Routing II: Protocols (RIP, EIGRP, OSPF, PNNI,...

Shivkumar Kalyanaraman1

Routing II: Protocols (RIP, EIGRP, OSPF, PNNI, IS-IS)

Shivkumar KalyanaramanRensselaer Polytechnic Institute

[email protected]

Based in part upon slides of Prof. Raj Jain (OSU), S. Keshav (Cornell), J. Kurose (U Mass)


Overview

RIP, RIPv2, EIGRPOSPF, PNNI, IS-IS: LS efficiency & robustness

Link state distribution, DB synchronization, NBMAs etcRefs: 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"

http://www.landfield.com/rfcs/rfc2328.html

http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp2001/notes/notes-for-ccn.pdf

http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp2001/notes/routing101.pdf

http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp2001/notes/routing101.doc

http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp2001/readings/landmark-routing.pdf





RIP: Routing Information ProtocolUses hop count as metric (max: 16 is infinity) Tables (vectors) “advertised” to neighbors every 30 s.

Each advertisement: upto 25 entriesNo advertisement for 180 sec: neighbor/link declared dead

routes via neighbor invalidatednew advertisements sent to neighbors (Triggered updates)neighbors in turn send out new advertisements (if tables changed)link failure info quickly propagates to entire netpoison reverse used to prevent ping-pong loops (infinite distance = 16 hops)


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


RIPv2Why ? Installed base of RIP routersProvides:

VLSM supportAuthenticationMulticasting“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.


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 updatesLoop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation

Freeze entry to particular destinationDiffuse a request for updatesOther nodes may freeze/propagate the diffusing computation (tree formation)Unfreeze when updates received.Tradeoff: temporary un-reachability for some destinations


Link State vs. Distance VectorLink State (LS) advantages:

More stable (aka fewer routing loops) Faster convergence than distance vectorEasier to discover network topology, troubleshoot network.Can do better source-routing with link-stateType & Quality-of-service routing (multiple route tables) possible

Caveat: With path-vector-type (paths instead of distances) DV routing, these differences blur…


Link State ProtocolsKey: 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 destinations4. Routing table = next-hops of these paths.5. Hierarchical routing: organization of areas, and filtered control plane information flooded.


Link State IssuesReliable Flooding: sequence #s, age LSA types, Neighbor discovery and maintainence (hello)

Efficiency in Broadcast LANs, NBMA, Pt-Mptsubnets: designated router (DR) concept

Areas and HierarchyArea types: Normal, Stub, NSSA: filteringExternal Routes (from other ASs), interaction with inter-domain routing.

Advanced topics: incremental SPF algorithms


Reliable Flooding…


Topology DisseminationA.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 duplicatesWhy? Routers/links may go down/upIssue: wrap-around, larger sequence number is not the most recent!


Sequence Number Space OrganizationCircular 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 handleLinear space: OSPFv2.

If Smax reached, expicitly delete Smax LSA before wrapping around


Topology Dissemination (Continued)Checksum field:

Drop packet if in error, get retransmission from neighbor

Age field (similar to TTL)Number of seconds since LSA originatedPeriodically incremented after acceptanceOriginating router refreshes LSA after 30 minDelete if Age = MaxAgeLow age field + large seq # => that LSA is flapping or frequently changing …


Recovering from a partitionOn partition, LSP databases can get out of synch

Databases described by database descriptor recordsRouters on each side of a newly restored link talk to each other to update databases (determine missing and out-of-date LSPs) => selective synchronization


LSA-types, Neighbor & flooding Adjacencies in Different Subnets


OSPF Router-LSA: Scenario


Neighbor Discovery & RelationshipEvery OSPF router sends out 'hello' packetsHello packets used to determine if neighbor is upHello 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 relationshipand is more than physical connection.It consumes bandwidth and computation resourcesBecomes an issue if large number of adj need to be maintained


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 synchronizationNew announcements are sent ONCE, and only updated if there's a changeOr every 45mins...


Hello: Packet Format


Router-LSA:


Database Synchronization LS Database (LSDB): collection of the Link State Advertisements (LSAs) accepted at a node.

This is the “map” for Dijkstra algorithmWhen 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 inconsistencyOSPF technique:

Source sends only LSA headers, thenNeighbor requests LSAs that are more recent.Those LSAs are sent overAfter sync, the neighbors are said to be “fully adjacent”


Recap: IP Subnet ModelEach subnet assigned one or more address prefixes. Each address prefix is called an IP subnetIP routes to subnets, not to individual hostsTwo hosts on different IP subnets have to go through one or more routers.

Even if they are on the same “physical” network


IP Subnet Model (Contd)Two hosts or routers on a common subnet can send packets “directly”to one anotherTwo routers cannot exchange routing information directly unless they have one or more IP subnets in commonAll these issues will be strained as we study OSPF adjacency operation over different subnets


Broadcast Media IssuesMultiple (N) OSPF routers attached to a common subnet

Problems: One “physical link” vs N*(N-1) “adjacencies”How many “links” to be counted for Dijkstra algo?


Broadcast net: # links for DIjkstraEach router is assumed to be “linked” to every other router for the purposes of Dijkstra.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.

What about “flooding adjacencies”, I.e., Whom to send (flood) LSAs when a router generates or learns a new LSA?Does it need to synchronize DBs with all nodes ?


Flooding Adjacencies : option 1Using Router-LSAs …

O(N) Router-LSAs, with O(N2) adjacency infoMulticast of Router-LSAs does not solve O(N2) DB synchronization issue


Flooding Adjacencies: option 2New LSA-type: Network-LSA …

O(N) Router-LSAs + 1 network-LSA+ O(N) adjacenciesConverted O(N2) adjacency problem into O(N) problem


Recap: O(N2) model ⇒ O(N) model

⇒

Question: Who creates the network-LSA?


Designated Router (DR)One router elected as a designated router (DR)

Each router maintains flooding adjacency with the DR, I.e., sends acks of LSAs to DRDR informs each router of other routers on LANDR generates the network-LSA on subnet’s behalf after synchronizing with all routers


DR, BDR … continuedBackup DR (BDR) also syncs with all routers, and takes over if DR dies (typically 5 s wait)Total: 2N – 1 adjacenciesMulticast-based optimization:

New LSAs, Hellos sent to AllSPFRouters avoids DR re-advertising new informationLSA acks sent to AllDRRouters avoids separate copies to be sent to DR and BDR

DR election:First router on net = DR, second = BDRRouterPriority: [0, 127] indicated in Hello packet=> highest priority router becomes DRIf network is partitioned and healed, the two DRs are reduced to one by looking at RouterPriority


Network-LSA Example: Summary

DR


What if subnet does not support broadcast?

Non-Broadcast Multiple Access (NBMA) mediaNBMA 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) communicationEach VC is costly => setting up full mesh for Hellos is prohibitively expensive

Two flooding adjacency models in OSPF:Non-Broadcast Multiple Access (NBMA) modelPoint-to-Multipoint (pt-mpt) Model

Different tradeoffs…


NBMA Subnet ModelNeighbor discovery: manually configuredDijkstra SPF views NBMA as a full mesh!Most routers assigned a RouterPriority = 0Other routers: eligible to become DRs =>

ID of all routers in the NBMA configuredMaintains 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 allrouters on NBMA

DB synchronization works same as broadcast subnetFlooding in NBMA always goes through DRMulticast not available to optimize LSA flooding.

DR generates network-LSA just like broadcast subnet


NBMA vs Pt-Mpt Subnet ModelKey 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


Partial Mesh F-Relay: NBMA model


Partial Mesh F-Relay: pt-mpt model


Pt-Mpt Subnet ModelEach router: single OSPF interface, but multiple neighbor relationshipsNote 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


Hierarchical Routing


Why Hierarchy?Information hiding (filtered) => computation, bandwidth, storage saved => efficiency => scalabilityAddress abstraction vs Topology Abstraction

Multiple paths possible between two areas

⇒


Hierarchical OSPF


AreaConfigured area IDA set of address prefixes

Do not have to be contiguousSo a prefix can be in only one area

A set of router IDsRouter functions may be interior, inter-area, or external


Hierarchical OSPFTwo-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)


Sample Area Configuration

10.2.0.0/24


Summary-LSA Example


Externals and Aggregation 1A 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


Externals & Aggregation 2

In an enterpriseLimit 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 differencesDo things with defaults where possible


Type 1 and Type 2 externalsType 2:

Default type for routes distributed into OSPFEGP costs very different from IGP costsExit based on external (EGP) cost only

Type 1Needs to be set explicitly: not defaultIGP costs can be compared and summedSelects exit based on internal + external costs


Stubbiness: A Means of Controlling Externals


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)


Stub Areas

AS-external-LSAs (type 5) not flooded into stub areasSummary-LSA flooded only optionallyDefault route to ABR for all non-area prefixesPaths may be inefficient, cannot place an ASBR in stub areas


Not-So-Stubby-Areas (NSSA)

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


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 IPIS-IS competed neck-to-neck with OSPF.

OSPF deployed in large enterprise networksIS-IS deployed in several large ISPs


IS-IS Terminology

Intermediate system (IS) - RouterDesignated Intermediate System (DIS) - Designated RouterPseudonode - Broadcast link emulated as virtual node by DISEnd System (ES) - Network Host or workstationNetwork Service Access Point (NSAP) - Network Layer AddressSubnetwork Point of attachment (SNPA) - Datalink interfacePacket data Unit (PDU) - Analogous to IP PacketLink State PDU (LSP) - Routing information packetLevel 1 and Level 2 – Area 0 and lower areas


Functional ComparisonProtocols are recognizably similar in function and mechanism (common heritage)Link state algorithms Two level hierarchiesDesignated Router on LANsWidely deployed (ISPs vs enterprises)Multiple interoperable implementationsOSPF 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)


Sample comparison pointsEncapsulation

OSPF runs on top of IP=> Relies on IP fragmentation for large LSAsIS-IS runs directly over L2 (next to IP) => fragmentation done by IS-IS

Media supportBoth protocols support LANs and point-to-point links in similar waysIS-IS supports NBMA in a manner similar to OSPF pt-mpt model: as a set of point-to-point linksOSPF NBMA mode is configuration-heavy and risky (all routers must be able to reach DR; bad news if VC fails)


Packet EncodingOSPF 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) encodedNo particular alignmentExtensible from the start (unknown types ignored but still flooded)All packet types are extensibleNested TLVs provide structure for more granular extension


IS-IS LS Database: Generic Packet Format

Intra-domain Routing Protocol Discriminator

Length Indicator

TLV Fields

Version/Protocol ID Extension

ID Length

R R R PDU Type

Version

Reserved

Maximum Area Addresses

Packet-Specific Header Fields

No. of Octets

1

1

1

1

1

1

1

1


More detailed comparison provided as a reference (not covered in

class)…


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…”


PNNI FeaturesScales 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.


PNNI Terminology (partial)Peer group: A group of nodes at the same hierarchyBorder node: one link crosses the boundaryLogical group node: Representation of a group as a single pointChild node: Any node at the next lower hierarchy levelParent node: LGN at the next higher hierarchy levelLogical links: links between logical nodesPeer 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


PNNI Terminology


Hierarchical Routing: PNNI


Source RoutingSource 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 groupSet of DTLs and manipulations implemented as a stack

DTL example: next slide


DTL Example


Crank back and Alternate Path RoutingIf a call fails along a particular route:

It is cranked back to the originator of the top DTLThe originator finds another route orCranks back to the generator of the higher level source route


Summary

DV Protocols: RIP, EIGRPLS Protocols: OSPF, IS-IS, PNNI

Date post:	26-May-2020
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Routing II: Protocols (RIP, EIGRP, OSPF, PNNI,...

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