Routing II: Protocols RIP, EIGRP, OSPF, PNNI, ISIS QoS Routing and Traffic Engineering

1
Routing II Protocols (RIP, EIGRP, OSPF, PNNI,
IS-IS)QoS Routing and Traffic Engineering

• Shivkumar Kalyanaraman
• Rensselaer Polytechnic Institute
• shivkuma_at_ecse.rpi.edu
• Based in part upon slides of Prof. Raj Jain
  (OSU), S. Keshav (Cornell),
• J. Kurose (U Mass), J. Rexford (Princeton)

2
Overview

• RIP, RIPv2, EIGRP
• OSPF, PNNI, IS-IS LS efficiency robustness
• Link state distribution, DB synchronization,
  NBMAs etc
• Refs Chap 16,14
• Books Interconnections by Perlman, OSPF by
  John Moy, Routing in Internet by Huitema.
• Reference RFC 2328 OSPF Version 2 In HTML
• Reading Notes for Protocol Design, E2e
  Principle, IP and Routing In PDF
• Reading Routing 101 Notes on Routing In PDF
  In MS Word
• Reference Tsuchiya, "The Landmark Hierarchy A
  New Hierarchy for Routing in Very Large Networks"

3
RIP Routing Information Protocol

• Uses hop count as metric (max 16 is infinity)
• Tables (vectors) advertised to neighbors every
  30 s.
• Each advertisement upto 25 entries
• No advertisement for 180 sec neighbor/link
  declared dead
• routes via neighbor invalidated
• new advertisements sent to neighbors (Triggered
  updates)
• neighbors in turn send out new advertisements (if
  tables changed)
• link failure info quickly propagates to entire
  net
• poison reverse used to prevent ping-pong loops
  (infinite distance 16 hops)

4
RIPv1 Problems (Continued)

• Split horizon/poison reverse does not guarantee
  to solve count-to-infinity problem
• 16 infinity gt RIP for small networks only!
• Slow convergence
• Broadcasts consume non-router resources
• RIPv1 does not support subnet masks (VLSMs)
• No authentication

5
RIPv2

• Why ? Installed base of RIP routers
• Provides
• VLSM support
• Authentication
• Multicasting
• Wire-sharing by multiple routing domains,
• Tags to support EGP/BGP routes.
• Uses reserved fields in RIPv1 header.
• First route entry replaced by authentication
  info.

6
E-IGRP (Interior Gateway Routing Protocol)

• CISCO proprietary successor of RIP (late 80s)
• Several metrics (delay, bandwidth, reliability,
  load etc)
• Uses TCP to exchange routing updates
• Loop-free routing via Distributed Updating Alg.
  (DUAL) based on diffused computation
• Freeze entry to particular destination
• Diffuse a request for updates
• Other nodes may freeze/propagate the diffusing
  computation (tree formation)
• Unfreeze when updates received.
• Tradeoff temporary un-reachability for some
  destinations

7
Link State vs. Distance Vector

• Link State (LS) advantages
• More stable (aka fewer routing loops)
• Faster convergence than distance vector
• Easier to discover network topology,
  troubleshoot network.
• Can do better source-routing with link-state
• Type Quality-of-service routing (multiple route
  tables) possible
• Caveat With path-vector-type (paths instead of
  distances) DV routing, these differences blur

8
Link State Protocols

• Key Create a network map at each node.
• 1. Node collects the state of its connected links
  and forms a Link State Packet (LSP)
• 2. Flood LSP gt reaches every other node in the
  network and everyone now has a network map.
• 3. Given map, run Dijkstras shortest path
  algorithm (SPF) gt get paths to all destinations
• 4. Routing table next-hops of these paths.
• 5. Hierarchical routing organization of areas,
  and filtered control plane information flooded.

9
Link State Issues

• Reliable Flooding sequence s, age
• LSA types, Neighbor discovery and maintainence
  (hello)
• Efficiency in Broadcast LANs, NBMA, Pt-Mpt
  subnets designated router (DR) concept
• Areas and Hierarchy
• Area types Normal, Stub, NSSA filtering
• External Routes (from other ASs), interaction
  with inter-domain routing.
• Advanced topics incremental SPF algorithms

10
Reliable Flooding
11
Topology Dissemination

• A.k.a LSP distribution
• 1. Flood LSPs on links except incoming link
• Require at most 2E transfers for n/w with E edges
• 2. Sequence numbers to detect duplicates
• Why? Routers/links may go down/up
• Issue wrap-around, larger sequence number is not
  the most recent!

12
Sequence Number Space Organization

• Circular space S1 gt S2 gt S3 gt S1
• Accidental bit errors in switch memory caused
  this problem in ARPANET
• Lollipop sequence Start with S0, increment till
  you reach circle and then view it as a circular
  space
• No ambiguity in lollipop handle
• Linear space OSPFv2.
• If Smax reached, expicitly delete Smax LSA before
  wrapping around

13
Topology Dissemination (Continued)

• Checksum field
• Drop packet if in error, get retransmission from
  neighbor
• Age field (similar to TTL)
• Number of seconds since LSA originated
• Periodically incremented after acceptance
• Originating router refreshes LSA after 30 min
• Delete if Age MaxAge
• Low age field large seq gt that LSA is
  flapping or frequently changing

14
Recovering from a partition

• On partition, LSP databases can get out of synch
• Databases described by database descriptor
  records
• Routers on each side of a newly restored link
  talk to each other to update databases (determine
  missing and out-of-date LSPs) gt selective
  synchronization

15
LSA-types, Neighbor flooding Adjacencies in
Different Subnets
16
OSPF Router-LSA Scenario
17
Neighbor Discovery Relationship

• Every OSPF router sends out 'hello' packets
• Hello packets used to determine if neighbor is
  up
• Hello packets sent periodically (short
  intervals)
• HelloInterval 10s (in example)
• Assumes neighbor dead if no response within
• RouterDeadInterval 40s (in example)
• This is also called an adjacency
• Note that adjacency is a logical routing
  relationship and is more than physical
  connection.
• It consumes bandwidth and computation resources
• Becomes an issue if large number of adj need to
  be maintained

18
Neighbor

• Once an adjacency is established, trade
  information
• Neighbor relationship is bi-directional as a
  result of OSPF hello packets
• Local topology information is packaged in a "link
  state announcement (LSA)
• Multiple types of LSAs (detail later)
• Initial DB synchronization
• New announcements are sent ONCE, and only updated
  if there's a change
• Or every 45mins...

19
Hello Packet Format
20
Router-LSA
21
Database Synchronization

• LS Database (LSDB) collection of the Link State
  Advertisements (LSAs) accepted at a node.
• This is the map for Dijkstra algorithm
• When the connection between two neighbors comes
  up, the routers must wait for their LS DBs to be
  synchronized.
• Else routing loops and black holes due to
  inconsistency
• OSPF technique
• Source sends only LSA headers, then
• Neighbor requests LSAs that are more recent.
• Those LSAs are sent over
• After sync, the neighbors are said to be fully
  adjacent

22
Problems mapping routing protocols over
underlying networks

• Note mapping IP to L2 networks (eg ethernet,
  ATM) is not the same as mapping routing protocols
  (eg OSPF)
• IP requires ARP and frag/reassembly
• Even this gets complicated in ATM networks
• OSPF requires
• Neighbor abstractions virtual link to each
  neighbor (hello messages)
• Flooding support to efficiently disseminate
  information such as LSAs.
• If neighbors lie on a shared L2 network (eg
  ethernet or ATM), do you use 1 link or N links in
  the Dijkstra algorithm?
• Support over large underlying networks (eg ATM)
• Mapping OSPF to L2 networks is far more
  complicated than mapping IP!

23
Recap IP Subnet Abstraction

• Each subnet assigned one or more address
  prefixes.
• Each address prefix is called an IP subnet
• IP routes to subnets, not to individual hosts
• Two hosts on different IP subnets have to go
  through one or more routers.
• Even if they are on the same physical network

24
IP Subnet Model (Contd)

• Two hosts or routers on a common subnet can send
  packets directly to one another
• Two routers cannot exchange routing information
  directly unless they have one or more IP subnets
  in common
• All these issues will be strained as we study
  OSPF adjacency operation over different subnets

25
OSPF -gt Broadcast Media

• Multiple (N) OSPF routers attached to a common
  subnet
• Problems
• One physical link or N(N-1) adjacencies ?
• How many links to be counted for Dijkstra algo?

26
Broadcast net Mapping Issues

• 1 Each router is assumed to be linked to
  every other router for the purposes of Dijkstra.
• 2 Hello protocol optimization
• Each node multicasts Hello to 224.0.0.5
  (multicast address AllSPFRouters)
• The Hello multicast message also indicates acks
  for other routers Hellos by listing their
  RouterIDs
• Link relationship for purposes of Dijkstra
  maintained by each node sending a single Hello
  packet, instead of N packets.
• 3 What about LSA structure flooding
  adjacencies,
• Can we optimize how this broadcast link is
  represented in an LSA? (Why? More LSAs gt more
  info flooded everywhere!)
• Whom to send (flood) LSAs when a router generates
  or learns a new LSA?
• Does it need to synchronize DBs with all nodes ?

27
LSA Structure option 1 (Router LSA)

• Using Router-LSAs
• O(N) Router-LSAs, with O(N2) adjacency info must
  be flooded everywhere!
• Multicast of Router-LSAs does not solve O(N2) DB
  synchronization issue when LAN comes up after
  failure

28
LSA Structure option 2 (Network LSA)

• New LSA-type Network-LSA
• O(N) Router-LSAs 1 network-LSA O(N)
  adjacencies
• Converted O(N2) adjacency problem into O(N)
  problem

Note Dijsktra algo (executed locally based upon
LSA DB) will interpret this to mean O(N2) links
But we have reduced the amount of control
traffic flooded everywhere!
29
Recap O(N2) model ? O(N) model
?
Dijkstra algo view
Encoding of LSAs, Flooding/DB sync model
New Question Who creates the network-LSA?
30
Ans Designated Router (DR)

• One router elected as a designated router (DR) on
  LAN
• Each router maintains flooding adjacency with the
  DR, I.e., sends acks of LSAs to DR
• DR informs each router of other routers on LAN
• DR generates the network-LSA on subnets behalf
  after synchronizing with all routers

31
Primary/Backup DR, BDR

• Backup DR (BDR) also syncs with all routers, and
  takes over if DR dies (typically 5 s wait)
• Total 2N 1 adjacencies
• Multicast-based optimization
• New LSAs, Hellos sent to AllSPFRouters avoids DR
  re-advertising new information
• LSA acks sent to AllDRRouters avoids separate
  copies to be sent to DR and BDR
• DR election
• First router on net DR, second BDR
• RouterPriority 0, 127 indicated in Hello
  packetgt highest priority router becomes DR
• If network is partitioned and healed, the two DRs
  are reduced to one by looking at RouterPriority

32
Network-LSA Example Summary
DR
33
What if subnet does not support broadcast?

• Non-Broadcast Multiple Access (NBMA) media
• NBMA segments may support more than 2 routers,
  and allow any two routers to communicate
  directly, but do not support data-link
  broadcast/mcast capability
• EgX.25, SMDS, Frame-Relay, ATM etc
• Connection-oriented (VC-based) communication
• Each VC is costly gt setting up full mesh for
  Hellos is prohibitively expensive
• Two flooding adjacency models in OSPF
• Non-Broadcast Multiple Access (NBMA) model
• Point-to-Multipoint (pt-mpt) Model
• Different tradeoffs not covered see extra
  slides

34
Hierarchical Routing
35
Why Hierarchy?

• Information hiding (filtered) gt computation,
  bandwidth, storage saved gt efficiency gt
  scalability
• But filtering in control plane, not data plane
• Address abstraction vs Topology Abstraction
• Multiple paths possible between two adj. areas

?
36
Hierarchical OSPF
37
Area

• Configured area ID
• A set of address prefixes
• Do not have to be contiguous
• So a prefix can be in only one area
• A set of router IDs
• Router functions may be interior, inter-area, or
  external

38
Hierarchical OSPF

• Two-level hierarchy local area, backbone.
• Link-state advertisements only in area
• each nodes has detailed area topology only know
  direction (shortest path) to nets in other areas.
• Two-level restriction avoids count-to-infinity
  issues in backbone routing.
• Area border routers (ABR) summarize distances
  to nets in own area, advertise to other Area
  Border routers.
• Backbone routers uses a DV-style routing between
  backbone routers
• Boundary routers (AS-BRs) connect to other ASs
  (generate external records)

39
Sample Area Configuration
10.2.0.0/24
40
Summary-LSA Example
41
IS-IS Overview

• The Intermediate Systems to Intermediate System
  Routing Protocol (IS-IS) was originally designed
  to route the ISO Connectionless Network Protocol
  (CLNP) . (ISO10589 or RFC 1142)
• Adapted for routing IP in addition to CLNP
  (RFC1195) as Integrated or Dual IS-IS (1990)
• IS-IS is a Link State Protocol similar to the
  Open Shortest Path First (OSPF). OSPF supports
  only IP
• IS-IS competed neck-to-neck with OSPF.
• OSPF deployed in large enterprise networks
• IS-IS deployed in several large ISPs

42
IS-IS Terminology
Intermediate system (IS) - Router Designated
Intermediate System (DIS) - Designated
Router Pseudonode - Broadcast link emulated as
virtual node by DIS End System (ES) - Network
Host or workstation Network Service Access Point
(NSAP) - Network Layer Address Subnetwork Point
of attachment (SNPA) - Datalink interface Packet
data Unit (PDU) - Analogous to IP Packet Link
State PDU (LSP) - Routing information
packet Level 1 and Level 2 Area 0 and lower
areas
43
Functional Comparison

• Protocols are recognizably similar in function
  and mechanism (common heritage)
• Link state algorithms
• Two level hierarchies
• Designated Router on LANs
• Widely deployed (ISPs vs enterprises)
• Multiple interoperable implementations
• OSPF more optimized by design (and therefore
  significantly more complex)
• IS-IS not designed from the start as an IP
  routing protocol (and is therefore a bit clunky
  in places)

44
Sample comparison points

• Encapsulation
• OSPF runs on top of IPgt Relies on IP
  fragmentation for large LSAs
• IS-IS runs directly over L2 (next to IP) gt
  fragmentation done by IS-IS
• Media support
• Both protocols support LANs and point-to-point
  links in similar ways
• IS-IS supports NBMA in a manner similar to OSPF
  pt-mpt model as a set of point-to-point links
• OSPF NBMA mode is configuration-heavy and risky
  (all routers must be able to reach DR bad news
  if VC fails)

45
Packet Encoding

• OSPF is efficiently encoded
• Positional fields, 32-bit alignment
• Only LSAs are extensible (not Hellos, etc.)
• Unrecognized types not flooded. Opaque-LSAs
  recently introduced.
• IS-IS is mostly Type-Length-Value (TLV) encoded
• No particular alignment
• Extensible from the start (unknown types ignored
  but still flooded)
• All packet types are extensible
• Nested TLVs provide structure for more granular
  extension

46
IS-IS LS Database Generic Packet Format
47
More detailed comparison provided as a reference
in a separate slide set(not covered in class)
48
PNNI, QoS Routing and Traffic Engineering
49
Private Network to Node Interface (PNNI)

• Link State Routing Protocol for ATM Networks
• A hierarchy mechanism ensures that this protocol
  scales well for large world-wide ATM networks. A
  key feature of the PNNI hierarchy mechanism is
  its ability to automatically configure itself in
  networks in which the address structure reflects
  the topology

50
PNNI Features

• Scales to very large networks.
• Supports hierarchical routing.
• Supports QoS.
• Supports multiple routing metrics and attributes.
• Uses source routed connection setup.
• Operates in the presence of partitioned areas.
• Provides dynamic routing, responsive to changes
  in resource availability.
• Separates the routing protocol used within a peer
  group from that used among peer groups.
• Interoperates with external routing domains, not
  necessarily using PNNI.
• Supports both physical links and tunneling over
  VPCs.

51
PNNI Terminology (partial)

• Peer group A group of nodes at the same
  hierarchy
• Border node one link crosses the boundary
• Logical group node Representation of a group as
  a single point
• Child node Any node at the next lower hierarchy
  level
• Parent node LGN at the next higher hierarchy
  level
• Logical links links between logical nodes
• Peer group leader (PGL) Represents a group at
  the next higher level.
• Node with the highest "leadership priority" and
  highest ATM address is elected as a leader.
• PGL acts as a logical group node.
• Uses same ATM address with a different selector
  value.
• Peer group ID Address prefixes up to 13 bytes

52
PNNI Terminology
53
Hierarchical Routing PNNI
54
Source Routing

• Source specifies route as a list of all
  intermediate systems in the route. Abstracts out
  area hops.
• Designated Transit List (DTL) Source route across
  each level of hierarchy
• Entry switch of each peer group specifies
  complete route through that group
• Set of DTLs and manipulations implemented as a
  stack
• DTL example next slide

55
DTL Example
56
Crank back and Alternate Path Routing

• If a call fails along a particular route
• It is cranked back to the originator of the top
  DTL
• The originator finds another route or
• Cranks back to the generator of the higher level
  source route

57
QoS Routing outline

• QoS routing involves route selection to meet user
  QoS constraints resource reservation/signaling
• PNNI supports QoS reservations w/ crankback
  after determining source route using a link-state
  approach
• Internet decouples routing (eg OSPF etc) from
  resource reservation/signaling (RSVP)
• Real issues
• How to modify dijsktras algo to compute QoS
  routes?
• Some modifications are complex (NP-hard!)
• How to convey QoS in link states/LSPs (extensions
  to OSPF)
• What to do about stale information? (no crank
  back support only retry)

58
Quality-of-Service Routing With Circuit Switching

• Traffic performance requirement
• Guaranteed bandwidth b per connection
• Link resource reservation
• Reserved bandwidth ri on link I
• Capacity ci on link i
• Signaling admission control on path P
• Reserve bandwidth b on each link i on path P
• Block if (ribgtci) then reject (or try again)
• Accept else ri ri b
• Routing ingress router selects the path

59
Source-Directed QoS Routing

• New connection with b 3
• Routing select path with available resources
• Signaling reserve bandwidth along the path (r
  r 3)
• Forward data packets along the selected path
• Teardown free the link bandwidth (r r -3)

r8, c10
r6, c7
b3
r1, c5
r15, c20
60
QoS Routing Path Selection

• Link-state advertisements
• Advertise available bandwidth (ci ri ) on link
  i
• E.g., every T seconds, independent of changes
• E.g., when metric changes beyond threshold
• Each router constructs view of topology
• Path computation at each router (modified
  Dijkstra!)
• E.g., Shortest widest path
• Consider paths with largest value of mini(ci-ri)
• Tie-break on smallest number of hops
• E.g., Widest shortest path
• Consider only paths with minimum hops
• Tie-break on largest value of mini(ci-ri) over
  paths

61
Ongoing Work on QoS Routing

• Standards activity
• Traffic-engineering extensions to the
  conventional routing protocols (e.g., OSPF and
  IS-IS)
• Use of MPLS to establish the circuits over the
  links
• New work on Path Computation Elements that
  compute the load-sensitive routes for the routers
• Research activity
• Avoid propagating dynamic link-state information
• Based decisions based on past success or failure
• Essentially inferring the state of the links

62
Traffic Engineering Motivation

• TE that aspect of Internet network engineering
  dealing with the issue of performance evaluation
  and performance optimization of operational IP
  networks
• 90s approach to TE was by changing link weights
  in IGP (OSPF, IS-IS) or EGP (BGP-4)
• Performance limited by the shortest/policy path
  nature
• Assumptions Quasi-static traffic, knowledge of
  demand matrix

63
Traffic Engineering

• What is traffic engineering?
• Control and optimization of routing, to steer
  traffic through the network in the most effective
  way
• Two fundamental approaches to adaptation
• Adaptive routing protocols
• Distribute traffic and performance measurements
• Compute paths based on load, and requirements
• Adaptive network-management system
• Collect measurements of traffic and topology
• Optimize the setting of the static parameters
• Big debates still today about the right answer

QoS routing optimization of user QoS
objectives TE optimization of user AND network
QoS objectives
64
Outline Three Alternatives

• Load-sensitive routing at packet level
• Routers receive feedback on load and delay
• Routers re-compute their forwarding tables
• Fundamental problems with oscillation
• Load-sensitive routing at circuit (or aggregate)
  level
• Routers receive feedback on load and delay
• Router compute a path for the next circuit
• Less oscillation, as long as circuits last for a
  while
• Traffic engineering as a management problem
• Routers compute paths based on static values
• Network management system sets the parameters to
  influence the mapping of traffic to paths
• Acting on network-wide view of traffic and
  topology

65
Connectionless Routing Today

• Internet connectionless routing protocols
  originally designed to find one route
• Eg shortest route or policy route)
• Connectionless routing relies upon a global
  consistency criterion (GCC)
• The GCC is constructed using globally known
  identifiers (Eg ASNs, link weights)

66
Limitations of Todays Connectionless TE

• Traffic mapping coupled with route availability
• Changing parameters changes routes AND changes
  the traffic mapped to the routes
• Priority rules only
• LOCAL-PREF, MED, longest-prefix match
• Cannot split traffic to same destination among
  two paths

67
Signaled Approach (eg MPLS)

• Nice features
• In MPLS, choice of a route (and its setup) is
  orthogonal to the problem of traffic mapping onto
  the route
• Signaling maps global IDs (addresses,
  path-specification) to local IDs (labels)
• Nice label stacking, tunneling features

68
Label-Switched Forwarding

• San Francisco prepends MPLS header to the IP
  packet
• MPLS label is swapped at each hop along the LSP
• Forwarding is done based on a label table

Seattle
New York (Egress)
San Francisco (Ingress)
5
1321
120
Miami
69
MPLS Signaling and Forwarding Model

• MPLS label is swapped at each hop along the LSP
• Labels LOCAL IDENTIFIERS
• Signaling maps global identifiers (addresses,
  path spec) to local identifiers

Seattle
New York (Egress)
San Francisco (Ingress)
5
1321
120
Miami
70
What Does MPLS Offer?

• Tunnels
• Drop a packet in, and out it comes at the other
  end without being IP routed
• Explicit (source) routing (circuits)
• Label stack
• 2-label stack outer label defines the tunnel
  inner label de-multiplexes
• Layer 2 independence
• Lot of flexibility (remember indirection?) in
  creating traffic aggregates and mapping them to
  routes.
• Decouples (keyword!) traffic mapping from route
  establishment

71
Limitations of Signaled TE Approach

• Requires extensive upgrades in the network
• Hard to inter-network beyond area boundaries
• Very hard to go beyond AS boundaries
• Even within the same organization/ISP !
• Note large ISPs (eg ATT) have several ASes
• Impossible for inter-domain routing across
  multiple organizations
• Inter-domain TE has to be connectionless

72
Traffic Engineering w/o Signaling?

• Fine-grained Traffic Engineering needs some form
  of source routing
• Specific incremental changes much easier with
  source routing
• Change a single city-pair flow
• Reacting to a link failure
• Can we do source-routing efficiently in
  connectionless protocols?
• (research topic eg BANANAS-TE)

73
Summary

• DV Protocols RIP, EIGRP
• LS Protocols OSPF, IS-IS, PNNI
• Why routing gets complicated to scale?
• Why routing gets complicated to map on different
  subnets?
• Source routing, QoS Routing and Traffic
  Engineering

74
Extra Slides not covered in class
75
Extra Slides For reference

• NBMA and Pt-Mpt mapping models of OSPF over
  telecom data networks (eg ATM, frame relay)
• More complicated than broadcast model, may
  break IP abstraction assumptions, but use similar
  mechanisms (DR, BDR etc)
• External routes (BGP routes in OSPF) and how to
  control the scope of their dissemination
• Used rarely now BGP has internal mechanisms
  (iBGP) for this

76
NBMA Subnet Model

• Neighbor discovery manually configured
• Dijkstra SPF views NBMA as a full mesh!
• Most routers assigned a RouterPriority 0
• Other routers eligible to become DRs gt
• ID of all routers in the NBMA configured
• Maintains VCs and Hellos with all routers
  eligible to become DRs (RouterPriority gt 0)
• Enables election of new DR if current one fails
• DR and BDR only maintain VCs and Hellos with all
  routers on NBMA
• DB synchronization works same as broadcast subnet
• Flooding in NBMA always goes through DR
• Multicast not available to optimize LSA flooding.
• DR generates network-LSA just like broadcast
  subnet

77
NBMA vs Pt-Mpt Subnet Model

• Key assumption in NBMA model
• Each router on the subnet can communicate with
  every other (same as IP model)
• But this requires a full mesh of expensive PVCs
  at the lower layer!
• Many organizations have a hub-and-spoke PVC
  setup, a.k.a. partial mesh
• Conversion into NBMA model requires multiple IP
  subnets, and complex configuration (see fig on
  next slide)
• OSPFs pt-mpt subnet model breaks the rule that
  two routers on the same network must be able to
  talk directly
• Can turn partial PVC mesh into a single IP subnet

78
Partial Mesh F-Relay NBMA model
79
Partial Mesh F-Relay pt-mpt model
80
Pt-Mpt Subnet Model

• Each router single OSPF interface, but multiple
  neighbor relationships
• Note that neighbor relationships not formed to
  nodes to which direct PVC does not exist.
• Key differences
• No DRs or BDRs! Just hellos over the PVCs. Make
  sure that the communication is bi-directional.
• I.e. Partial mesh is viewed in Dijkstra as a
  partial mesh. Full mesh view not forced like in
  NBMA model.
• Sometimes auto-configuration is possible.
• Loss in efficiency because the DB synchronization
  has to be done between every peer.
• O(n2) if full mesh. So, in true full PVC mesh
  situations, it is better to operate subnet as an
  NBMA

81
Externals and Aggregation 1

• A full ISP routing table has approximately 100K
  routes!
• But will you do anything differently if you know
  all of them and have a single ISP?
• Multiple ISP situations call for complex OSPF and
  BGP design
• Never redistribute IGPs into BGP! (later)
• Redistribute BGP into IGPs with extreme care

82
Externals Aggregation 2

• In an enterprise
• Limit externals from subordinate domains (e.g.,
  RIP) to be within area (area-scope)
• Flood only in area 0 and in area with ASBR
• Allow externals from Internet, peer domains to go
  outside Area 0
• Only when there will be significant path
  differences
• Do things with defaults where possible

83
Type 1 and Type 2 external routes

• Information from BGP in OSPF used rarely
• Type 2
• Default type for routes distributed into OSPF
• EGP costs very different from IGP costs
• Exit based on external (EGP) cost only
• Type 1
• Needs to be set explicitly not default
• IGP costs can be compared and summed
• Selects exit based on internal external costs

84
Stubbiness A Means of Controlling Externals
85
Normal Areas

• Flood AS-external-LSAs (type 5) across
  area-boundaries (AS flooding scope)
• ASBR-summary-LSAs (type 4) advertises location of
  ASBR (area flooding scope)

86
Stub Areas

• AS-external-LSAs (type 5) not flooded into stub
  areas
• Summary-LSA flooded only optionally
• Default route to ABR for all non-area prefixes
• Paths may be inefficient, cannot place an ASBR in
  stub areas

87
Not-So-Stubby-Areas (NSSA)

• A subset of external LSAs may be flooded
• Use Type-7 LSAs for such external routes
• Used to import RIP domain routes and flood it
  externally, but keep default route for BGP routes