Link State Routing. In particular OSPF (Open Shortest Path First)

Transcription

1 Link State Routing In particular OSPF (Open Shortest Path First)

2 Link State Protocol Complete information about network topology Uses Dijkstra s Shortest Path Tree algorithm Replaces RIP for more complex topologies Faster convergence time

3 Link State Packets LSPs represent the state of a router and its links to the rest of the network Works for point to point links Broadcast networks (LANs) and NBMA networks are represented by virtual nodes inside the topology Designated routers speak on their behalf

4 Learn to know your neighbours Easy on point to point links Just send out greetings (hello packets) to the other side A little more advanced on broadcast networks Send out HELLOs using multicast Not all neighbours will become adjacent

5 LSP generation Periodic announcements (long period) Triggered updates when there are changes New neighbour Change of link cost Link failure

6 LSP distribution LSPs are distributed throughout the network Cannot use the routing database (chicken/egg problem) Ordinary flooding is problematic Needs TTL to prevent loops Generates exponential behavior Smart flooding recognizes identical LSPs

7 Most recent LSP problem LSPs may arrive out of order Needs a mechanism to recognize older packets Timestamps? Can lock up if clocks are not synchronised or out of order! Sequence numbers with an aging procedure

8 ARPANET lockup Sequence numbers may wrap Sequence number comparison is not a total linear order Cyclic behaviour is possible c a < b < c < a a b

9 Intermezzo The devilish dice There are three dice, A, B and C such that A is better than B B is better than C C is better than A A: B: C:114444

10 Lockup solution Large sequence number space Wait for timeouts after overflow Age field Always increase at least by one Increase periodically

11 Consistent LSP databases LSPs must be acknowledged LSPs are queued/flagged for transmission LSPs must time out at about the same time triggered distribution of age MaxAge packets which must be honored

12 OSPF advantages (1) Introduces hierarchical routing Supports load balancing Supports subnets Supports unnumbered interfaces/networks

13 OSPF advantages (2) Built in authentication Uses multicast for flooding Metrics built on cost, per interface, per TOS Easily extendable for multicast routing (MOSPF)

14 OSPF fun :) IETF T-shirt IS - IS = 0 Old time IETF versus ISO controversy But OSPF really builds on BBN s research on SPF Early versions of OSI s IS-IS

15 Some OSPF facts OSPF builds directly upon IP Uses protocol type 89 Current version is 2 RFC 2328 (Moy, 1998) RFC 2740 (OSPF for IPv6) -> version 3 OSPF uses LSA (Link State Advertisement) terminology in stead of LSP

16 Hierarchical routing Backbone Area 0 Limbs (non-backbone subsystems) Areas 0 Backbone (area 0)

17 Router roles Backbone router Only inside area 0 Internal router Only inside a single area 0 Area Border Router (ABR) Inside area 0 and one or more other areas Autonomous System Border Router (ASBR) Participates in other (external) routing protocol

18 OSPF packet types Hello Database Description Link State Request Link State Update Link State Acknowledgement

19 (Backup) Designated Router A Designated Router (DR) and a Backup Designated Router (BDR) is elected on every multi-access network using Hello packets The (B)DR represents the network as a (virtual) node and acts on its behalf The DR/BDR election process is sticky The priority of routers can be configured

20 Multicast usage On LANs every router becomes adjacent to the Designated Router Link State Update Link State Ack R DR (BDR) S Multicast to Multicast to AllDRouters ( ) AllSPFRouters ( )

21 Timers and Overflow These must be the same for all OSPF routers HelloInterval (default 10 sec) RouterDeadInterval (default 40 sec) In case of database overflow external routing information is dropped first

22 Link State Advertisements Multiple LSAs may be found inside a Link State Update packet Every LSA consists of LSA Header (20 bytes) LSA type specific content

23 LSA types Router LSA Network LSA Network Summary LSA AS Boundary Router Summary LSA AS External LSA

24 OSPF packet header version packet type packet length router ID area ID checksum authentication type authentication data

25 OSPF hello packet network mask hello interval options priority router dead interval designated router backup designated router neighbor #1 neighbor #N

26 OSPF DD packet MTU options flags DD sequence number LSA header #1 LSA header #N

27 OSPF LS request packet LS type #1 link state ID #1 advertising router #1 LS type #N link state ID #N advertising router #N

28 OSPF LS update packet Number of LSA s LSA #1 LSA #N

29 OSPF LS Ack packet LSA header #1 LSA header #N

30 LS(A) header LS age options LS type link state ID advertising router LS sequence number LS checksum length

31 Router LSA (LS type = 1) reserved E B reserved number of links link ID link data link type # of TOS TOS 0 metric TOS #1 reserved metric #1 TOS #N reserved metric #N repeat for every link Originated by every router Flooded throughout the area(s)

32 Network LSA (LS type = 2) network mask attached router #1 attached router #N Originated by Designated Router Flooded throughout the area

33 Summary LSA (type = 3, 4) network mask for type 3 (0 for type 4) TOS #1 metric #1 TOS #N metric #N Originated by Area Border Router Flooded throughout the area(s)

34 External LSA (LS type 5) network mask E TOS #1 metric #1 forwarding address #1 external route tag #1 E TOS #N metric #N forwarding address #N external route tag #N Originated by AS Border Router Flooded throughout the whole AS