IS-IS i Detail ISP Workshops These materials are licesed uder the Creative Commos Attributio-NoCommercial 4.0 Iteratioal licese (http://creativecommos.org/liceses/by-c/4.0/) Last updated 27 th November 2016 1
Ackowledgemets p This material origiated from the Cisco ISP/IXP Workshop Programme developed by Philip Smith & Barry Greee p Use of these materials is ecouraged as log as the source is fully ackowledged ad this otice remais i place p Bug fixes ad improvemets are welcomed Please email workshop (at) bgp4all.com Philip Smith 2
IS-IS Stadards History p ISO 10589 specifies OSI IS-IS routig protocol for CLNS traffic A Lik State protocol with a 2 level hierarchical architecture Type/Legth/Value (TLV) optios to ehace the protocol p RFC 1195 added IP support Itegrated IS-IS I/IS-IS rus o top of the Data Lik Layer 3
IS-IS Stadards History p RFC5308 adds IPv6 address family support to IS-IS p RFC5120 defies Multi-Topology cocept for IS-IS Permits IPv4 ad IPv6 topologies which are ot idetical (Required for a icremetal roll-out of IPv6 o existig IPv4 ifrastructure) 4
ISIS Levels p IS-IS has a 2 layer hierarchy Level-2 (the backboe) Level-1 (the areas) p A router ca be Level-1 (L1) router Level-2 (L2) router Level-1-2 (L1L2) router 5
ISIS Levels p Level-1 router Has eighbours oly o the same area Has a level-1 LSDB with all routig iformatio for the area p Level-2 router May have eighbours i the same or other areas Has a Level-2 LSDB with all routig iformatio about iter-area p Level-1-2 router May have eighbours o ay area. Has two separate LSDBs: level-1 LSDB & level-2 LSDB 6
Backboe & Areas p IS-IS does ot have a backboe area as such (like OSPF) p Istead the backboe is the cotiguous collectio of Level-2 capable routers p IS-IS area borders are o liks, ot routers p Each router is idetified with a uique Network Etity Title (NET) NET is a Network Service Access Poit (NSAP) where the - selector is 0 (Compare with each router havig a uique Router-ID with IP routig protocols) 7
Example: L1, L2, ad L1L2 Routers L1-oly L1L2 Area 3 Area 2 L1L2 L1-oly L1L2 L2-oly Area 4 Area 1 L1L2 L1L2 L1-oly L1-oly 8
NSAP ad Addressig IDP DSP AFI IDI High Order DSP System ID NSEL Variable legth Area address 6 bytes 1 byte p NSAP: Network Service Access Poit Total legth betwee 8 ad 20 bytes Area Address: variable legth field (up to 13 bytes) System ID: defies a ES or IS i a area. NSEL: N-selector; idetifies a etwork service user (trasport etity or the IS etwork etity itself) p NET: the address of the etwork etity itself 9
Addressig Commo Practices p ISPs typically choose NSAP addresses thus: First 8 bits pick a umber (usually 49) Next 16 bits area Next 48 bits router loopback address Fial 8 bits zero p Example: NSAP: 49.0001.1921.6800.1001.00 Router: 192.168.1.1 (loopback) i Area 1 10
Addressig & Desig Practices p ISPs usually oly use oe area Multiple areas oly come ito cosideratio oce the etwork is several hudred routers big p NET begis with 49 Private address rage p All routers are i L2 oly Note that Cisco IOS default is L1L2 Set L2 uder IS-IS geeric cofiguratio (ca also be doe per iterface) 11
Typical ISP Desig 49.0001.1921.6800.1004.00 PoP 2 49.0001.1921.6800.1006.00 PoP 3 49.0001.1921.6800.1003.00 49.0001.1921.6800.1007.00 PoP 4 49.0001.1921.6800.1002.00 PoP 1 49.0001.1921.6800.1008.00 49.0001.1921.6800.1001.00 All routers are i L2 oly ad oly oe area is used 12
Adjacecies p Hello Protocol Data Uits (PDUs) are exchaged betwee routers to form adjacecies ISIS adjacecy through IIH p Area addresses are exchaged i IIH PDUs Itermediate-System to Itermediate System Hello PDUs (PDU is IS-IS equivalet of a packet) 13
Lik State PDU (LSP) p Each router creates a LSP ad floods it to eighbours p A level-1 router will create level-1 LSP(s) p A level-2 router will create level-2 LSP(s) p A level-1-2 router will create level-1 LSP(s) ad level-2 LSP(s) 14
The IS-IS LSP p LSPs have a Fixed Header ad TLV coded cotets p The LSP header cotais LSP-id (Sequece umber) Remaiig Lifetime (Checksum) Type of LSP (level-1, level-2) Attached bit (Overload bit) p The LSP cotets are coded as TLV (Type, Legth, Value) Area addresses IS eighbours Autheticatio Iformatio 15
Lik State Database Cotet p Each router maitais a separate LSDB for level-1 ad level-2 LSPs p The LSDB cotais: LSP headers ad cotets SRM bits: set per iterface whe router has to flood this LSP SSN bits: set per iterface whe router has to sed a PSNP for this LSP 16
Floodig of LSPs p New LSPs are flooded to all eighbors p All routers get all LSPs p Each LSP has a sequece umber p There are 2 kids of floodig: Floodig o a p2p lik Floodig o LAN 17
Floodig o a p2p lik p Oce the adjacecy is established both routers sed CSNP packet p Missig LSPs are set by both routers if ot preset i the received CSNP p Missig LSPs may be requested through PSNP 18
Floodig o a LAN p Each LAN has a Desigated Router (DIS) p The DIS has two tasks Coductig the floodig over the LAN Creatig ad updatig a special LSP describig the LAN topology (Pseudoode LSP) p DIS electio is based o priority Best practice is to select two routers ad give them higher priority the i case of failure oe provides determiistic backup for the other Tie break is by the highest MAC address 19
Floodig o a LAN p DIS coducts the floodig over the LAN p DIS multicasts CSNP every 10 secods p All routers o the LAN check the CSNP agaist their ow LSDB (ad may ask specific re-trasmissios with PSNPs) 20
Complete Sequece Number PDU p Describes all LSPs i your LSDB (i rage) p If the LSDB is large, multiple CSNPs are set p Used o 2 occasios: Periodic multicast by DIS (every 10 secods) to sychroise the LSDB over LAN subets O p2p liks whe lik comes up 21
Partial Sequece Number PDUs p PSNPs Exchaged o p2p liks (ACKs) p Two fuctios Ackowledge receipt of a LSP Request trasmissio of latest LSP p PSNPs describe LSPs by its header LSP idetifier Sequece umber Remaiig lifetime LSP checksum 22
Network Desig Issues p As i all IP etwork desigs, the key issue is the addressig lay-out p ISIS supports a large umber of routers i a sigle area p Whe etwork is so large requirig the use of areas, use summary-addresses p >400 routers i the backboe is quite doable 23
Network Desig Issues p Lik cost Default o all iterfaces is 10 (Compare with OSPF which sets cost accordig to lik badwidth) Maually cofigured accordig to routig strategy p Summary address cost Equal to the best more specific cost Plus cost to reach eighbour of best specific p Backboe has to be cotiguous Esure cotiuity by redudacy p Area partitioig Desig so that backboe ca NOT be partitioed 24
Scalig Issues p Areas vs. sigle area Use areas where p sub-optimal routig is ot a issue p areas with oe sigle exit poit p Start with L2-oly everywhere Future implemetatio of level-1 areas will be easier Backboe cotiuity is esured from start 25
IS-IS i Detail ISP Workshops 26