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Choosing an IGP 101. James Blessing, NetMCR, 9 th Feb PDF


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Choosing an IGP 101 James Blessing, NetMCR, 9 th Feb 2017 Why an IGP? Get packets round your network This is 101, there are other things you can do (like ibgp in DC, see RFC7938) Networks change (usually)
Choosing an IGP 101 James Blessing, NetMCR, 9 th Feb 2017 Why an IGP? Get packets round your network This is 101, there are other things you can do (like ibgp in DC, see RFC7938) Networks change (usually) Choices, choices, choices Static Routes RIP EIGRP OSPF IS-IS Static Routes Go over there Everyone uses it (somewhere) Could be used with SDN* *joke RIP Created in 1988 in RFC 1058, updated in RFC1388 (RIPv2 93) and RFC2483 (98) Distance Vector based solution Update all routes periodically High bandwith Slow conversion times Poor control of DV Simple to Implement, low computational requirement Infinity starts at 16 RIPv2 added support for CIDR/VSM EIGRP CISCO Proprietary Metric for each link based on delay and bandwidth (and other things) General form: Metric = [K1 * Bw + K2 * Bw/(256 - Load) + K3 * Delay] * [K5/(Reliability + K4)] Works for stable networks, only updates on changes Redistributing Metric can cause issues with other IGPs Comparing ISIS and OSPF p Both are Link State Routing Protocols using the Dijkstra SPF Algorithm p So what s the difference then? p And why do ISP engineers end up arguing so much about which is superior? What is IS-IS? Intermediate System to Intermediate System An IS is ISO terminology for a router IS-IS was originally designed for use as a dynamic routing protocol for ISO CLNP, defined in the ISO standard Later adapted to carry IP prefixes in addition to CLNP (known as Integrated or Dual IS-IS) as described in RFC 1195 Predominantly used in ISP environment ITU-T mandates it for SDH What is OSPF? Open Shortest Path First Link State Protocol using the Shortest Path First algorithm (Dijkstra) to calculate loop-free routes Used purely within the TCP/IP environment Designed to respond quickly to topology changes but using minimal protocol traffic Used in both Enterprise and SP Environment Timeline IS-IS 1978ish New Arpanet Algorithm 1986 to 90 Decnet Phase V 1987 ISO (IS-IS) 1990 RFC 1195 (Integrated IS- IS) 2008 RFC5308 adds IPv6 support and RFC5120 adds Multi-Topology Routing support OSPF 1987 Development by IETF 1989 OSPFv1 RFC OSPFv2 RFC RFC2740 introduced OSPFv3 (for IPv6) 2008 replaced by RFC5340 IS-IS & OSPF: Similarities Both are Interior Gateway Protocols (IGP) They distribute routing information between routers belonging to a single Autonomous System (AS) With support for: Classless Inter-Domain Routing (CIDR) Variable Subnet Length Masking (VLSM) Authentication Multi-path IP unnumbered links IS-IS and OSPF Terminology OSPF Host Router Link Packet Designated router (DR) Backup DR (BDR) Link-State Advertisement (LSA) Hello packet Database Description (DBD) ISIS 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 Complete sequence number PDU (CSNP) IS-IS and OSPF Terminology (Cont.) OSPF Area Non-backbone area Backbone area Area Border Router (ABR) Autonomous System Boundary Router (ASBR) ISIS Sub domain (area) Level-1 area Level-2 Sub domain (backbone) L1L2 router Any IS Transport OSPF uses IP Protocol 89 as transport Data Link Header IP Header OSPF Header OSPF Data IS-IS is directly encapsulated in Layer 2 Data Link Header IS-IS Header IS-IS Data Which should I choose? Both OSPF and ISIS use Dijkstra SPF algorithm Exhibit same convergence properties ISIS less widely implemented on router platforms ISIS runs on data link layer, OSPF runs on IP layer Biggest ISPs tend to use ISIS why? In early 90s, Cisco implementation of ISIS was much more solid than OSPF implementation ISPs naturally preferred ISIS Main ISIS implementations more tuneable than equivalent OSPF implementations because biggest ISPs using ISIS put more pressure on Cisco to implement knobs Moving forward a decade Early Cisco OSPF implementation substantially rewritten Now competitive with ISIS in features and performance Router vendors wishing a slice of the core market need an ISIS implementation as solid and as flexible as that from Cisco Those with ISIS & OSPF support tend to ensure they exhibit performance and feature parity How to choose an IGP? OSPF Rigid area design all networks must have area 0 core, with sub-areas distributed around Suits networks with central high speed core network linking regional PoPs Teaches good routing protocol design practices How to choose an IGP? ISIS Relaxed two level design L2 routers must be linked through the backbone Suits ISPs with stringy networks, diverse infrastructure, etc, not fitting central core model of OSPF More flexible than OSPF, but easier to make mistakes too Other considerations ISIS runs on link layer Not possible to attack the IGP using IP as with OSPF ISIS s NSAP addressing scheme avoids dependencies on IP as with OSPF Because biggest ISPs use ISIS, major router vendors tend to apply new optimisation features before they are added to OSPF There is an RFC that says it should be OSPF - it s wrong (probably) Mashup Noting to stop you using different IGP in different parts of network Questions?
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