IOS assumes that the NBMA environment features a full mesh of PVCs, when implemented in broadcast mode.
• When creating static maps to map the remote IP address to the local DLCI on a Frame Relay interface, it is recommended that you use the broadcast keyword.
  • This is because broadcast mode uses multicast updates, which will not be sent without this keyword.
• When using multipoint WAN links such as multipoint Frame Relay interfaces, you must also allow CLNS broadcast and multicasts.
• This can be done by using the following comands (in addition to creating the IP mappping):

frame-relay map clns dlci-number broadcast

Implementing IS-IS in Broadcast Networks

• In IS-IS, broadcast networks are LAN interfaces or multipoint WAN interfaces.
• Use broadcast mode only for LANs.
  • Although it is default for multipoint WANs, broadcast mode is recommended for use only on LAN interfaces.
• Separate IS-IS adjacencies are established for L1 and L2 processes. If neighboring routers are L1/L2 routers, they establish two separate adjacencies for each level, using specific Layer 1 and Layer 2 IIH PDUs.
• Routers on a LAN establish adjacencies with all the other routers on the LAN, unlike OSPF with the DR/BDR concept.
• IIH PDUs announce the area address.
  • Adjacencies form based on the area address communicated in the incoming IIH and the type of router (L1 or L2).

Pseudonode and DIS

• Designated Intermediate System (DIS)
  • The DIS is the router that creates the pseudonode and acts on behalf of the pseudonode.
  • On broadcast multiaccess networks, a single router is elected as the DIS.
  • There is no backup DIS elected.
  • Selection of the DIS follows the criteria:
    1. Highest priority
    2. Highest SNPA (on LANs the SNPA is the MAC Address)
  • Cisco routers have a default L1 and L2 priority of 64
    • You can configure the priority from 0 to 127 usng the following interface configuration command: isis priority number-value [level-1 | level-2].
    • Because an interface can have different L1 and L2 priorities, the L1 DIS and L2 DIS on a LAN may or may not be the same router.
  • Does not guarantee to keep the DIS role. If there is an IS with a higher priority on the LAN, that IS automatically takes over as DIS. This is called preemptive behavior.
• Pseudonode
  • Is a logical representation of the LAN which is generated by a DIS.
  • In order to reduce the number of full mesh adjacencies between nodes on multiaccess links, the multiaccess link itself is modeled as a pseudonode that connects all attached routers to a star-shaped topology.
  • All routers on a broadcast link, including the DIS, form adjacencies with the pseudonode.

Figure 3: Physical and Logical Representation of the Pseudonode

• Rather than having each router connected to the LAN advertise an adjacency with every router on the LAN, each router, including the DIS, advertise a single adjacency to the pseudo-node.
• The DIS generates the pseudo-node LSPs.
• A Pseudo-node LSP details only the adjacent ISs.
• The pseudo-node LSP is used to build the map of the network and to calculate the SPF tree.
• The pseudo-node LSP is equivalent to a network LSA in OSPF.

L1 and L2 LSPs

• Each IS orginates its owl LSPs: One for L1 and one for L2
• On a LAN, the DIS (representing the pseudo-node) sends out LSP information on behalf of the LAN.
  • The DIS sends out separate L1 and L2 LSPs for the pseudo-node.
• LSPs are sent differently in different media types:
  • Broadcast type – send out as multicast
  • Point-to-point links – sent out as unicast

L1 and L2 IIHs

• IIHs establish and maintain adjacency between ISs.
  • Defualt Hello = 10 seconds; 3.3 sec for DIS
  • Hold Time = default multiplier (3) x hello time = 3 x 10 = 30 sec.
• On a LAN separate L1 and L2 IIHs are sent periodically as multicasts to a multicast MAC address:
  • L1 – sent to AllL1IS multicast MAC address 0180.C200.0014.
  • L2 – sent to AllL2IS multicast MAC address 0180.C200.0014.
• Point-to-point links have a common point-to-point IIH format that specifies whether hello relates to L1 or L2 or both.
  • Point-to-point hellos are sent to the unicast address of the connected router.

Link State Database Synchronization

LSP Flooding

• LSPs are flooded throughout the IS-IS domain. LSPs are typically flooded to all adjacent routers except the neighbor from which the LSP was received.
  • L1 LSPs are flooded within their local areas.
  • L2 LSPs are flooded throughout the backbone.
• LSPs originated by each ISs are identified by the originator’s system ID and an LSP fragment number starting at 0.
  • If an LSP is bigger than the maximum transmission unit (MTU), it is fragmented into several LSPs, numbered 1, 2, 3, and so on.
• When an IS receives an LSP, it examines the checksum and discards any invalid LSPs, by expiring the lifetime age.
  • If the LSP is valid and newer than what is currently in the LSDB, it is retained, acknowledged with a PSNP, and given a lifetime of 1200 seconds (20 min).
  • When the LSP expires after 1200 seconds, it is kept for an additional 60 seconds before it is flooded as an expired LSP.

LSDB Synchronization

• In order to acknowledge the receipt of LSPs and to maintain LSDB synchronization, sequence number PDUs (SNPs) are used. The use of SNPs differ between point-to-point and broadcast media.
• There are two types of SNPs:
  1. Complete Sequence Number PDUs (CSNPs)
     • Used to inform other routers of LSPs that may be outdated or missing from their own databases. This ensures all the routers have the same information and are synchronized.
     • Similar to an OSPF database description packet.
  2. Partial Sequence Number PDUs (PSNPs)
     • PSNPs are used to request an LSP (or LSPs),
     • and acknowledge receipt of an LSP (or LSPs).
• Separate CSNPs and PSNPs are used for L1 and L2 adjacencies.
• In broadcast networks, only the DIS transmits CSNPs.
  • CSNP multicasts are sent every 10 sec by the DIS on a LAN to ensure LSDB accuracy.
• In point-to-point networks, CSNPs are sent when the link comes up to synchronize the LSDB.
  • This is sent only once.
  • After the first transmission, LSPs are only sent if there topology changes.
  • CSNP receipt is acknowledged with PSNP.

LAN Adjacencies

• IIH PDUs announce the area address.
• Routers from one area accept L1 IIH PDUs only from their own area and therefore establish adjacencies only with their own area routers
• Similarly, L2 routers accept only L2 IIH PDUs and establish L2 adjacencies.

WAN Adjacecies

• On point-to-point WAN links, the IIH PDUs have common formats to both levels. The level type and area address are announced in the IIH.
• L1 routers receive IIH that speficify the L1 level and form L1 adjacency.
• L2 routers exchange IIH PDUs that specify L2 levels and form level 2 adjacency.
• L1/L2 establish a separate level 1 and level 2 adjacencies
• Two L1 routers that are physically connected but are not in the same area can exchange IIHs, but they do not establish an adjacency, because their area ID do not match.

Resources:

1. Intermediate System-to-Intermediate System Protocol – Cisco Technical White Paper
2. IS-IS Route Leaking Overview
3. Intermediate System-to-Intermediate System (IS-IS) TLVs
4. IS-IS Network Types and Frame Relay Interfaces
5. Understanding IS-IS Pseudonode LSP

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.