1. Routing II: Protocols (RIP, EIGRP, OSPF, PNNI, IS-IS): Brief Version
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
Based in part upon slides of Prof. Raj Jain (OSU), S. Keshav (Cornell), J. Kurose (U Mass)

2. Overview RIP, RIPv2, EIGRP
OSPF, PNNI, IS-IS: LS efficiency & robustness
Link state distribution, DB synchronization, NBMAs etc
Refs: Chap 16,14
Suggested Ref. 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
A.k.a Triggered updates
Link failure info quickly propagates to entire net

4. RIPv1 Problems (Continued)
Recall: Poisoned Reverse:
If Z routes through Y to get to X :
Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)
Poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
Split horizon/poison reverse does not guarantee to solve count-to-infinity problem
16 = infinity => RIP for small networks only!
Slow convergence
RIPv1 does not support subnet masks (VLSMs)
No authentication

5. RIPv2 Why ? Installed base of RIP routers Key new features:
VLSM support
Authentication
Multicasting

6. E-IGRP (Interior Gateway Routing Protocol)
CISCO proprietary; successor of RIP (late 80s)
Key idea:
Loop-free routing via Distributed Updating Alg. (DUAL) based on diffusing 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 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.

8. Link State Protocol Issues
Reliable Flooding: sequence #s, age
Neighbor discovery and Neighbor maintenance (hello)
Efficiency in different types of networks: Broadcast LANs, NBMA, Pt-Mpt subnets
Hierarchy of areas
Area types: Normal, Stub, NSSA: filtering
Advanced topics: incremental SPF algorithms

9. Reliable Flooding…

10. 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!

11. 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, explicitly delete Smax LSA before wrapping around

12. 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 …

13. Neighbor Relationships & LSA: Basic OSPF Models

14. Neighbor Discovery & Relationship
OSPF routers periodically send out 'hello' packets
Used to determine if neighbor is up
HelloInterval = 10s (in example)
Assumes neighbor dead if no response within
RouterDeadInterval = 40s (in example)
A.k.a: “adjacency”
Note that adjacency is a “software link” abstraction
Less reliable than a physical link
Becomes an issue if large number of adjacencies need to be maintained

15. Hello: Packet Format

16. Neighbor Relationships…
Once an adjacency is established, information is traded
Neighbor relationship: bi-directional
Local topology information is packaged in a "link state announcement“ (LSA)
Multiple types of LSAs: (details later)
Initial DB synchronization
New announcements sent ONCE, and only updated if there's a change, or every 45mins...

17. 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”

18. Recovering from a partition
On partition, LSP databases can get out of synch
Databases described by database descriptor records
Restored link => talk to each other to update databases (determine missing and out-of-date LSPs) => selective synchronization

19. OSPF Router-LSA: Scenario

20. Router-LSA:

21. Issues in Mapping OSPF Hellos, LSAs, Dijkstra semantics onto Different Types of Sub-Networks

22. Recap: IP Subnet Model
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 subnets have to go through routers…
Even if they are on the same “physical” network

23. IP Subnet Model (Contd)
Two hosts or routers must be able to send packets “directly” to one another IFF they are on a common subnet
=> Two routers cannot exchange routing information directly unless they have one or more IP subnets in common
=> Two hosts on the same “subnet” cannot be indirectly connected through switches etc!
All these issues will be strained as we study OSPF adjacency operation over different subnets

24. Broadcast Media: Adjacency Maintenance
Issue: Hellos and LSAs optimized for pt-pt links
Multiple (N) OSPF routers on a common subnet (bus)
One “physical link” vs N*(N-1) “adjacencies”
How many “links” to be counted for Dijkstra algo?
How many Hellos to be exchanged on the shared bus?

25. Broadcast net: Adjacency Maintenance
Ans: Each router is assumed to be “linked” to every other router
Dijkstra algorithm views the bus as a full mesh, I.e. counts O(N2) adjacencies.
Hello protocol optimization:
Each node multicasts Hello to (multicast address “AllSPFRouters”) & piggybacks its acks
LSA optimization:
Since we have O(N2) adjacencies for Dijkstra, should we create N Router LSAs, with a total of O(N2) adjacency information?
Or 1 new type of LSA to represent this “bus”?

26. Flooding Adjacencies : option 1
Using Router-LSAs …
O(N) Router-LSAs, with O(N2) adjacency info
Multicast of Router-LSAs does not solve O(N2) DB synchronization issue

27. Flooding Adjacencies: option 2
New LSA-type: Network-LSA …
O(N) Router-LSAs + 1 network-LSA+ O(N) adjacencies
Converted O(N2) adjacency problem into O(N) problem

28. Recap: O(N2) model  O(N) model
Question: Who creates the network-LSA?

29. Designated Router (DR)
One router elected as a designated router (DR)
Each router maintains “flooding adjacency” with the DR, I.e., DR responsible for collecting acks of LSAs
DR informs each router of other routers on LAN
DR generates the network-LSA on subnet’s behalf after synchronizing with all routers

30. DR, BDR … continued Backup DR (BDR) and takes over if DR dies
It also has flooding adjacencies w/ other routers
=> Total: 2N – 1 adjacencies
Multicast-based optimization:
LSAs from other networks & Hellos sent to AllSPFRouters
LSA acks sent to AllDRRouters avoids separate copies to be sent to DR and BDR

31. DR, BDR … continued 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. Non-Broadcast Subnets: OSPF Optimizations

34. What if subnet does not support broadcast?
Non-Broadcast Multiple Access (NBMA) media
NBMA segments may support > 2 routers
Allow any two routers to communicate directly,
But do not support data-link broadcast/mcast capability
Eg:X.25, SMDS, Frame-Relay, ATM etc
Issues:
Connection-oriented (VC-based) communication
Each VC is costly => setting up full mesh for Hellos is prohibitively expensive

35. OSPF models Two flooding adjacency models in OSPF:
1. Non-Broadcast Multiple Access (NBMA) model
Simple extension of broadcast subnet model
2. Point-to-Multipoint (pt-mpt) Model
Different tradeoffs…

36. NBMA Model Preliminaries: Neighbor discovery: manually configured
Dijkstra SPF views NBMA as a full mesh!
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

37. Partial Mesh F-Relay: NBMA model

38. 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 => multiple IP subnets, and complex configuration
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

39. Partial Mesh F-Relay: pt-mpt model

40. Pt-Mpt Subnet Model
Key: Partial mesh is viewed in Dijkstra as a partial mesh. Full mesh view not forced like in NBMA model.
Neighbor relationships are not formed w/ nodes to which direct PVC does not exist.
No DRs or BDRs! Just hellos over the PVCs. Make sure that the communication is bi-directional.
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

41. Hierarchical Routing

42. Why Hierarchy?
Information hiding (filtered) => computation, bandwidth, storage saved => efficiency => scalability
Address abstraction vs Topology Abstraction
Multiple paths possible between two areas 

43. Hierarchical OSPF

44. 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.

45. 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)

46. Sample Area Configuration
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47. Summary-LSA Example

48. Stubbiness: A Means of Controlling External Routes

49. 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)

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

51. 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

52. Recap: Some Key Differences
External routes vs Summary LSAs
External routes summarize routes outside the domain, while summary LSAs summarize routes outside an area.
NSSA vs Stubby Areas:
Both are special cases of OSPF hierarchies
They differ in their treatment of external routes
Stubby areas filter ALL external routes, while NSSAs selectively filter external routes.

53. Other Link State Protocols: IS-IS, PNNI

54. IS-IS Overview
The Intermediate Systems to Intermediate System Routing Protocol (IS-IS): 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

55. 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)

56. 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

57. 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…”

58. 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 PVCs.

59. PNNI Terminology

60. PNNI Terminology … 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

61. Hierarchical Routing: PNNI

62. 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

63. DTL Example

64. 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

65. Summary
DV Protocols: RIP, EIGRP
LS Protocols: OSPF, IS-IS, PNNI