Comparing IS-IS and OSPF

Transcription

1 Comaring IS-IS and OSPF ISP Workshos Last udated 8 th Setember

2 Comaring IS-IS and OSPF Both are Link State Routing Protocols using the Dijkstra SPF Algorithm So what s the difference then? And why do ISP engineers end u arguing so much about which is suerior? 2

3 OSPF Oen Shortest Path First Oen: n Meaning an Oen Standard n Develoed by IETF (OSPF Working Grou) for IP RFC1247 n Current standard is OSPFv2 (RFC2328) Shortest Path First: n Edsger Dijkstra s algorithm for roducing shortest ath tree through a grah Dijkstra, E. W. (1959). A note on two roblems in connexion with grahs. Numerische Mathematik 1:

4 IS-IS Intermediate System to Intermediate System ISO secifies OSI IS-IS routing rotocol for CLNS traffic n A Link State rotocol with a 2 level hierarchical architecture n Tye/Length/Value (TLV) otions to enhance the rotocol RFC 1195 added IP suort n Integrated IS-IS n I/IS-IS runs on to of the Data Link Layer 4

5 IS-IS & OSPF: Similarities Both are Interior Gateway Protocols (IGP) n They distribute routing information between routers belonging to a single Autonomous System (AS) With suort for: n Classless Inter-Domain Routing (CIDR) n Variable Subnet Length Masking (VLSM) n Authentication n Multi-ath n IP unnumbered links 5

6 IS-IS and OSPF Terminology OSPF Host Router Link Packet Designated router (DR) Backu DR (BDR) Link-State Advertisement (LSA) Hello acket Database Descrition (DBD) IS-IS End System (ES) Intermediate System (IS) Circuit Protocol Data Unit (PDU) Designated IS (DIS) N/A (no BDIS is used) Link-State PDU (LSP) IIH PDU Comlete sequence number PDU (CSNP) 6

7 IS-IS and OSPF Terminology (Cont.) OSPF Area Non-backbone area Backbone area IS-IS Sub domain Level-1 (station) Level-2 (area) Area Border Router (ABR) Autonomous System Boundary Router (ASBR) L1L2 (station & area) Any IS 7

8 Transort OSPF uses IP Protocol 89 as transort Data Link Header IP Header OSPF Header OSPF Data IS-IS is directly encasulated in Layer 2 Data Link Header IS-IS Header IS-IS Data 8

9 For Service Providers Which IGP should an ISP choose? n Both OSPF and IS-IS use Dijkstra SPF algorithm n Exhibit same convergence roerties n IS-IS less widely imlemented on router latforms n IS-IS runs on data link layer, OSPF runs on IP layer Why do we kee discussing the merits of each IGP? 9

10 For Service Providers Biggest ISPs tend to use IS-IS why? n In early 1990s, Cisco imlementation of IS-IS was much more stable and reliable than OSPF imlementation ISPs naturally referred IS- IS n Main IS-IS imlementations more tuneable than equivalent OSPF imlementations because biggest ISPs using IS-IS ut more ressure on Cisco to imlement knobs 10

11 For Service Providers Moving forward a decade n Early Cisco OSPF imlementation substantially rewritten Now cometitive with IS-IS in features and erformance n Router vendors wishing a slice of the core market need an IS-IS imlementation as solid and as flexible as that from Cisco Those with IS-IS & OSPF suort tend to ensure they exhibit erformance and feature arity 11

12 How to choose an IGP? OSPF n Rigid area design all networks must have area 0 core, with sub-areas distributed around n Suits ISPs with central high seed core network linking regional PoPs 12

13 How to choose an IGP? IS-IS n Relaxed two level design L2 routers must be linked through the backbone n Suits ISPs with stringy networks, diverse infrastructure, etc, not fitting central core model of OSPF n More flexible than OSPF, but easier to make mistakes too 13

14 Considerations Security n IS-IS runs on link layer n Not ossible to attack the IGP using IP as with OSPF Not deendent on IP addressing n IS-IS s NSAP addressing scheme avoids deendencies on IP as with OSPF Reliability n IS-IS has long been used by the majority of the world s biggest ISPs n Belief that equiment vendors ay more attention to IS- IS reliability, scalability, and features 14

15 More considerations Migration to IPv6 n Adding IPv6 means OSPFv2 and OSPFv3 in network Two rotocols, two sets of identical configuration n IS-IS simly requires the addition of the IPv6 addressfamily Most networks oerate single toology for IPv4 and IPv6 n Is this why there is now RFC5838 describing suort of multile address families in OSPFv3? Vendor suort? 15

16 Comaring IS-IS and OSPF ISP Workshos 16