dynamic routing overview - uopvclass.uop.gr/.../itcom654/lecture4_advanced_routing.pdf · 2019. 1....

Dynamic Routing

Overview

Desirable Characteristics of

Dynamic Routing Automatically detect and adapt to

topology changes

Provide optimal routing

Scalability

Robustness

Simplicity

Rapid convergence

Some control of routing choices

E.g. which links we prefer to use

Interplay between routing & forwarding

1

23

0111

value in arriving

packet’s header

routing algorithm

local forwarding table

header value output link

0100

0101

0111

1001

3

2

2

1

IP Routing – finding the path

Path is derived from information received from the routing protocol

Several alternative paths may exist

best next hop stored in forwarding table

Decisions are updated periodically or as topology changes (event driven)

Decisions are based on:

topology, policies and metrics (hop count, filtering, delay, bandwidth, etc.)

Convergence – why do I care?

Convergence is when all the routers have a stable view of the network

When a network is not converged there is network downtime

Packets don’t get to where they are supposed to go

Black holes (packets “disappear”)

Routing Loops (packets go back and fore between the same devices)

Occurs when there is a change in status of router or the links

Internet Routing Hierarchy

The Internet is composed of Autonomous Systems

Each Autonomous System is an administrative entity that

Uses Interior Gateway Protocols (IGPs) to determine routing within the Autonomous System

Uses Exterior Gateway Protocols (EGPs) to interact with other Autonomous Systems

Internet Routing Architecture

Autonomous

System (AS)

Autonomous

System (AS)Autonomous

System (AS)

Autonomous

System (AS)

Autonomous

System (AS)

Autonomous System: A collection of IP subnets and routersunder the same administrative authority.

Interior Routing Protocol

Exterior Routing Protocol

Interior Gateway Protocols

Four well known IGPs today

RIP

EIGRP

OSPF

ISIS

Exterior Gateway Protocols

One single de-facto standard:

BGP

Routing’s 3 Aspects

Acquisition of information about the IP subnets that are reachable through an internet

static routing configuration information

dynamic routing information protocols (e.g., BGP4, OSPF, RIP, ISIS)

each mechanism/protocol constructs a Routing Information Base (RIB)

Routing Aspect #2

Construction of a Forwarding Table

synthesis of a single table from all the Routing Information Bases (RIBs)

information about a destination subnet may be acquired multiple ways

a precedence is defined among the RIBs to arbitrate conflicts on the same subnet

Also called a Forwarding Information Base (FIB)

Routing #3

Use of a Forwarding Table to forward individual packets

selection of the next-hop router and interface

hop-by-hop, each router makes an independent decision

Routing versus Forwarding

Routing = building maps and giving directions

Forwarding = moving packets between interfaces according to the “directions”

IP Forwarding

Forwarding decisions:

Destination address

class of service (fair queuing, precedence, others)

local requirements (packet filtering)

S

D

IP Subnet

IP Subnet

IP Subnet

IP Subnet

Source

Destination

Routing Tables Feed the

Forwarding Table

BGP 4 Routing Table

ISIS – Link State Database

Static Routes

Ro

uti

ng

In

form

ati

on

Base (

RIB

)

Fo

rwa

rdin

g In

form

ati

on

Base (

FIB

)

Routing Tables Feed the

Forwarding Table

RIB Construction

Each routing protocol builds its own Routing Information Base (RIB)

Each protocol has its own “view” of “costs”

e.g., ISIS is administrative weights

e.g., BGP4 is Autonomous System path length

FIB Construction

There is only ONE forwarding table!

An algorithm is used to choose one next-hop toward each IP destination known by any routing protocol

the set of IP destinations present in any RIB are collected

if a particular IP destination is present in only one RIB, that RIB determines the next hop forwarding path for that destination

FIB Construction

Choosing FIB entries, cont..

if a particular IP destination is present in multiple RIBs, then a precedence is defined to select which RIB entry determines the next hop forwarding path for that destination

This process normally chooses exactly one next-hop toward a given destination

There are no standards for this; it is an implementation (vendor) decision

FIB Contents

IP subnet and mask (or length) of destinations

can be the “default” IP subnet

IP address of the “next hop” toward that IP subnet

Interface id of the subnet associated with the next hop

Optional: cost metric associated with this entry in the forwarding table

IP routing

Default route

where to send packets if there is no entry for the destination in the routing table

most machines have a single default route

often referred to as a default gateway

0.0.0.0/0

matches all possible destinations, but is usually not the longest match

10.0.0.0/8 R310.1.0.0/16 R420.0.0.0/8 R50.0.0.0/0 R1

R2’s IP forwarding table

IP route lookup:

Longest match routing

R2

R3

R4

Most of 10.0.0.0/8 except for10.1.0.0/16

10.1.0.0/16

Based on destination IP address

Packet: DestinationIP address: 10.1.1.1

R1

10.0.0.0/8 R310.1.0.0/16 R420.0.0.0/8 R50.0.0.0/0 R1


IP route lookup:


R2

R3

R4


10.1.0.0/16



10.1.1.1 & FF.00.00.00vs.

10.0.0.0 & FF.00.00.00Match! (length 8)

R1

10.0.0.0/8 R310.1.0.0/16 R420.0.0.0/8 R50.0.0.0/0 R1


IP route lookup:


R2

R3

R4


10.1.0.0/16



10.1.1.1 & FF.FF.00.00vs.

10.1.0.0 & FF.FF.00.00Match! (length 16)

R1

10.0.0.0/8 R310.1.0.0/16 R420.0.0.0/8 R50.0.0.0/0 R1


IP route lookup:


R2

R3

R4

10.1.0.0/16



10.1.1.1 & FF.00.00.00vs.

20.0.0.0 & FF.00.00.00No Match!

R1


10.0.0.0/8 R310.1.0.0/16 R420.0.0.0/8 R50.0.0.0/0 R1


IP route lookup:


R2

R3

R4

10.1.0.0/16



10.1.1.1 & 00.00.00.00vs.

0.0.0.0 & 00.00.00.00Match! (length 0)

R1


10.0.0.0/8 R310.1.0.0/16 R420.0.0.0/8 R50.0.0.0/0 R1


IP route lookup:


R3

R4


10.1.0.0/16



This is the longest matching prefix (length 16). “R2” will send the packet to “R4”.

R2R1

IP route lookup:

Longest match routing Most specific/longest match always

wins!!

Many people forget this, even experienced ISP engineers

Default route is 0.0.0.0/0

Can handle it using the normal longest match algorithm

Matches everything. Always the shortest match.

Distance Vector and Link

State Distance Vector

Accumulates a metric hop-by-hop as the protocol messages traverse the subnets

Link State

Builds a network topology database

Computes best path routes from current node to all destinations based on the topology

RIP (Routing Information

Protocol) Distance vector algorithm

Included in BSD-UNIX Distribution in 1982

Distance metric: # of hops (max = 15 hops)

DC

BA

u vw

x

yz

destination hopsu 1v 2w 2x 3y 3z 2

From router A to subsets:

RIP advertisements

Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)

Each advertisement: list of up to 25 destination nets within AS

RIP: link failure and recovery

If no advertisement heard after 180 sec, neighbor/link declared dead

Routes via the neighbor are invalidated

New advertisements sent to neighbors

Neighbors in turn send out new advertisements (if their tables changed)

Link failure info quickly propagates to entire net

Poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

Why not use RIP?

RIP is a Distance Vector Algorithm

Listen to neighbouring routes

Install all routes in routing table

Lowest hop count wins

Advertise all routes in table

Very simple, very stupid

Only metric is hop count

Network is max 16 hops (not large enough)

Slow convergence (routing loops)

Poor robustness

EIGRP

“Enhanced Interior Gateway Routing Protocol”

Predecessor was IGRP which was classfull IGRP developed by Cisco in mid 1980s to overcome

scalability problems with RIP

Cisco proprietary routing protocol

Distance Vector Routing Protocol Has very good metric control

Still maybe used in some enterprise networks? Multi-protocol (supports more than IP)

Exhibits good scalability and rapid convergence

Supports unequal cost load balancing

OSPF

Open Shortest Path First

“Open” means it is public domain

Uses “Shortest Path First” algorithm – sometimes called “the Dijkstra algorithm”

Current generation interior routing protocol based on “link state” concepts (RFC 1131, 10/1/89, obsoleted by OSPF v2, RFC 1723, 11/15/94)

Supports hierarchies for scalability

Fast convergence and loop avoidance

OSPFv3 based on OSPFv2 designed to support IPv6

Hierarchical OSPF

Hierarchical OSPF

Two-level hierarchy: local area and backbone.

Link-state advertisements only in respective areas.

Nodes in each area have detailed area topology; only know direction (shortest path) to networks in other areas.

Area Border routers “summarize” distances to networks in the area and advertise them to other Area Border routers.

Backbone routers: run an OSPF routing algorithm limited to the backbone.

Boundary routers: connect to other ASs.

Border Gateway Protocol

Introduction

BGP Protocol Basics

Routing Protocol used between ASes

If you aren’t connected to multiple ASes you don’t need BGP

Runs over TCP

AS 100 AS 101

AS 102

E

B D

A C

Peering

Consider a typical small ISP

Local network in one country

May have multiple POPs in different cities

Line to Internet

International line providing transit connectivity

Very, very expensive international line

Doesn’t yet need BGP

Small ISP with one upstream

provider

Provider

Small ISP

Static default route to provider

Static routes or IGP routes to small customers

Static or IGP routes inside

IGP routes inside

BGP to other large ISPs

What happens with other ISPs

in the same region/country Similar setup

Traffic between you and them goes over

Your expensive line

Their expensive line

Traffic can be significant

Your customers want to talk to their customers

Same language/culture

Local email, discussion lists, web sites

Keeping Local Traffic Local

Upstream ISP

Small ISP

Small ISP

UK

Mainland Europe or USA

Consider a larger ISP with

multiple upstreams Large ISP multi-homes to two or more

upstream providers

multiple connections

to achieve:

redundancy

connection diversity

increased speeds

Use BGP to choose a different upstream for different destination addresses

A Large ISP with more than

one upstream provider

Upstream ISP

Upstream ISP

MainlandEurope

USA

ISP UK

Terminology: “Policy”

Where do you want your traffic to go?

It is difficult to get what you want, but you can try

Control of how you accept and send routing updates to neighbours

Prefer cheaper connections

Prefer connections with better latency

Load-sharing, etc

“Policy” (continued)

Implementing policy:

Accepting routes from some ISPs and not others

Sending some routes to some ISPs and not to others

Preferring routes from some ISPs over those from other ISPs

“Policy” Implementation

You want to use a local line to talk to the customers of other local ISPs

local peering

You do not want other local ISPs to use your expensive international lines

no free transit!

So you need some sort of control over routing policies

BGP can do this

Terminology:

“Peering” and “Transit” Peering: getting connectivity to the

network of other ISPs

… and just that network, no other networks

Usually at zero cost (zero-settlement)

Transit: getting connectivity though the other ISP to other ISP networks

… getting connectivity to rest of world (or part thereof)

Usually at cost (customer-provider relationship)

Terminology: “Aggregation”

Combining of several smaller blocks of address space into a larger block

For example:

192.168.4.0/24 and 192.168.5.0/24 are contiguous address blocks

They can be combined and represented as 192.168.4.0/23…

…with no loss of information!

Customers and Providers

Customer pays provider for access to the Internet

provider

customer

IP trafficprovider customer

Big tier-1 providers

customerprovider

Large providers can charge twice for traffic… $$$

traffic

$$$$$$

The “Peering” Relationship

Peerings are mutual agreements.Both partners benefit…

traffic

$$$$$$

peer peer

customerprovider

The “Peering” Relationship

peer peer

customerprovider

Peers provide transit between their respective customers

Peers do not provide transit between peers

Peers (often) do not exchange $$$

trafficallowed

traffic NOTallowed

Economic Relationships can get complex

Peering Wars

Reduces upstream transit costs

Can increase end-to-end performance

May be the only way to connect your customers to some part of the Internet (“Tier 1”)

You would rather have customers

Peers are usually your competition

Peering relationships may require periodic renegotiation

Peering struggles are by far the most contentious issues in the ISP world!

Peering agreements are often confidential.

Peer Don’t Peer

Structure of the Internet

IXP

“Hyper Giants”Large Content,

Consumer, Hosting CDN

Global Transit /“tier-1”

Glo

bal

Inte

rnet

Core

Regio

nal

Tie

r 2

Pro

vid

ers

IXP

ISP 1ISP 2

Custo

mer

IPN

etw

ork

s

IXP

Summary:

Why do I need BGP? Multi-homing – connecting to multiple

providers

upstream providers

local networks – regional peering to get local traffic

Policy discrimination

controlling how traffic flows

do not accidentally provide transit to non-customers

BGP Part II

BGP Building Blocks

Autonomous System (AS)

Collection of networks with same policy

Single routing protocol

Usually under single administrative control

IGP to provide internal connectivity

AS 100

Establish Peering on

TCP port 179

Peers Exchange

All Routes

Exchange Incremental

Updates

AS1

AS2While connection

is ALIVE exchange

route UPDATE messages

BGP

BGP Route = network prefix + attributes

BGP Operations Simplified

BGP Messages

OPEN:

opens TCP conn. to peer

authenticates sender

UPDATE:

“Announcement”: prefix is reachable

“Withdraw”: prefix is not reachable

KEEPALIVE:

keeps connection alive in absence of UPDATES

serves as ACK to an OPEN request

NOTIFICATION:

reports errors in previous msg;

closes a connection

Next

Hop

AS

Path...MED

Local-

Pref.... Community

BGP Attributes

Attributes are “knobs” for

traffic engineering

capacity planning

BGP Protocol Basics

Uses Incremental updates

sends one copy of the RIB at the beginning, then sends changes as they happen

Path Vector protocol

keeps track of the AS path of routing information

Many options for policy enforcement

Terminology

Neighbour

Configured BGP peer

NLRI/Prefix

NLRI – network layer reachability information

Reachability information for an IP address & mask

Router-ID

32 bit integer to uniquely identify router

Comes from Loopback or Highest IP address configured on the router

Route/Path

NLRI advertised by a neighbour

Terminology

Transit – carrying network traffic across a network, usually for a fee

Peering – exchanging routing information and traffic

your customers and your peers’ customers network information only.

not your peers’ peers; not your peers’ providers.

Peering also has another meaning:

BGP neighbour, whether or not transit is provided

Default – where to send traffic when there is no explicit route in the routing table

BGP Basics …

Each AS originates a set of NLRI (routing announcements)

NLRI is exchanged between BGP peers

Can have multiple paths for a given prefix

BGP picks the best path and installs in the IP forwarding table

Policies applied (through attributes) influences BGP path selection

Interior BGP vs.

Exterior BGP

Interior BGP (iBGP)

Between routers in the same AS

Often between routers that are far apart

Should be a full mesh: every iBGP router talks to all other iBGP routers in the same AS

Exterior BGP (eBGP)

Between routers in different ASes

Almost always between directly-connected routers (ethernet, serial line, etc.)

AS 100 AS 101

AS 102

A C

BGP Peers

E

B D

100.100.8.0/24 100.100.16.0/24

100.100.32.0/24

BGP Peers exchange Update messages containing Network Layer Reachability Information (NLRI)

BGP Update

Messages

BGP Peers – External (eBGP)

AS 100 AS 101

AS 102

A C

BGP speakers are called peers

Peers in different AS’sare called External Peers

Note: eBGP Peers normally should be directly connected.

E

B D

100.100.8.0/24 100.100.16.0/24

100.100.32.0/24eBGP TCP/IP

Peer Connection

AS 100 AS 101

AS 102

A C

BGP speakers are called peers

BGP Peers – Internal (iBGP)

Peers in the same ASare called Internal Peers

Note: iBGP Peers don’t have to be directly connected.

E

B D

100.100.8.0/24 100.100.16.0/24

100.100.32.0/24iBGP TCP/IP

Peer Connection

interface Serial 0

ip address 110.110.10.2 255.255.255.252

router bgp 100

network 100.100.8.0 mask 255.255.255.0

neighbor 110.110.10.1 remote-as 101

interface Serial 0

ip address 110.110.10.1 255.255.255.252

router bgp 101

network 100.100.16.0 mask 255.255.255.0


eBGP TCP Connection

110.110.10.0/30

B C DA

AS 100 AS 101

.2100.100.8.0/30 100.100.16.0/30.2 .1 .2 .1.1

Configuring eBGP peers

BGP peering sessions are established using the BGP “neighbor” command

eBGP is configured when AS numbers are different

AS 100 AS 101

110.110.10.0/30

.2

interface Serial 1

ip address 100.100.16.2 255.255.255.252

router bgp 101

network 100.100.16.0 mask 255.255.255.0


B

interface Serial 1

ip address 100.100.16.1 255.255.255.252

router bgp 101

network 100.100.16.0 mask 255.255.255.0


C

iBGP TCP Connection

D100.100.8.0/30 100.100.16.0/30A .2 .1 .2 .1.1

Configuring iBGP peers

BGP peering sessions are established using the BGP “neighbor” command

iBGP is configured when AS numbers are the same

iBGP TCP/IP

Peer Connection

AS 100

AB

C

Configuring iBGP peers:

Full mesh Each iBGP speaker must peer with every other

iBGP speaker in the AS

iBGP TCP/IP

Peer Connection

AS 100

AB

C

105.10.7.1105.10.7.2

105.10.7.3

Configuring iBGP peers:

Loopback interface Loopback interfaces are normally used as the

iBGP peer connection end-points


AS 100

AB

C

105.10.7.1105.10.7.2

105.10.7.3

interface loopback 0

ip address 105.10.7.1 255.255.255.255

router bgp 100

network 105.10.7.0 mask 255.255.255.0


neighbor 105.10.7.2 update-source loopback0




AS 100

AB

C

105.10.7.1105.10.7.2

105.10.7.3


ip address 105.10.7.2 255.255.255.255

router bgp 100

network 105.10.7.0 mask 255.255.255.0





iBGP TCP/IP

Peer Connection


AS 100

AB

C

105.10.7.1105.10.7.2

105.10.7.3


ip address 105.10.7.3 255.255.255.255

router bgp 100

network 105.10.7.0 mask 255.255.255.0





Route Reflectors

• Route reflectors can pass on iBGP updates to clients

• Each RR passes along

ONLY best routes

• ORIGINATOR_ID and CLUSTER_LIST attributes are needed to avoid loops

RR RR

RR

BGP Part III

BGP Protocol – A little more detail

BGP Updates — NLRI

Network Layer Reachability Information

Used to advertise feasible routes

Composed of:

Network Prefix

Mask Length

BGP Updates — Attributes

Used to convey information associated with NLRI

AS path

Next hop

Local preference

Multi-Exit Discriminator (MED)

Community

Origin

Aggregator

AS 100

AS 300

AS 200

AS 500

AS 400

170.10.0.0/16 180.10.0.0/16

150.10.0.0/16

Network Path

180.10.0.0/16 300 200 100

170.10.0.0/16 300 200

150.10.0.0/16 300 400

Network Path

180.10.0.0/16 300 200 100

170.10.0.0/16 300 200

AS-Path Attribute

Sequence of ASes a route has traversed

Loop detection

Apply policy

AS-Path (with 16 and 32-bit

ASNs) Internet with 16-bit

and 32-bit ASNs

AS-PATH length maintained 180.10.0.0/16 300 23456 23456

170.10.0.0/16 300 23456

AS 80000

AS 300

AS 70000

AS 90000

AS 400

170.10.0.0/16 180.10.0.0/16

150.10.0.0/16

180.10.0.0/16 300 70000 80000

170.10.0.0/16 300 70000

150.10.0.0/16 300 400

Shorter Doesn’t Always Mean

ShorterIn fairness: could you do this “right” and still scale?

Exporting internalstate would dramatically increase global instability and amount of routingstate

AS 4

AS 3

AS 2

AS 1

Mr. BGP says that path 4 1 is betterthan path 3 2 1

Duh!

ASPATH Padding

Padding will (usually) force inbound traffic from AS 1to take primary link

AS 1

192.0.2.0/24ASPATH = 2 2 2

customer

AS 2

provider

192.0.2.0/24

backupprimary

192.0.2.0/24ASPATH = 2

160.10.0.0/16

150.10.0.0/16

192.10.1.0/30

.2

AS 100

AS 200

Network Next-Hop Path

160.10.0.0/16 192.20.2.1 100

C .1

B

A

.1

.2

AS 300

E

D

140.10.0.0/16

BGP Update

Messages

Next Hop Attribute

Next hop to reach a network

Usually a local network is the next hop in eBGP session

160.10.0.0/16

150.10.0.0/16

192.10.1.0/30

.2

AS 100

AS 200C .1

B

A

.1

.2

AS 300

E

D

140.10.0.0/16

BGP Update

MessagesNetwork Next-Hop Path

150.10.0.0/16 192.10.1.1 200

160.10.0.0/16 192.10.1.1 200 100

Next Hop Attribute

Next hop to reach a network

Usually a local network is the next hop in eBGP session

Next Hop updated betweeneBGP Peers

160.10.0.0/16

150.10.0.0/16

192.10.1.0/30

.2

AS 100

AS 200C .1

B

A

.1

.2

AS 300

E

D

140.10.0.0/16

BGP Update

Messages Network Next-Hop Path

150.10.0.0/16 192.10.1.1 200

160.10.0.0/16 192.10.1.1 200 100

Next Hop Attribute

Next hop not changedbetween iBGP peers

Next Hop Attribute (more)

IGP is used to carry route to next hops

Recursive route look-up

BGP looks into IGP to find out next hop information

BGP is not permitted to use a BGP route as the next hop

Unlinks BGP from actual physical topology

Allows IGP to make intelligent forwarding decision

Next Hop Best Practice

Cisco IOS default is for external next-hop to be propagated unchanged to iBGP peers

This means that IGP has to carry external next-hops

Forgetting means external network is invisible

With many eBGP peers, it is extra load on IGP

ISP best practice is to change external next-hop to be that of the local router

neighbor x.x.x.x next-hop-self

Community Attribute

32-bit number

Conventionally written as two 16-bit numbers separated by colon

First half is usually an AS number

ISP determines the meaning (if any) of the second half

Carried in BGP protocol messages

Used by administratively-defined filters

Not directly used by BGP protocol (except for a few “well known” communities)

BGP Updates:

Withdrawn Routes Used to “withdraw” network reachability

Each withdrawn route is composed of:

Network Prefix

Mask Length

BGP Updates:

Withdrawn Routes

AS 321AS 123

192.168.10.0/24

192.192.25.0/24

.1 .2

x

Connectivity lost

BGP Update

Message

Withdraw Routes192.192.25.0/24

Network Next-Hop Path150.10.0.0/16 192.168.10.2 321 200192.192.25.0/24 192.168.10.2 321

BGP Routing Information Base

BGP RIB

D 10.1.2.0/24

D 160.10.1.0/24

D 160.10.3.0/24

R 153.22.0.0/16

S 192.1.1.0/24


router bgp 100 network 160.10.1.0 255.255.255.0network 160.10.3.0 255.255.255.0no auto-summary

Route Table

*>i160.10.1.0/24 192.20.3.1 i

*>i160.10.3.0/24 192.20.3.1 i

BGP ‘network’ commands are normally used to populate the BGP RIB with routes from the Route Table

•S: Identifies that the route was manually created

by an administrator to reach a specific network.

This is known as a static route.

•D: Identifies that the route was learned

dynamically from another router using the

EIGRP routing protocol.

•O: Identifies that the route was learned

dynamically from another router using the OSPF

routing protocol.

•R: Identifies that the route was learned

dynamically from another router using the RIP

routing protocol.

•B - BGP

BGP Routing Information

Base

BGP RIBIN Process

Network Next-Hop Path173.21.0.0/16 192.20.2.1 100

Update * 173.21.0.0/16 192.20.2.1 100 i

• BGP “in” process• receives path information from peers

• results of BGP path selection placed in the BGP table

• “best path” flagged (denoted by “>”)

Update


*>i160.10.1.0/24 192.20.3.1 i

*>i160.10.3.0/24 192.20.3.1 i

OUT Process

>


Base

OUT Process

Network Next-Hop Path160.10.1.0/24 192.20.3.1 200160.10.3.0/24 192.20.3.1 200173.21.0.0/16 192.20.2.1 200 100

BGP RIB

> 173.21.0.0/16 192.20.2.1 100


*>i160.10.1.0/24 192.20.3.1 i

*>i160.10.3.0/24 192.20.3.1 i

*

IN Process

Update Update

• BGP “out” process• builds update using info from RIB

• may modify update based on config

• Sends update to peers


Base

BGP RIB

D 10.1.2.0/24

D 160.10.1.0/24

D 160.10.3.0/24

R 153.22.0.0/16

S 192.1.1.0/24


*>i160.10.1.0/24 192.20.3.1 i

*>i160.10.3.0/24 192.20.3.1 i

*> 173.21.0.0/16 192.20.2.1 100

• Best paths installed in routing table if:

B 173.21.0.0/16

Route Table

• prefix and prefix length are unique• lowest “protocol distance”

An Example…

Learns about 35.0.0.0/8 from F & D

AS3561

B

E

C

D

F

A

AS200

AS101

AS21

AS675

35.0.0.0/8

dynamic routing overview - uopvclass.uop.gr/.../itcom654/lecture4_advanced_routing.pdf · 2019. 1....

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