Shivkumar Kalyanaraman 1 Routing II: Protocols (RIP, EIGRP, OSPF, PNNI, IS-IS) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute [email protected]

Shivkumar Kalyanaraman

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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)


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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"

Overview


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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)


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RIPv1 Problems (Continued)

Split horizon/poison reverse does not guarantee to solve count-to-infinity problem16 = infinity => RIP for small networks only!Slow convergence

Broadcasts consume non-router resources RIPv1 does not support subnet masks (VLSMs)

No authentication


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RIPv2 Why ? Installed base of RIP routers Provides:

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.


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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


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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…


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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.


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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: filteringExternal Routes (from other ASs), interaction

with inter-domain routing. Advanced topics: incremental SPF algorithms


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Reliable Flooding…


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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 duplicatesWhy? Routers/links may go down/up Issue: wrap-around, larger sequence number

is not the most recent!


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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


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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 …


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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


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LSA-types, Neighbor & flooding Adjacencies in Different Subnets


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OSPF Router-LSA: Scenario


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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


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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...


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Hello: Packet Format


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Router-LSA:


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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”


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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 IP

subnets have to go through one or more routers. Even if they are on the

same “physical” network


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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


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Broadcast Media Issues Multiple (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?


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Broadcast net: # links for DIjkstra Each 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 ?


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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


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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


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Recap: O(N2) model O(N) model

Question: Who creates the network-LSA?


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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 DR

DR informs each router of other routers on LAN DR generates the network-LSA on subnet’s behalf

after synchronizing with all routers


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DR, BDR … continued 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 = BDRRouterPriority: [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


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Network-LSA Example: Summary

DR


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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…


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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


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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


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Partial Mesh F-Relay: NBMA model


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Partial Mesh F-Relay: pt-mpt model


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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


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Hierarchical Routing


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Why Hierarchy? Information hiding (filtered) => computation,

bandwidth, storage saved => efficiency => scalability Address abstraction vs Topology Abstraction

Multiple paths possible between two areas


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Hierarchical OSPF


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Area

Configured area ID A 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


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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)


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Sample Area Configuration

10.2.0.0/24


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Summary-LSA Example


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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


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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

differences Do things with defaults where possible


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Type 1 and Type 2 externals

Type 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 default IGP costs can be compared and summedSelects exit based on internal + external costs


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Stubbiness: A Means of Controlling Externals


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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)


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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


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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


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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


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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


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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)


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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)


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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


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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


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More detailed comparison provided as a reference (not covered in class)

…


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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…”


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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.


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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


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PNNI Terminology


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Hierarchical Routing: PNNI


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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 hierarchyEntry switch of each peer group specifies

complete route through that groupSet of DTLs and manipulations implemented

as a stack DTL example: next slide


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DTL Example


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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


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Summary

DV Protocols: RIP, EIGRP LS Protocols: OSPF, IS-IS, PNNI

Documents

Shivkumar Kalyanaraman 1 Routing II: Protocols (RIP, EIGRP, OSPF, PNNI, IS-IS) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute [email protected]