© 2006 Cisco Systems, Inc. All rights reserved. BSCI v The IS-IS Protocol Performing IS-IS Routing Operations

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSI Addresses OSI network layer addressing is implemented with NSAP addresses. An NSAP address identifies a system in the OSI network; an address represents an entire node, not an interface. Various NSAP formats are used in various systems, because different protocols may use different representations of NSAP. NSAP addresses are a maximum of 20 bytes: –Higher-order bits identify the interarea structure. –Lower-order bits identify systems within area.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Integrated IS-IS NSAP Address Structure The Cisco implementation of Integrated IS-IS distinguishes only the following three fields in the NSAP address: –Area address: Variable-length field (1 to 13 octets) composed of the higher-order NSAP octets, excluding system ID and NSEL. –System ID: ES or IS identifier in an area; fixed length of six octets in Cisco IOS software. –NSEL: One octet NSAP selector, service identifier. Total length of NSAP is from 8 (minimum) to 20 octets (maximum).

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Typical NSAP Address Structure The simplest NSAP format used by most companies running IS-IS as their IGP is as follows: Area address (must be at least 1 byte) –AFI set to 49 Locally administered; thus, you can assign your own addresses. –Area ID The octets of the area address after the AFI. System ID –Cisco routers require a 6-byte system ID. NSEL –Always set to 0 for a router.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Identifying Systems in IS-IS: Area Address The area address uniquely identifies the routing area, and the system ID identifies each node. All routers within an area must use the same area address. An ES may be adjacent to a router only if they share a common area address. Area address is used in Level 2 routing.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Identifying Systems in IS-IS: System ID System ID in the address used to identify the IS; it is not just an interface. Cisco supports only a 6-byte system ID. System ID is used in Level 1 routing and has to be unique within an area. System ID has to be unique within Level 2 routers that form the routing domain. General recommendation: use domain-wide unique system ID. –This may be MAC (for example, c ) or IP address (for example, ) taken from an interface.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSI Addressing: NET Addresses NSAP address includes NSEL field (process or port number) NET: NSAP with a NSEL field of 0 –Refers to the device itself (equivalent to the Layer 3 OSI address of the device) –Used in routers because they implement the network layer only (base for SPF calculation)

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Subnetwork Point of Attachment (SNPA) and Circuit SNPA is equivalent to Layer 2 address; for example: Virtual circuit ID (DLCI on Frame Relay) MAC address on LAN interfaces Interfaces uniquely identified by circuit ID: On point-to-point interfaces, SNPA is used. On LANs, circuit ID concatenated with six-octet system ID of a designated IS to form seven-octet LAN ID (for example, ) is used. Cisco routers use host name instead of system ID (for example, R1.01).

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Level 1, Level 2, and Level 1-2 Routers Level 1 (like OSPF internal nonbackbone routers): Intra-area routing enables ESs to communicate. Level 1 area is a collection of Level 1 and Level 1-2 routers. Level 1 IS keeps copy of the Level 1 area LSDB. Level 1-2 (like OSPF ABR): Intra-area and interarea routing. Level 1-2 IS keeps separate Level 1 and Level 2 LSDBs and advertises default route to Level 1 routers. Level 2 (like OSPF backbone routers): Interarea routing. Level 2 (backbone) area is a contiguous set of Level 1-2 and Level 2 routers. Level 2 IS keeps a copy of the Level 2 area LSDB.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Addressing and Routing Area address is used to route between areas; system ID is not considered. System ID is used to route within an area; area address is not considered.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSI IS-IS Routing Logic Level 1 router: For a destination address, compare the area address to this area. If not equal, pass to nearest Level 1-2 router. If equal, use Level 1 database to route by system ID. Level 1-2 router: For a destination address, compare the area address to this area. If not equal, use Level 2 database to route by area address. If equal, use Level 1 database to route by system ID.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Example: Identifying Systems: OSI Addressing in Networks

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Example: OSI Area Routing

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Route Leaking Available since Cisco IOS Sofware Release 12.0 Helps reduce suboptimal routing by allowing Level 2 information to be leaked into Level 1 Uses up/down bit in Type, Length, and Value (TLV) field

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSI PDUs PDUs between peers: Network PDU = datagram, packet Data-link PDU = frame Examples:

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v IS-IS PDUs IS-IS PDUs are encapsulated directly into a data-link frame. There is no CLNP or IP header in a PDU. IS-IS PDUs are as follows: –Hello (ESH, ISH, IIH) –LSP –PSNP (partial sequence number PDU) –CSNP (complete sequence number PDU)

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSP Header IS Neighbors ES Neighbors ………….. A Link-State Packet Represents Router Router describes itself with an LSP LSP header contents include: –PDU type, length, LSP ID, sequence number, remaining lifetime TLV variable-length fields: –IS neighbors –ES neighbors –Authentication information –.... TLV

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSP Header LSPs are sequenced to prevent duplication of LSPs. Assists with synchronization. Sequence numbers begin at 1. Sequence numbers are increased to indicate the newest LSP. LSPs in LSDB have a remaining lifetime. Allows synchronization. Decreasing timer.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v TLV Type Code Length FieldValue Variable Length Area address1 Area ID length + 1 Areas Intermediate system neighbors 2 Neighbor count + 1 IS neighbors IP internal reachability 128 Number of connected prefixes Connected IP prefixes 4-byte metric, 4-byte prefix, 4-byte mask IP external reachability 130 Number of redistributed prefixes Redistributed IP prefixes 4-byte metric, 4-byte prefix, 4-byte mask LSP TLV Examples Each set of information, called a tuple, includes a type code, length field, and value.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v IS-IS Network Representation Generally, physical links can be placed in the following two groups: –Broadcast: Multiaccess subnetworks that support addressing of a group of attached systems –Point-to-point: Permanent or dynamically established links Only two link-state representations are available in IS-IS: –Broadcast for LANs and multipoint WANs –Point-to-point for all other topologies IS-IS has no concept of NBMA networks.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Implementing Network Types in NBMA When implementing IS-IS in NBMA (such as Frame Relay or ATM): Broadcast mode assumes fully meshed connectivity. In broadcast mode, you must enable CLNS mapping and include the broadcast keyword, in addition to creating IP maps with the broadcast keyword. Point-to-point mode is highly recommended (using subinterfaces).

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Broadcast Mode Used for LAN and multipoint WAN interfaces. Adjacency is recognized through hellos; separate adjacencies for Level 1 and Level 2. Designated IS (DIS) creates a pseudonode and represents LAN. DIS for Level 1 and Level 2 may be different. DIS is elected based on these criteria: –Only routers with adjacencies are eligible. –Highest interface priority. –Highest SNPA (MAC) breaks ties. There is no backup DIS.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSP Representing Routers: LAN Representation

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Level 1 and Level 2 LSPs and IIHs The two-level nature of IS-IS requires separate types of LSPs: Level 1 and Level 2 LSPs. DIS is representative of LAN: –DIS sends pseudo-Level 1 and pseudo-Level 2 LSPs for LAN. –Separate DIS for Level 1 and Level 2. LSPs are sent as unicast on point-to-point networks. LSPs are sent as multicast on broadcast networks. LAN uses separate Level 1 and Level 2 IIHs; sent as multicast. Point-to-point uses a common IIH format; sent as unicast.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BroadcastPoint-to-Point Usage LAN, full-mesh WAN PPP, HDLC, partial-mesh WAN Hello timer 3.3 sec for DIS else 10 sec 10 sec Adjacenciesn (n-1) / 2n-1 Uses DISYesNo IIH type Level 1 IIH, Level 2 IIH Point-to-point IIH Comparing Broadcast and Point-to-Point Topologies

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSP Flooding Single procedure for flooding, aging, and updating of LSPs. Level 1 LSPs are flooded within an area. Level 2 LSPs are flooded throughout the Level 2 backbone. Large PDUs are divided into fragments that are independently flooded. –Each PDU is assigned an LSP fragment number, starting at 0 and incrementing by 1. Separate LSDBs are maintained for Level 1 and Level 2 LSPs.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSDB Synchronization SNP packets are used to ensure synchronization and reliability. –Contents are LSP descriptions PSNP is used for the following: –For acknowledgment of LSPs on point-to-point links –To request missing pieces of LSDB CSNP is used for the following: –Periodically by DIS on LAN to ensure LSDB accuracy –On point-to-point link when the link comes up

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSDB Synchronization: LAN

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSDB Synchronization: Point-to-Point

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LAN Adjacencies Adjacencies are established based on the area address announced in the incoming IIHs and the type of the router.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Example: WAN Adjacencies

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary An NSAP is the OSI network-layer address. Cisco uses the area address (comprising the AFI and area ID), system ID, and NSEL fields. The system ID must be 6 bytes. A NET address is an NSAP with an NSEL value of 0 and is used to identify the device itself. IS-IS defines three types of routers: Level 1, Level 2, and Level 1-2. The area address is used to route between areas; the system ID is used to route within an area. The four types of IS-IS PDUs are hello, LSP, PSNP, and CSNP.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary (Cont.) LSPs are used by routers to describe their characteristics. LSPs contain a header and TLV fields. The header ensures unique sequenced packets; the TLV fields include information about the network and stations attached to the router. IS-IS recognizes two topology types: point-to-point and broadcast. Broadcast networks are LAN interfaces or multipoint WAN interfaces. The two-level nature of IS-IS requires separate types of LSPs: Level 1 and Level 2. Level 1 LSPs are flooded within an area; Level 2 LSPs are flooded throughout the Level 2 backbone.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v