IS-IS

1. Connectionless Network Service (CLNS)

   1. Intro

      1. Based on OSI suite

      2. Layer 3 protocol is CLNP

      3. Originally developed for DECnet Phase V

   2. ES-IS Protocol

      1. Hosts are referred to as 'end systems'. Routers are 'Intermediate systems'

      2. ES-IS identifies the area prefix to end systems, creates adjacencies with ES's, and creates data link to network address mappings

      3. End system hellos are sent to well-know addresses by End systems to announce their presence

      4. routers transmit intermediate system hellos to well-known addresses

      5. ES's listen to ISH's and randomly pick an IS to forward all their packets

      6. Routers use IS-IS hellos (IIH's) to establish and maintain adjacencies between IS's

   3. Addressing

      1. Network service access point (NSAP) addresses - OSI address

         1. Area address

            • Authority Format Identifier (AFI) - Typically set to 49

            • IDI (initial domain identifier)

            • High Order Domain Specific part

            • IDI and HODSP typically combined to form an area address or 'prefix'

         2. system id

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            • 6 bytes on cisco equipment

            • MAC address is correct length and ensures uniqueness

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         1. NSAP Selector (NSEL)

            • one octet

            • Identifies upper layer process, almost like a port in IP

            • set to 0 for the router, or the device itself

            • Network Entity Title (NET) is NSAP w/ NSEL set to 0

         2. example NET - 49.0001.0015.5e00.1234.00

            • AFI of 49

            • area 1

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            • systemID of 0015.5e00.1234

            • NSEL of 0

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      1. Subnetwork point of attachment (snpa)

         1. Layer 2 equivalent in OSI

         2. assigned by:

            • MAC on Ethernet

            • virtual circuit id on atm or x.35

            • dlci on frame relay

            • 'hdlc' on hdlc devices

      2. circuit ID

         1. the actual interface

         2. 1 octet

         3. for point-to-point, snpa is used

         4. on Ethernet, circuit ID is tagged onto the system ID of the designated IS to from a 7 byte LAN ID. Cisco uses the router hostname instead of circuit ID

      3. link - physical medium

2. IS-IS routing

   1. Level 1 - Intra-area

      1. Enables ES's to communicate

      2. based on System ID's

      3. L1 routers are only aware of the routers in the area - they pass traffic destined for another area to the closest L1/L2 router. Note that this can result in asymmetrical routing and sub-optimal path selection

      4. Collects system ID's and adjacencies of all devices in the area and then uses spf to find the best path between devices

   2. Level 2 - Inter-area and backbone

      1. Based on Area ID's. System ID's are not considered

      2. L2 routers exchange area information and use spf to compute best paths

   3. Level 1-2 - Routers that sit on the boarder of an area

      1. form adjacencies on both levels

      2. maintain separate adjacencies and topology for both levels

      3. default route injected into the area by these routers

   4. Level 3 - Inter-domain routing

      1. not accomplished by IS-IS

      2. OSI specifies Inter-domain Routing Protocol - not implemented by cisco

      3. normally accomplished through BGP

   5. Designated router

      1. broadcast link is modeled as a pseudo-node that connects all attached routers in a star-shape

      2. pseudo-node is represented by the Designated IS

      3. Selection criteria

         1. highest priority

            • default of 64

            • configurable from 0 to 127 using isis priority pri [level-1 | level-2] interface command

         2. highest SNPA (MAC address on Ethernet)

      4. all routers establish adjacencies with all routers + the DIS. No backup DIS

      5. DIS can be preempted

      6. All routers just advertise adjacency with the pseudo-node - this includes the DIS

      7. hello interval is 3 times faster by default for DIS

   6. Route leaking

      1. helps avoid asymmetric routing and reduce sub-optimal routing by leaking L2 routes into L1

      2. route-leak bit set in TLV to prevent routing loops - L1/L2 router does not re-advertise leaked routes

   7. OSI routing table

      1. Routers synch their LSDB, then run spf for each level (L1 and L2)

      2. Once the routes to each IS are determined, partial route calculations are computed to each ES

      3. IP prefixes are treated as ES's and use PRCs

3. OSI PDU's

   1. Layer 2 = data-link PDU

   2. Layer 3 = network PDU

   3. ES-IS and IS-IS use data-link PDU's

   4. Uses TLVs to carry information

   5. Types of PDU

      1. Hello PDU - establish and maintain adjacencies

         1. AllL1IIS MAC 0180.c200.0014

         2. AllL2IIS MAC 0180.c200.0015

         3. dead timer is product of hello multiplier and hello time

         4. default hello is 10 sec

         5. default multiplier is 3

      2. LSP (link state packet) - distributes link-state info

         1. header

            • PDU type and length

            • LSP ID

            • LSP sequence number

            • LSP's remaining lifetime (starts at 1200 seconds by default)

         2. TLV's

            • router's neighbor ISs

            • neighbor ESs

            • Authentication information

            • Attached IP subnets

         3. flooding

            • LSP's are flodded to all adjacent nei except the one that sent it.

            • lsp's are identified by the originator's system ID and an LSP fragment number (in case the LSP exceeds the MTU)

            • LSP's with an invalid checksum are re-flooded with and expired lifetime age

            • valid LSP's are acknowledged and given a lifetime of 1200 sec

            • expired LSP's are kept 60 seconds and then flooded as expired

      3. Complete sequence number PDU (CSNP)

         1. transmitted between DIS and all neighbors to compare LSDB

         2. multicast every 10 seconds on LAN

         3. sent when link comes up on point to point

      4. Partial sequence number PDU (PSNP)

         1. used to acknowledge receipt of LSP's and to request transmission of missing or newer LSP's

4. IS-IS in different network types

   1. point-to-point networks - IIH's are unicast

   2. broadcast networks - IIH's are multicast from the DIS

   3. NBMA

      1. IS-IS does not work in NBMA.

      2. configure point-to-point sub-interfaces.

      3. IOS automatically assumes a full mesh exists in broadcast mode

      4. be sure to use broadcast keyword when configuring static mappings to remote IPs through a local DLCI on frame relay so that multicast packets will be sent

5. Configuring IS-IS

   1. clns routing - global and interface config commands (both needed)

   2. router isis [process tag] - global command

   3. net NET - router command sets the network entity title

   4. ip router isis [process tag] - interface config command

      1. enables IS-IS on that interface for adjacencies

      2. also enables advertisements of subnets on that interface, so must also be run on loopback interfaces (no network command exists in IS-IS)

   5. Change router level

      1. all routers are L1/L2 by default

      2. change with is-type {level-1 | level-1-2 | level-2-only} router config command

      3. for L1/L2 routers, you can also set each interface with the isis circuit-type interface config command

   6. Set the link metric

      1. all links have a metric of 10 by default

      2. isis metric metric {level-1 | level-2} interface command

      3. can be set from 1 to 63

      4. only cost is used by default, but delay, expense, and error are also implemented (although not by cisco)

   7. summarize routes with summary-address address mask [level-2 | level-2 | level-1-1] [tag tag] [metric metric] router configuration command. Level 2 is default if not specified

6. Verifying and troubleshooting

   1. routing information

      1. show ip protocols - shows active routing protocols

      2. show ip route isis

   2. CLNS

      1. show clns

      2. show clns [process-tag] protocol

      3. sh clns interface

      4. sh clns neighbors

   3. troubleshooting

      1. show isis [process-tag] route - shows L1 routes

      2. sh clns route [nsap] - shows L2 routes

      3. sh isis [process-tag] database

      4. sh isis topology

      5. show isis spf-log

      6. debug isis adjacencies-packets

      7. debug isis spf-statistics

      8. debug isis update-packets

7. Advantages and disadvantages

   1. Advantages

      1. groups all updates and sends them as one LSP - this makes it capable of scaling up to 1000 routers per area

      2. Partial Route Calculations reduce CPU usage

      3. converges faster than ospf using default timers

      4. easily extended through new TLVs

   2. disadvantages

      1. not optimized for IP like OSPF

      2. only one area type, unlike OSPF's stubby area's and such

      3. IS-IS metric defaults to 10 on all interfaces regardless of bandwidth

      4. expertise and documentation are harder to find for IS-IS

8. Sources

   1. Cisco Press BSCI Authorized Self-study guide

   2. Sybex CCNP complete

   3. Cisco Press BSCI Official Exam Certification Guide

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LSDB的同步

OSPF分组

IS-IS分组

Hello

Hello（ESH、ISH）

DBD

CSNP

LSR

PSNP

LSU

LSP（L1、L2）

LSAck

PSNP

CNSP和PSNP的格式相同，都包含LSP摘要信息。主要差别是，CSNP包含LSDB中所有LSP的摘要信息，而PSNP只包含部分LSP

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OSI的NSAP addresss对应了TCP/IP中的2，3层地址

                  OSI                          TCP/IP

2层      System address            MAC address

3层        Area assress                IP address