Link State Routing. In particular OSPF. dr. C. P. J. Koymans. Informatics Institute University of Amsterdam. March 4, 2008
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1 Link State Routing In particular OSPF dr. C. P. J. Koymans Informatics Institute University of Amsterdam March 4, 2008 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
2 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
3 Outline Link State Protocols 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
4 Outline Link State Protocols Basic ideas 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
5 Link State Protocols Basic ideas Link State Protocol A Link State Protocol builds on complete information about the network topology uses Dijkstra s Shortest Path Tree algorithm replaces RIP for more complex topologies has a faster convergence time than distance vector protocols dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
6 Link State Protocols Basic ideas Link State Packets Links State Packets (LSPs) represent the state of a router and its links to the rest of the network This representation works for point to point links Broadcast networks (LANs) and NBMA networks are represented by virtual nodes inside the topology Designated routers (DRs) speak on their behalf dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
7 Link State Protocols Basic ideas Non-broadcast networks NBMA Non-broadcast Multiple Access Full mesh of connectivity Connectivity via elected DR Point-to-Multipoint A subset of the collection of all point-to-point links No full mesh of connectivity No DR is elected dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
8 Link State Protocols Basic ideas Learn to know your neighbours Easy on point to point links Just send out greetings (hello packets) Both ends will become adjacent A little more advanced on broadcast networks Send out hellos using multicast Not all neighbours will become adjacent dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
9 Link State Protocols Basic ideas LSP generation Periodic announcements with a long period (like 30 minutes) Triggered updates when there are changes, which can be Detection of a new neighbour (link or node coming up) Detection of a link or node failure Change of a link cost dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
10 Link State Protocols Basic ideas LSP distribution LSPs are distributed throughout the network Distribution can t use the routing database for distant nodes Why? What is the difference with RIP here? Ordinary flooding is problematic dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
11 Link State Protocols Basic ideas LSP distribution LSPs are distributed throughout the network Distribution can t use the routing database for distant nodes Why? What is the difference with RIP here? Ordinary flooding is problematic It would need a TTL to prevent loops dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
12 Link State Protocols Basic ideas LSP distribution LSPs are distributed throughout the network Distribution can t use the routing database for distant nodes Why? What is the difference with RIP here? Ordinary flooding is problematic It would need a TTL to prevent loops It could cause exponential multiplication of packets dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
13 Link State Protocols Basic ideas LSP distribution LSPs are distributed throughout the network Distribution can t use the routing database for distant nodes Why? What is the difference with RIP here? Ordinary flooding is problematic It would need a TTL to prevent loops It could cause exponential multiplication of packets Smart flooding recognizes identical LSPs dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
14 Link State Protocols Basic ideas LSP distribution LSPs are distributed throughout the network Distribution can t use the routing database for distant nodes Why? What is the difference with RIP here? Ordinary flooding is problematic It would need a TTL to prevent loops It could cause exponential multiplication of packets Smart flooding recognizes identical LSPs Causing propagation to be tree-like dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
15 Outline Link State Protocols Problems and pitfalls 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
16 Link State Protocols Problems and pitfalls Most recent LSP problem LSPs may arrive out of order Therefore we need a mechanism to recognize older packets dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
17 Link State Protocols Problems and pitfalls Most recent LSP problem LSPs may arrive out of order Therefore we need a mechanism to recognize older packets What about using timestamps? dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
18 Link State Protocols Problems and pitfalls Most recent LSP problem LSPs may arrive out of order Therefore we need a mechanism to recognize older packets What about using timestamps? Timestamps cause trouble if clocks are not synchronised or out of order! dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
19 Link State Protocols Problems and pitfalls Most recent LSP problem LSPs may arrive out of order Therefore we need a mechanism to recognize older packets What about using timestamps? Timestamps cause trouble if clocks are not synchronised or out of order! What about using sequence numbers? dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
20 Link State Protocols Problems and pitfalls Most recent LSP problem LSPs may arrive out of order Therefore we need a mechanism to recognize older packets What about using timestamps? Timestamps cause trouble if clocks are not synchronised or out of order! What about using sequence numbers? Sequence numbers need an aging procedure to protect from stale information, for instance when a router reboots and starts from scratch dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
21 Pitfalls Pitfalls Sequence numbers may wrap Sequence number comparison is not a total linear order Cyclic behaviour is possible a c b a < b < c < a 9
22 Link State Protocols Problems and pitfalls 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 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
23 Link State Protocols Problems and pitfalls 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: dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
24 Link State Protocols Problems and pitfalls 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: dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
25 Link State Protocols Problems and pitfalls 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: dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
26 ARPANET lockup ARPANET lockup S xcba P c ba c ba a bc a bc Q b c a b c a R 11
27 Link State Protocols Problems and pitfalls Lockup solution dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
28 Link State Protocols Problems and pitfalls Lockup solution One could use a very large sequence number space dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
29 Link State Protocols Problems and pitfalls Lockup solution One could use a very large sequence number space and wait for timeouts after overflow dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
30 Link State Protocols Problems and pitfalls Lockup solution One could use a very large sequence number space and wait for timeouts after overflow One could use an age (or ttl) field dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
31 Link State Protocols Problems and pitfalls Lockup solution One could use a very large sequence number space and wait for timeouts after overflow One could use an age (or ttl) field and always increase the age (decrease the ttl) by at least one dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
32 Link State Protocols Problems and pitfalls Lockup solution One could use a very large sequence number space and wait for timeouts after overflow One could use an age (or ttl) field and always increase the age (decrease the ttl) by at least one and furthermore increase (decrease) it periodically dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
33 Outline OSPF 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
34 Outline OSPF Rationale 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
35 OSPF Rationale OSPF advantages (1) OSPF Introduces hierarchical routing Supports load balancing Supports subnets Supports unnumbered interfaces/networks Supports point-to-point, broadcast, NBMA and point-to-multipoint networks Supports virtual links for backbone connectivity dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
36 OSPF Rationale OSPF advantages (2) OSPF Has built in authentication Uses efficient multicast for flooding Uses metrics built on cost, per interface Is easily extendable for multicast routing (MOSPF) dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
37 OSPF Rationale OSPF fun :) IETF T-shirt IS IS = 0 Old time IETF versus ISO controversy But OSPF in fact really builds on BBN s research on SPF Early versions of OSI s IS-IS dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
38 OSPF Rationale Some OSPF facts OSPF builds directly upon IP OSPF uses protocol type 89 Current OSPF 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 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
39 Outline OSPF Parameters 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
40 OSPF Parameters 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 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
41 Outline OSPF Databases 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
42 OSPF Databases Consistent LSP databases LSPs must be acknowledged LSPs are queued/flagged for transmission LSPs must time out at about the same time by using triggered distribution of age MaxAge packets which must be honored dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
43 Outline OSPF Network representation and architecture 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
44 OSPF Network representation and architecture (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 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
45 Multicast usage Multicast usage On LANs every router becomes adjacent to the Designated Router R Link State Update DR (BDR) Link State Ack S Multicast to AllDRouters ( ) Multicast to AllSPFRouters ( ) 20
46 Hierarchical routing Hierarchical routing Backbone Area 0 Limbs (non-backbone subsystems) Areas 0 ABRs Backbone (area 0)
47 Virtual Links Area 5 Virtual links Not physically linked to the backbone Virtual link created to a real ABR Behaves as an unnumbered point to point link inside the backbone area Backbone (area 0)
48 OSPF Network representation and architecture Inter Area Summaries IASs are injected by ABRs IASs make use of a hub and spoke topology Summary information is spread RIP -like There are no loops or slow convergence Virtual links maintain the hub and spoke topology Some spokes are point to point links tunneled through an area dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
49 OSPF Network representation and architecture Router roles Backbone router Has at least one interface inside area 0 Internal router Exists completely inside a single area Area Border Router (ABR) Exists inside area 0 and one or more other areas Autonomous System Border Router (ASBR) Participates in other (external) routing protocol dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
50 OSPF Network representation and architecture Area Border Router (ABR) Attaches to multiple areas Runs multiple copies of the basic algorithm, once for each area Summarizes area data towards the backbone Receives summarized data from the backbone about other areas dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
51 OSPF Network representation and architecture Autonomous System Border Router (ASBR) Can be part of any area Interfaces with other routing protocols BGP, RIP, IS-IS,... Injects external routes into OSPF ASBR notion is independent of backbone, internal or ABR router dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
52 OSPF Network representation and architecture Stub(by) area A stub(by) area is an area into which no external routing information is injected by the ABRs It uses a default route for all external destinations A default route is injected by all ABRs If even inter-area summaries are not injected the stub area is called totally stubby dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
53 Outline OSPF packet details 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
54 Outline OSPF packet details OSPF packet header 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
55 OSPF packet details OSPF packet header OSPF packet header Version Type Packet length Router ID Area ID Checksum AuType Authentication dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
56 OSPF packet details OSPF packet header OSPF packet header fields OSPF packet header fields Version 2 Type Packet length Router ID Area ID Checksum AuType Authentication 1 5 (see next slide) total length, including this header ID of packet originating router The area a packet belongs to one s complement checksum Null, Simple or Crypto Pointer to message digest (Crypto) dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
57 OSPF packet details OSPF packet header OSPF packet types OSPF packet types Type Meaning 1 Hello 2 Database Description 3 Link State Request 4 Link State Update 5 Link State Acknowledgement dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
58 Outline OSPF packet details The different kinds of OSPF packets 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
59 OSPF packet details The different kinds of OSPF packets OSPF Hello packet Network Mask HelloInterval Options Rtr Pri RouterDeadInterval Designated Router Backup Designated Router Neighbor #1 Repeated for. each neighbor Neighbor #N dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
60 OSPF packet details The different kinds of OSPF packets a deprecated because of lack of experience with TOS dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70 OSPF Hello packet fields Hello fields Network Mask HelloInterval Options Rtr Pri RouterDeadInterval Designated Router Backup Designated Router Neighbor subnet mask of link interval in seconds between hellos multiple metrics (T a ); stub area (E) router priority, used for DR election interval to consider a silent neighbor dead IP address of designated router IP address of backup designated router (living) neighbor IDs
61 OSPF packet details The different kinds of OSPF packets OSPF DD packet Interface MTU Options I M M S DD sequence number LSA header #1. LSA header #N Partial list of database dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
62 OSPF packet details The different kinds of OSPF packets OSPF DD packet fields DD fields Interface MTU Options I M MS DD sequence number LSA header detection of MTU mismatch same as for Hello packets init bit (first packet) more bit (more packets follow) master/slave bit used for ordering DD packets identification of link state database piece dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
63 OSPF packet details The different kinds of OSPF packets OSPF LS request packet 0 31 LS type Link State ID Advertising Router Multiple occurrences dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
64 OSPF packet details The different kinds of OSPF packets OSPF LS request packet fields LS request fields LS type Link State ID Advertising Router Link type of the LSA ID of (link type specific) part of the LSA Router ID of originating router Together these entries uniquely identify an LSA, but not an LSA instance dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
65 OSPF packet details The different kinds of OSPF packets OSPF LS update packet 0 31 # LSAs (N) LSA #1 N occurrences LSA #N dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
66 OSPF packet details The different kinds of OSPF packets OSPF LS update packet fields LS update fields # LSAs Number of LSAs inside the update packet LSA A complete Link State Advertisement dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
67 OSPF packet details The different kinds of OSPF packets OSPF LS Acknowledgement packet 0 31 LSA header #1 N occurrences LSA header #N dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
68 OSPF packet details The different kinds of OSPF packets OSPF LS acknowledgement packet fields LS acknowledgement fields LSA header A complete Link State Header dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
69 Outline OSPF packet details Link State Advertisement packets 1 Link State Protocols Basic ideas Problems and pitfalls 2 OSPF Rationale Parameters Databases Network representation and architecture 3 OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
70 OSPF packet details Link State Advertisement packets 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 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
71 OSPF packet details Link State Advertisement packets LSA header LS age Options LS type Link State ID Advertising Router LS sequence number LS Checksum length dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
72 OSPF packet details Link State Advertisement packets LSA header fields LSA header fields LS age Options LS type Link State ID Advertising Router LS sequence number LS Checksum Length time in seconds since the LSA was originated same as for Hello packets link type of the LSA (see next slide) ID of (link type specific) part of the LSA router ID of originating router used for most recent check of LSA packets the Fletcher checksum of the LSA (without age) length in bytes of the LSA, including the header dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
73 OSPF packet details Link State Advertisement packets LS types LS Types Type Meaning 1 Router LSA 2 Network LSA 3 Network Summary LSA 4 AS Boundary Router Summary LSA 5 AS External LSA 7 NSSA LSA (see next slide) dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
74 OSPF packet details Link State Advertisement packets NSSA NSSA stands for Not So Stubby Area Support for certain external routes throughout the area itself NSSA uses its own special type (7) These LSAs are translated at the ABR into ordinary external LSAs (type 5) for the rest of the OSPF domain dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
75 OSPF packet details Link State Advertisement packets Link State IDs Link State IDs Type Meaning 1 ID of originating router 2 IP address of the network s DR 3 The destination s IP network address 4 ID of described ASBR 5 The destination s IP network address dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
76 OSPF packet details Link State Advertisement packets Router LSA without header (LS type = 1) repeated # links times V E B 0 # links Link ID Link Data Type # TOS metric } TOS 0 TOS metric # TOS times 1 Originated by every router Flooded throughout the area(s) 1 For backward compatibility dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
77 Router LSA fields OSPF packet details Link State Advertisement packets Router LSA fields V Router is virtual link endpoint E Router is AS Boundary Router B Router is Area Border Router # links Number of router links described Link ID ID of the connected network Link Data Extra information on network Type Type of connected network # TOS Number of extra TOSs (usually 0) metric Cost of link dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
78 OSPF packet details Link State Advertisement packets Router LSA type field Router LSA type field Type Meaning 1 Point-to-point link 2 Transit network 3 Stub network 4 Virtual link dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
79 OSPF packet details Link State Advertisement packets Router LSA Link ID field Router LSA Link ID field Type Meaning 1 ID of neighbor router 2 IP address of designated router 3 IP (sub)network number 4 ID of neighbor router dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
80 OSPF packet details Link State Advertisement packets Router LSA Link Data field Router LSA Link Data field Type Meaning 1 Originating router s interface IP address 2 Originating router s interface IP address 3 Connected (sub)network mask 4 Originating router s interface IP address dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
81 OSPF packet details Link State Advertisement packets Network LSA without header (LS type = 2) 0 31 Network Mask Attached Router } Repeated for each attached router Originated by Designated Router Flooded throughout the area dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
82 OSPF packet details Link State Advertisement packets Network LSA fields Network LSA fields Network Mask Attached Router (sub)network mask Router ID of router(s) on network dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
83 OSPF packet details Link State Advertisement packets Summary LSA without header (type = 3, 4) Network Mask 2 0 metric TOS TOS metric } for each desired TOS 3 Originated by Area Border Router Flooded throughout the area(s) 2 Only relevant for type 3 3 For backward compatibility dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
84 OSPF packet details Link State Advertisement packets Summary LSA fields Summary LSA fields Network Mask a metric Address mask for the advertised destination Cost to advertised destination a Only relevant for type 3 dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
85 OSPF packet details Link State Advertisement packets AS External LSA without header (LS type 5) Network Mask E 0 metric Forwarding address External Route Tag E TOS TOS metric Forwarding address External Route Tag for each desired TOS 4 Originated by AS Border Router Flooded throughout the whole AS 4 For backward compatibility dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
86 OSPF packet details Link State Advertisement packets AS External LSA fields AS External LSA fields Network Mask E metric Forwarding address External Route Tag Address mask for the advertised destination External cost is higher than internal cost Cost to advertised destination Comparable to Next Hop in RIP Comparable to Route Tag in RIP dr. C. P. J. Koymans (UvA) Link State Routing March 4, / 70
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