1. Routing II: Protocols (RIP, EIGRP, OSPF, PNNI, IS-IS) + QoS Routing and Traffic Engineering
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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 => 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 => reaches every other node in the network and everyone now has a network map.
3. Given map, run Dijkstra’s shortest path algorithm (SPF) => 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 > S2 > S3 > 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 # => 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) => 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 -> 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 (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 => 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 subnet’s 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 packet=> 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: SummaryDR

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
Eg:X.25, SMDS, Frame-Relay, ATM etc
Connection-oriented (VC-based) communication
Each VC is costly => 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) => computation, bandwidth, storage saved => efficiency => 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
An area contains a set of cooperating routers that share a synchronized and distributed topological database. Routers connected to multiple areas have multiple databases.
The ability to hide details of areas from other areas makes a significant reduction in routing traffic possible. Additional refinements are possible, furhter hiding the detals of knowledge about areas outside the local area.

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
/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 IP=> Relies on IP fragmentation for large LSAs
IS-IS runs directly over L2 (next to IP) => 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
Intra-domain Routing Protocol Discriminator
Length Indicator
TLV Fields
Version/Protocol ID Extension
ID Length R
PDU Type
Version
Reserved
Maximum Area Addresses
Packet-Specific Header Fields
No. of Octets 1

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 dijsktra’s 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 (ri+b>ci) 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)
r=8, c=10
r=6, c=7
b=3
r=1, c=5
r=15, c=20

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 …’’
90’s 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 A B C D 1 2 4 E
Links AC and CD are overloaded A B C D 1 2 E
Can not do this with OSPF A B C D 1 2 E
Links AB and BD are overloaded

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
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 Today’s 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) IP IP
1321
San
Francisco
(Ingress) 5 IP
120
1321
120
Miami IP
Label

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) IP IP
1321
San
Francisco
(Ingress) 5 IP
120
1321
120
Miami IP
Label

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 AS’es
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 =>
ID of all routers in the NBMA configured
Maintains VCs and Hellos with all routers eligible to become DRs (RouterPriority > 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)
OSPF’s 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(n^2) 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
Type 2 is the default. The external LSA type is set on the redistribute statement.

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