FÉDÉRALE DE LAUSANNE ExternalRouting BGPica · A router that runs BGP is called a BGP speaker At...

1

External Routing

ÉCOLE POLYTECHNIQUEFÉDÉRALE DE LAUSANNE

External RoutingBGP

Jean‐Yves Le BoudecFall 2009

SelfOrganization

1

2

Contents

A. What Inter‐Domain Routing does1 Inter‐Domain Routing1. Inter Domain Routing

2. Policy Routing

3. Route Aggregation

B. How BGP works1. How it works

2. Interaction BGP—IGP—Packet Forwarding

3. Other Bells and Whistles

C. ExamplesD. Illustrations and Statistics

2

3

A. What BGP Does1. Inter‐Domain Routing

Why invented ?The Internet is too large to be run by one routing protocolThe Internet is too large to be run by one routing protocol

Hierarchical routing is usedthe Internet is split into Domains, or Autonomous Systems

with OSPF: large domains are split into Areas

Routing protocols are saidinterior: (Internal Gateway Protocols, IGPs): inside ASs: RIP, OSPF (standard), IGRP (Cisco)

exterior: between ASs:

3

e te oEGP (old) and BGP‐1 to BGP‐4 (today), IDRP (tomorrow, maybe)

4

What is an ARD ? An AS ?

ARD = Autonomous Routing Domain= routing domain under one single administration

one or more border routersall subnetworks inside an ARD should be connectedshould learn about other subnetwork prefixes ‐ the routing tables of internal routers should contain entries of all destination of the Internet

AS = Autonomous System= ARD with a number (“AS number”)

AS number is 16 bitspublic: 1 ‐ 64511private: 64512 – 65535

ARDs that are do not need a number are typically those with default route to the rest of the worldExamples

4

ExamplesAS1942 ‐ CICG‐GRENOBLE, AS2200 ‐ RenaterAS559 ‐ SWITCH Teleinformatics Services AS5511 – OPENTRANSITEPFL: one ARD, no number

5

the figure shows three domains, or ARDs.

ARDs can be transit (B and D), stub (A) or multihomed (C). Only non stub ( ), ( ) ( ) ydomains need an AS number.

ARD D

3

D1

4

area 0

C2

C1

C4

IGRP

B1A2

A1

A4 BGP-4

BGP-4

OSPF BGP-4

BGP-4D2

D3D4

D6OSPF

area 2area 1

D5

5

ARD C

C4C3

B2

B1B4

B3

A3

Autonomous Routing Domain A

ARD B

6

What does BGP do ?What does BGP do ?BGP is a routing protocol between ARDs It is used to computeBGP is a routing protocol between ARDs. It is used to compute paths from one router in one ARD to any network prefix anywhere in the world

There are only two levels for BGP :Inter‐domain

within domain

6

The method of routing isPath vector

With policy

7

Path Vector RoutingWhat is the requirement ?

Find best routes, in a sense that can be decided by every ARD using their own criteriaHow does it work ?

a route between neighbours is (path: dest) where path is a sequence of AS numbers and dest isa route between neighbours is (path: dest) where path is a sequence of AS numbers and dest is an IP prefixexample: B A:n1every AS uses its own rules for deciding which path is betterBGP table keeps a record of best paths to all destinationsAS announces only the best paths it knows

Q. Explain how E can choose the best paths to n1 and n2 ?Q. How can loops be avoided ? solution BGP table in EQ p

B

En1, n2

A:n1,n2B A:n1,n2

B:n5 dest AS path

n1 B An2 B An3 D Cn4 D

BGP table in En5

n4

7

A

CA:n1,n2

C A:n1,n2C:n3

D

D C A:n1,n2D C: n3D: n4

n4 D n5 B

n3

8

Border Gateways, E‐ and I‐BGPA router that runs BGP is called a BGP speakerAt the boundary between 2 ARDs there are 2 BGP speakers, one in each domain

Q: compare to OSPF

Inside one ARD there are usually several BGP speakersThey all talk to each other, to exchange what they have learntUsing “Internal BGP” (I‐BGP)Over TCP connections, full mesh called the “BGP mesh”I‐BGP is the same as E‐BGP except for one rule: routes learned from a node in theI BGP is the same as E BGP except for one rule: routes learned from a node in the mesh are not repeated inside the mesh

D1 D2A B

C D

X:n1 X:n1

A->C: D1 X:n1

E-BGP

8

D4 D5

D3

G H

E F

C->E: D1 X:n1 E->G: D3 D1,X:n1

E-BGP

I-BGP

9

Q1: Which of the following announcements do you think router D will send to router E ? 1. D‐>E: D2 X:n1 2. D‐>E: D1 X:n1

A: 1 only; 2 cannot be sent because over I‐BGP we do not repeat a route learnt f th BGP k i th ARDfrom another BGP speaker in the same ARD

Q2: Assume that in the figure below the router R does not run BGP. Is this possible ?A: yes.

Q3: Do BGP speakers in the same ARD need to be on‐link ?Q3: Do BGP speakers in the same ARD need to be on link ?A: no; C and F are not directly connected (are not on‐link)

D1 D2A B

C D

X:n1 X:n1

E-BGPsolution

9D4 D5

D3

G H

C D

E F

E-BGP

I-BGP R

10

2. Policy Routing

Why invented ?

Interconnection of ASs is self‐organizedpoint to point links between networks: ex: EPFL to Switch, Switch to Telianet

interconnection points: NAP (Network Access Point), MAE (Metropolitan Area Ethernet), CIX (Commercial Internet eXchange), GIX (Global Internet eXchange), IXP, SFINX, LINX

Mainly 3 types of relations, depending on money flowscustomer: EPFL is customer of Switch. EPFL pays Switch

provider: Switch is provider for EPFL; Switch is paid by EPFL

10

peer: EPFL and CERN are peers: costs of interconnection is shared

the type of relation is negotiated in bilateral agreementsthere is no architecture rule, just business

11

What is the Goal of Policy Routing ?

Example: ISP3‐ISP2 is transatlantic link cost shared betweenISP3 ISP2 is transatlantic link, cost shared between ISP2 and ISP 3

ISP 3‐ ISP 1 is a local, inexpensive link

Ci is customer of ISPi, ISPs are peers

It is advantageous for ISP3 to send traffic to 2 i ISP1

ISP 1

ISP 3 ISP 2

n2 via ISP1ISP1 does not agree to carry traffic from C3 to C2

ISP1 offers a “transit service” to C1 and a “non‐transit” service to ISP 2 and ISP3

C1

C2C3

n2

11

The goal of “policy routing” is to support this and other similar requirements

provider

customerpeers

12

How does Policy Routing Work ?Policy routing is implemented by rules imposed to BGP routers inside an AS, who may

refuse to import or announce some pathsrefuse to import or announce some paths

modify the attributes that control which path is preferred (see later)

ExampleISP 1 announces to ISP 3 all networks of C1 – so that C1 can

ISP 1

ISP 3 ISP 2

be reached by all sources in the world

ISP 1 announces to C1 all routes it has learnt from ISP3 and ISP2 – so that C1 can send traffic to all destinations in the world

ISP2 announces “ISP2 n2” to ISP3 and ISP1 ; assume that ISP1 annouces “ISP1 ISP2 n2” to ISP3.

C1

C2C3

n2n3

12

ISP1 annouces ISP1 ISP2 n2 to ISP3.

ISP 3 has two routes to n2: “ISP2 n2” and “ISP1 ISP2 n2”; assume that ISP3 gives preference to the latter

packets from n3 to n2 are routed via ISP1 – undesired

solution: ISP 1 announces to ISP3 only routes to ISP3’s customers

13

Typical Policy Routing RulesProvider (ISP1) to customer (C1)

announce all routes learnt from other ISs

import only routes that belong to domain C1example: import from EPFL only one route 128.178/15

Customer (C1) to Provider (ISP1)announce all routes that belong to domain C1ISP 1

ISP 3 ISP 2

import all routes

Peers (ISP1 to ISP3)announce only routes to all customers of ISP1

import only routes to ISP3’s customer

these routes are defined as part of peering

C1

C2C3

13

these routes are defined as part of peering agreement

The rules are defined by every AS (self‐organization) and implemented in all BGP speakers in one AS

14

3. Aggregation

Domains that do not have a default route (i.e. all transit ISPs) must know all routes in the world (> 170 000 prefixes)must know all routes in the world (> 170 000 prefixes)

in IP routing tables unless default routes are used

in BGP announcements

Aggregation is a way to reduce the number of routes

14

15

Aggregation Example 1

AS1 193.212.0.0/24AS1

AS2

AS3 AS4

193.212.1.0/24

193.212.0.0/23 193.212.0.0/23

Assume AS3 aggregates the routes received from AS1 and AS2AS1: 193.212.0.0/24 AS_PATH: 1AS2: 193.212.1.0/24 AS_PATH: 2AS3: 193.212.0.0/23 AS_PATH: 3 {1 2}AS4: 193 212 0 0/23 AS PATH: 4 3 {1 2}

15

AS4: 193.212.0.0/23 AS_PATH: 4 3 {1 2}

16


AS1 197.8.2.0/24AS1

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23

197.8.3.0/24

197.8.2.0/23

n2= 197.8.3.0/24

n4

AS4 receives197.8.2.0/23 AS_PATH: 3 {1 2}197.8.3.0/24 AS_PATH: 2

and assume that AS4 exports only the first route (aggregation is done)But assume both routes are injected into AS4’s local routing tables; Q:

16

j gwhat happens to packets from n4 to n2 ?

If AS4 injects only the aggregate route into its local routing tables, what is the answer now ?solution

17

Example Without Aggregation

AS1 197.8.2.0/24 197.8.2.0/23 197.8.2.0/23AS1

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23

197.8.3.0/24

197.8.2.0/24

197.8.3.0/24

197.8.2.0/23

197.8.2.0/24

197.8.3.0/24

Q: If AS3 does not aggregate, what are the routes announced by AS 4 ? Is there any benefit ?solution

17

18

Morale

Aggregation should be performed whenever possiblewhen all aggregated prefixes have the same path (example 1)when all aggregated prefixes have the same path (example 1)

when all aggregated prefixes have the same path before the aggregation point (example 2 )

An AS can decide toAggregate several routes when exporting them

But still maintain different routing entries inside its domain (example 2)

18

19

B. BGP (Border Gateway Protocol)1. How it works, Fundamental Examples

BGP‐4, RFC 1771BGP routers talk to each other over TCP connectionsBGP routers talk to each other over TCP connections

Maintain adjacency to peers (over TCP connection): open, keep‐alive

Transmit only modifications

How a BGP router works (see also next slide)Receives routes from neighbours

Accepts / rejects them (import policy)

Processes attributes

Stores in Adj‐RIB‐in (one per BGP peer, internal or external)Applies decision process and stores results in Loc‐RIB (global to BGP speaker) (= decides which routes to accept among all present in the different Adj‐RIB‐ins)

For every subnetwork prefix, at most 1 route is selectedSends to neighbours

Decides whether to send or not (export policy)A l i l i if li bl

19

Aggregate multiple routes into one, if applicable Store result in Adj‐RIB‐out (one per BGP peer) and send to neighbourOnly routes learnt from E‐BGP are sent to an I‐BGP neighbourSends updates when Adj‐RIB‐out changes

Write forwarding entries in its routing table, either directly or by re‐distribution into IGP ; this is explained in detail in Section 2.

20

Inside BGP

updates in d t t

One BGP Router

Adj RIB out

updates in updates out

Adj-RIB-inAdj RIB in

Import Policy

DecisionProcess:

b t Export

Routing

Adj-RIB-outAdj-RIB-inAdj-RIB-outLoc-RIB

IGPWrite forwarding

entries

one best route /

destination

pPolicy

Internal routes

20

RoutingTable

Static Configuration

21

Routes, RIBs, Routing TableThe records sent in BGP messages are called “Routes”. Routes + their attributes are stored in the Adj‐RIB‐in, Loc‐RIB, Adj‐RIB‐out. A route is made of:A route is made of:

destination (subnetwork prefix)path to the destination (AS‐PATH)Attributes

Well‐known MandatoryORIGIN (route learnt from IGP, BGP or static)AS‐PATHAS PATHNEXT‐HOP

Well‐known DiscretionaryLOCAL‐PREF (see later)ATOMIC‐AGGREGATE (= route cannot be dis‐aggregated)

Optional TransitiveMULTI‐EXIT‐DISC (MED)(see later)AGGREGATOR (who aggregated this route)

21

AGGREGATOR (who aggregated this route)

Optional NontransitiveWEIGHT (see later)

In addition, like any IP host or router, a BGP router also has a Routing Table = IP forwarding table

Used for packet forwarding, in real timeThis is not the same as a RIB, we will see the interaction later

22

The Decision ProcessThe decision process decides which route is selected; its output is written into Loc‐RIBAt most one best route to exactly the same prefix is chosen

Only one route to 2.2/16 can be chosenBut there can be different routes to 2.2.2/24 and 2.2/16

Routes are compared against each other using the p g gfollowing sequence of criteria, until only one route remains:0. Highest weight (Cisco proprietary)1. Highest LOCAL‐PREF2. Shortest AS‐PATH

22

3. Lowest MED, if taken seriously by this network4. E‐BGP > I‐BGP5. Shortest path to NEXT‐HOP, according to IGP6. Lowest BGP identifier

23

Fundamental Examples

AS x

R2R1I-BGP

R4

R2R1

10.1/16 AS=(y)10.2/16 AS=(y)

10.1/16 AS=(y)10.2/16 AS=(y)

10 2/16

E-BGP E-BGP

R3

1.1.1.1 2.2.2.2

1.1.1.2 2.2.2.1

AS y

10.1/16 10.2/16

Skr

iva

ñ w

ar

an

da

ol

23

In this simple example there are 4 BGP routers. They communicate directly or indirectly via E‐BGP or I‐BGP, as shown on the figure. There are 2 ASs, x and y. We do not show the details of the internals of y. R3 and R4 send the BGP messages shown.In the next slide, we show only the following attributes in the RIBs: destination, path, NEXT‐HOP

24

R1 has the following RIBs, initially empty:Adj-RIB-In from R3 Adj-RIB-In from R2Loc-RIBAdj-RIB-Out to R3 Adj-RIB-Out to R2

R1 receives from R3 two announcements, made of the routes 10.1/16 AS=(y) and 10.2/16 AS=(y). R1 associates the NEXT‐HOP attribute 1 1 1 2 h d h Ad

R1 prepares to send these new routes to all its I‐BGP peers by writing them into Adj‐RIB‐Out to R2:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.210.2/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-In from R2Loc-RIB

10 1/16 AS=y NEXT HOP=1 1 1 21.1.1.2 to these routes and writes them in its Adj‐RIB‐In:Adj-RIB-In from R3


Adj-RIB-In from R2Loc-RIBAdj-RIB-Out to R3 Adj-RIB-Out to R2

R1 applies its decision process, and selects both


Adj-RIB-Out to R3 Adj-RIB-Out to R2


Since Adj‐RIB‐Out to R2 changed, a BGP message is sent to R2Similarly, at some time R1 will receive from R2 a BGP message with the routes 10.1/16 AS=y pp p ,

routes:Adj-RIB-In from R3





BGP message with the routes 10.1/16 AS y

NEXT-HOP=2.2.2.1 and 10.2/16 AS=y NEXT-HOP=2.2.2.1:

Adj-RIB-In from R310.1/16 AS=y NEXT-HOP=1.1.1.210.2/16 AS=y NEXT-HOP=1.1.1.2


Loc-RIB

24

R1 prepares to send these new routes to all its E‐BGP peers by writing them into Adj‐RIB‐Out. However, the message is not sent to R3 as it would create a loop in the AS‐path.




25

R1 applies again its decision process. Now it has several possible routes to each prefix. The first applicable rule in the decision process (slide “The Decision Process”) says that if a route is learnt from E‐BGP it has precedence over a route learnt from I‐BGP. Since all routes in Adj-RIB-In from R2 are learnt from I‐BGP, and all routes in Adj-learnt from I BGP, and all routes in AdjRIB-In from R3 are learnt from E‐BGP, the winners are the latter, so there is no change in Loc-RIB:


Adj-RIB-In from R210.1/16 AS=y NEXT-HOP=2.2.2.110 2/16 AS y NEXT HOP 2 2 2 1 10.2/16 AS=y NEXT-HOP=2.2.2.1

Loc-RIB10.1/16 AS=y NEXT-HOP=1.1.1.210.2/16 AS=y NEXT-HOP=1.1.1.2



Since there is no change in Loc-RIB there is h i dj d h f

25

no change in Adj-RIB-Out and therefore no message is sent by R1.

26

AS x

R22R1I-BGP R21 I-BGP

I-BGP

Fundamental Examples, continued

R4

R22R1

10.1/16 AS=(y) 10.2/16 AS=(y)

10 2/16

E-BGP E-BGP

R3

1.1.1.1 2.2.2.2

1.1.1.2 2.2.2.13.3.3.1

3.3.3.2

10.2/16 AS=(y)

AS y

10.1/16 10.2/16

There are now 3 BGP routers in AS x Note that the 3 BGP in AS x routers must have

26

There are now 3 BGP routers in AS x. Note that the 3 BGP in AS x routers must have TCP connections with each other (same in AS y, but not shown on figure).An IGP (for example OSPF) also runs on R1, R21 and R22. All link costs are equal to 1.The announcements made by R3 and R4 are different, as shown on the figure.

27

Here is a possible sequence of messages and the resulting states of RIBs, at R1, after message is processed:R1 receives message 10.1/16 AS=(y) from R3 over E_BGP:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-In from R22Adj-RIB-In from R21Loc-RIB

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-Out to R3 Adj-RIB-Out to R22 Adj-RIB-Out to R21

Assume R21 also received the E‐BGP messages shown on the figure and did its job. R1 receives message 10.2/16 AS=y, NEXT-HOP=3.3.3.1 from R21 over I‐BGP:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-In from R22

10.2/16 AS=y, NEXT-HOP=2.2.2.1Adj-RIB-In from R21

10.2/16 AS=y, NEXT-HOP=3.3.3.1Loc-RIB

10.1/16 AS=y NEXT-HOP=1.1.1.210 2/16 AS y NEXT HOP 2 2 2 1 Adj-RIB-Out to R21

R1 applies its decision process, accepts the route and sends it to R22 and R21Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-In from R22Adj-RIB-In from R21Loc-RIB

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-Out to R3 Adj-RIB-Out to R22

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-Out to R21

10.1/16 AS=y NEXT-HOP=1.1.1.2

10.2/16 AS=y NEXT-HOP=2.2.2.1 Adj-RIB-Out to R3 Adj-RIB-Out to R22


10.1/16 AS=y NEXT-HOP=1.1.1.2

The decision process now has to choose between two routes with same destination prefix 10.2/16. Both were learnt from I‐BGP, so we apply criterion 5 in slide “The Decision Process”.The distance, computed by the IGP, to 2.2.2.1 is 3 and the distance to 3.3.3.1 is 2. Thus the route that has NEXT-HOP=3 3 3 1 is preferred by the decision process:0 / 6 S y O

Assume R22 also received the E‐BGP messages shown on the figure and did its job. R1 receives message 10.2/16 AS=y, NEXT-HOP=2.2.2.1 from R22 over I‐BGP, and the decision process accepts it:

Adj-RIB-In from R310.1/16 AS=y NEXT-HOP=1.1.1.2

Adj-RIB-In from R2210.2/16 AS=y, NEXT-HOP=2.2.2.1


10.1/16 AS=y NEXT-HOP=1.1.1.2

HOP=3.3.3.1 is preferred by the decision process:Adj-RIB-In from R3

10.1/16 AS=y NEXT-HOP=1.1.1.2 Adj-RIB-In from R22

10.2/16 AS=y, NEXT-HOP=2.2.2.1Adj-RIB-In from R21

10.2/16 AS=y, NEXT-HOP=3.3.3.1Loc-RIB




27

/ y10.2/16 AS=y NEXT-HOP=2.2.2.1



10.1/16 AS=y NEXT-HOP=1.1.1.2

Since the new message is learnt from I‐BGP it is not put into any Adj‐RIB‐Out

j10.1/16 AS=y NEXT-HOP=1.1.1.2

Since the new message is learnt from I‐BGP it is not put into any Adj‐RIB‐Out.

28

Questions

What is the difference between I‐BGP and IGP ?

What is the difference between I‐BGP and E‐BGP ?

What is the difference between a RIB and a Routing Table ?

28

29

How are routes injected into BGP

BGP propagates route information, but how is this bootstrapped ?bootstrapped ?Two methods

Static configuration: tell this BGP router which are the prefixes owned by this domain

Import from IGP: tell this router to copy all the prefixes that the IGP has learntlearnt

Assumes the IGP either does not propagate external prefixes, or has a way to differentiate them

Such routes are sent to E‐BGP neighbours only, with ORIGIN=IGP

29

30

2. How Forwarding Entries learnt by BGP are written into Routing Tables

So far, we have seen how BGP routers learn about all the prefixes in the world It remains to see how they inform all theprefixes in the world. It remains to see how they inform all the routers in their AS about these prefixes (« write forwarding entries » in slide « Inside BGP »). This is the topic of this section.There are two possible ways for this, usually mutually

l iexclusive:Redistribution: routes learnt by BGP are passed to IGP (ex: OSPF)

Called “redistribution of BGP into OSPF”

OSPF propagates the routes using type 4 LSAs to all routers in OSPF cloud

30

p p g g yp

Not found much in practice

Injection: routes learnt by BGP are written into the forwarding table of this router

Routes do not propagate; this helps only this router

31

Redistribution Example

AS xE-BGPR5 R6

AS z

R5 advertises 18.1/16 to R6 via E‐BGPR6 i i R2 i I BGP

E-BGP

R1

R218.1/16 I-BGP

IGP(OSPF)

IGP(OSPF)

2.2.2.2

AS y

R6 transmits it to R2 via I‐BGPTCP connection between R6 and R2

(redistribute BGP into IGP) R6 injects 18.1/16 into IGP (OSPF)

OSPF propagates 18.1/16 (type 4 LSA) and updates forwarding tablesAfter OSPF converges, R1, R2 now have a route to 18.1/6R2

R4

31

g , , /advertises route to R4 via E‐BGP

(synchronize with IGP) R2 must wait for the OSPF entry to 18.1/6 before advertising via E‐BGPPacket to 18.1/16 from AS y finds forwarding table entries in R2, R1 and R6

32

Example with Re‐Distributionby ___, F learns from G the route D2‐D1‐n1

C redistributes the external route D2:n2 into OSPF

b D l th t D2 2 b D l th t DYD2 2 Dby ____, D learns the route D2:n2; by ____ D learns the route DYD2:n2; D selects D2:n2 and does not redistribute it to OSPF

by ___, B learns the route D2:n2

by ___, A learns the route DX:D2:n2

by ___, L learns the route to n2 via C

D1 = EPFL D2 = UCB

n1

D1: n1

n2

A C

FD2 : n2

G

32

DX DYD2 : n2DY D2: n2

D1: n1B

D EL

physical linkTCP connectionsolution

33

Re‐Distribution is Considered Harmful

In practice, operators avoid re‐distribution of BGP into IGPLarge number of routing entries in IGPLarge number of routing entries in IGP

Convergence time after failures is large if IGP has many routing table entries

Therefore, injection is used insteadInjection assumes that routers in the AS employ recursive table lookupp

When IP packet is submitted to router, the forwarding table may indicate a “NEXT‐HOP” which is not on‐link with router

A second table lookup needs to be done to resolve the next‐hop into an on‐link neighbour

in practice, second lookup may be done in advance – not in real time– by pre‐

33

processing the routing table

34

Example: Recursive Table Lookup

At R1, data packet to 10.1.x.y is receivedThe forwarding table at R1 is looked upThe forwarding table at R1 is looked up

Q: what are the next events ?

T NEXT HOP l 2 dd

Forwarding Table at R1

R1

To NEXT-HOP layer-2 addr10.1/16 2.2.2.63 N/A

2.2.2.63 2.2.2.33 x09:F1:6A:33:76:21

2.2.2.33 2.2.2.6310.1/16R2 R3

34

R

2.2.2.93

R3

solution

35

Injection Example

AS xE-BGPR5 R6

AS z

R5 advertises 18.1/16, NEXT-HOP = 2.2.2.2 E-BGP

R1

R218.1/16 I-BGP

IGP

IGP(OSPF)

2.2.2.2

AS y

to R6 via E‐BGPR6 injects 18.1/16, NEXT-HOP = 2.2.2.2 into its local forwarding table (does not re‐distribute into OSPF)

R2 learns route from R6 via I‐BGPR2 injects 18 1/16 NEXT-HOP = 2 2 2 2 into

R4

35

R2 injects 18.1/16, NEXT HOP = 2.2.2.2 into its local forwarding table

Data packet to 18.1.2.3 is received by R2Recursive table lookup at R2 can be used

Q: there is a problem at R1: how can we solve it ?solution

36

Injection in Practice Requires all Routers to Run BGPAS xE-BGP

R2

R5 R6

18.1/16 I-BGPIGP(OSPF)

2.2.20.12.2.2.2

AS z

Practical solution often deployedAll core routers also run I‐BGP (in addition to IGP)

E-BGP

R4

R1

R2/

IGP

(OSPF)

AS y

Routes learnt from BGP are injected into local forwarding table

Recursive table lookup is done at all routers

Q: repeat the sequence of previous slide with this new assumption

Potential problem: I‐BGP mesh ‐> use reflectors

36

Potential problem: I BGP mesh > use reflectorsIGP handles only internal networks – very few

solution

37

Questions

What is the difference between injection and re‐distribution ?

Can BGP routes be learnt by a non BGP router ?

What is recursive table lookup ? Where is it needed ?

37

38

3. Other Bells and Whistles MULTI‐EXIT‐DISC (MED)

packet to 10.1.2.3

packet to 10 2 3 4

AS x

R2R1

10.1/16 MED=1010 2/16 MED=50

10.1/16 MED=5010.2/16 MED=10E-BGP E-BGP

packet to 10.2.3.4

1.1.1.1 2.2.2.2

AS y

R4

10.2/16 MED=50

10.1/16 10.2/16R3

38

One AS connected to another over several linksex: multinational company connected to worldwide ISP

AS y advertises its prefixes with different MEDs (low = preferred)

If AS x accepts to use MEDs put by ASy: traffic goes on preferred link

39

MED Example

Q1: by which mechanisms will R1 and R2 make sure that packets to ASy use the preferred links ?packets to ASy use the preferred links ?

Q2: router R3 crashes; can 10.1/16 still be reached ? explain the sequence of actions.

solution

39

40

MED Question

Q1: Assume now ASx and ASy are peers (ex: both are ISPs). Explain why ASx is not interested in taking MED into accountExplain why ASx is not interested in taking MED into account.

Q2: By which mechanisms can ASx pick the nearest route to ASy ?

solution

40

41

LOCAL‐PREF

f 100AS x

E-BGP

R1 R2

R6

I-BGP

I-BGP E-BGP

E-BGP pref=10

pref=100

Used inside an AS to select a best AS pathAssigned by border router when receiving route over E‐BGP

Propagated without change over I‐BGP

41

ExampleR6 associates pref=100, R2 pref=10

R1 chooses the largest preferencebgp default local-preference pref-value

42

LOCAL‐PREF Example

Q1: The link AS2‐AS4 is expensive. How should AS 4 set local‐prefs on routes received from AS 3 and AS 2 in order to route traffic preferably through AS 3 ? Q2 E l i th f t f R1 R2 d R3Q2:Explain the sequence of events for R1, R2 and R3.

AS 1

3AS 2

AS1: 10.1/1610.1/16AS1: 10.1/16

AS 3AS 2

AS 4R1 R2

R3

42

AS 5

solution

43

LOCAL‐PREF Question

Q: Compare MED to LOCAL‐PREFsolutionsolution

43

44

Weight

This is a route attribute given by Cisco or Zebra routerIt remains local to this routerIt remains local to this router

Never propagated to other routers, even in the same cloud

Therefore there is no weight attribute in route announcements

44

45

Route Flap Dampening

Route modification propagates everywhereSometimes routes are flappingSometimes routes are flapping

successive UPDATE and WITHDRAW

caused for example by BGP speaker that often crashes and reboots

Solution:decision process eliminates flapping routes

Howwithdrawn routes are kept in Adj‐RIN‐in

if comes up again soon (ie : flap), route receives a penaltyif penalty ≥ suppress‐limit route is not selected

45

penalty fades out exponentially, see next slide

46

Route Flap Dampening

penalty

suppress-limit

penalty

reuse-limit

time

t1 t2

46

Route suppressed at t1, restored at t2

47

Avoid I‐BGP Mesh: Confederations

AS P1 AS P2 AS P3

AS z

E-BGP

AS P1 AS P2 AS P3

I-BGP I-BGP I-BGP

E-BGP

AS decomposed into sub‐ASprivate AS number

AS zE-BGPE-BGP E-BGP

47

similar to OSPF areasI‐BGP inside sub‐AS (full interconnection)E‐BGP between sub‐AS

48

Avoid I‐BGP Mesh : Route reflectors

I-BGP

AS z

I-BGP

I-BGP I-BGP I-BGP

I-BGP

RR RR RR

cluster 1 cluster 2 cluster 3

Cluster of routersone I‐BGP session between one client and RRCLUSTER_ID

R fl

AS zE-BGPE-BGP E-BGP

48

Route reflectorre‐advertises a route learnt via I‐BGPto avoid loops

ORIGINATOR_ID attribute associated with the advertisement

49

I‐BGP configuration

lo0lo0I-BGP

I-BGP

I‐BGP configured on loopback interface (lo0)i f l

AS z

49

interface always upIP address associated with the interfaceIGP routing guarantees packet forwarding to the interface

50

Avoid E‐BGP mesh: Route server

At i t ti i t

E-BGP

50

At interconnection pointInstead of n(n‐1)/2 peer to peer E‐BGP connectionsn connections to Route ServerTo avoid loops ADVERTISER attribute indicates which router in the AS generated the route

51

Communities

Other attributes can be associated with routes in order to simplify rules They are called « communities »simplify rules. They are called « communities »

Pre‐defined: Example: NO‐EXPORT ( a well known, pre‐defined attribute) –see later for an example

Defined by one AS (a label of the form ASN:x where AS= AS number, x = a 2 byte—number)

51

52

NO—EXPORT

Written on E‐BGP by one AS, transmitted on I‐BGP by accepting AS, not forwardedExample: AS2 has different routes to AS1 but AS2 sends only one aggregate route to AS3

simplifies the aggregation rules at AS2What is the route followed by a packet sent to 2.2.48 received by R4 ?

2.2.0/17 NO-EXPORT 2 2/16R1

2.2.0/17

/

2.2/16

2.2/16

R1

R5

R3

R4

52

2.2.128/17

2.2.128/17 NO-EXPORT 2.2/16R2

R5

solution

53

C. Examples

A network operator implements their policy by selecting theA network operator implements their policy by selecting the rules for

Import policy (accept route or not, if required modify/add/delete attributes)

Export policy (export or not a route, decide which attribute value)

We illustrate two scenarios:

Dual HomingHot potato routing

53

Hot potato routing

54

Ex1: Stub Area

Operator ASp

R1

R2

BGP not needed between Client and Operator No AS number for clientR2 learns all prefixes in Client by static configuration or RIP on link R1—R2

Client AS

54

Example: EPFL and Switchwhat if R1 fails ?

55

Ex2: Stub Area, Dual Homing to Single ProviderAS x

R2R1

10 1/16 MED=10 10.1/16 MED=50E BGP E BGP

Provider

AS

R4

10.1/16 MED=1010.2/16 MED=50 10.2/16 MED=10

10.1/16 10.2/16

E-BGP E-BGP

R3

Client

With numbered Client ASUse MED to share traffic from ISP to Client on two links

Use Client IGP configuration to share traffic from Client to two links

AS y

55

Q1: is it possible to avoid distributing BGP routes into Client IGP ?

Q2: is it possible to avoid assigning an AS number to Client ?

Q3: is it possible to avoid BGP between Client and Provider ?

solution

56

Ex3: Stub Area, Dual Homing to Several Providers

AS x

R2R1 P dAS y

AS z

R4

R2R1

/ 2 1/17

E-BGP E-BGP

R3

Cli t

Providers

Client has own address space and AS numberQ: how can routes be announced between AS 100 and AS x ? AS x and AS z ?

AS 100

2.0/17 2.1/17Client

56

Q: assume Client wants most traffic to favour AS x. How can that be done ?

solution

57

Ex4: Hot Potato Routing

Customer 1ISP1

R11 R12

ISP2R21 R22

Customer 2

Packets from Customer 2 to Customer 1Both R21 and R22 have a route to Customer 1Shortest path routing favours R21Q1: by which mechanism is that done ?

Q2: what is the path followed in the reverse direction ?

57

solution

58

D. Illustrations: The Switch Networkwww.switch.ch

58

59

An Interconnection Point

59

60

60

61

Type Next HOP MED Origin Community RRC ID

A 128.178.0.0/15

2003-10-02 05:05:49Z

129.250.0.232

129.250.0.232 9 Not defined 2914 1299

559

2914:420 2914:2000 2914:3000

RIPE NCC

from www.ris.ripe.net: all routes to 128.178.0.0/15 on RIPE Route Collectors

A 128.178.0.0/15

2003-10-02 06:16:00Z

193.10.252.5

193.10.252.5 0 IGP 2603 3356

1299 559

2603:666 3356:2 3356:86 3356:507 3356:666 3356:2076

Netnod

A 128.178.0.0/15

2003-10-02 06:16:17Z 194.68.48.1 194.68.48.1 0 IGP

12381 1653 2603 20965 559

12381:1653 Netnod

A 128.178.0.0/15

2003-10-02 06:16:37Z 194.68.48.1 194.68.48.1 0 IGP

12381 1653 2603 3356 1299 559

12381:1653 Netnod

A 128.178.0.0/15

2003-10-02 06:21:08Z

193.10.252.5

193.10.252.5 0 IGP 2603 20965

559

2603:222 2603:666 20965:155

Netnod

A 128.178.0.0/15

2003-10-02 06:21:17Z 194.68.48.1 194.68.48.1 0 IGP

12381 1653 2603 20965 559

12381:1653 Netnod

61

559

A 128.178.0.0/15

2003-10-02 07:24:06Z

129.250.0.232

129.250.0.232 9 Not defined 2914 3549

559 2914:420

P r e f i xT i m eP e e rA S p a th

62

The World seen from EPFL

62

http://www.ris.ripe.net/bgpviz/

63

Some statistics

Source: http://www.cidr‐report.org

Address prefixes

ASs

63

64

Number of hosts

64

65

65

66

Exercise

What ASs does EPFL receive service from ?

What ASs does Switch receive service from ?

Find the names of the networks that have these AS numbers

66

67

Exercise

Lookup http://rpsl.info.ucl.ac.be. to find out the relationships between Switch and other providersbetween Switch and other providersHow does the software on this site decide whether a relationship is client, provider or peer ?

67

68

Further Reading

Slow convergence after route suppression – “BGP path exploration” similar to (but worse than) distance vector slowexploration , similar to (but worse than) distance vector slow convergence. Is in the nature of path vector routing with explicit suppression.

Craig Labovitz, Abha Ahuja, Abhijit Bose, Farnam Jahanian: Delayed Internet routing convergence. IEEE/ACM Trans. Netw. 9(3): 293‐306 (2001)

R t fl d i l dRoute flap dampening slows down convergenceZhuoqing Morley Mao, Ramesh Govindan, George Varghese, Randy H. Katz: Route flap damping exacerbates internet routing convergence. SIGCOMM 2002: 221‐233

Path vector + policy may suffer from incompatibilities (loops)

68

The stable paths problem and interdomain routingGriffin, T.G.; Shepherd, F.B.; Wilfong, G.ACM/IEEE ToN April 2002, Page(s): 232‐243

69

References

Timothy Griffin’s home page at IntelRFC 1771 (BGP 4)RFC 1771 (BGP‐4)C. Huitema, “Le Routage dans l’Internet”John W. Stewart III “ BGP 4”www.ris.ripe.net : AS paths

id t ti t ti tiwww.cidr‐report.org aggregation statisticswww.caida.org map of Internetrpsl.info.ucl.ac.be relations between ASs

69

70

Solutions

70

71

Path Vector RoutingQ. Explain how E can choose the best paths to n1 and n2

A. E receives the routes “B A n1” and “D C A n1”. E selects as best routes the ones with shorter AS path.ones with shorter AS path.

Q. How can loops be avoided ?A: BGP routers recognize looping announcements by the repetition of the same AS in the path. Such announcements are discarded

backBGP table in E

B

En1, n2

A:n1,n2B A:n1,n2

B:n5 dest AS path

n1 B An2 B An3 D Cn4 D

BGP table in En5

n4

71

A

CA:n1,n2

C A:n1,n2C:n3

D

D C A:n1,n2D C: n3D: n4

n4 D n5 B

n3

72

Q1: Which of the following announcements do you think router D will send to router E ? 1. D‐>E: D2 X:n1 2. D‐>E: D1 X:n1

A: 1 only; 2 cannot be sent because over I‐BGP we do not repeat a route learnt f th BGP k i th ARDfrom another BGP speaker in the same ARD

Q2: Assume that in the figure below the router R does not run BGP. Is this possible ?A: yes.

Q3: Do BGP speakers in the same ARD need to be on‐link ?Q3: Do BGP speakers in the same ARD need to be on link ?A: no; C and F are not directly connected (are not on‐link)

D1 D2A B

C D

X:n1 X:n1

E-BGPback

72D4 D5

D3

G H

C D

E F

E-BGP

I-BGP R

73


AS1 197.8.2.0/24AS1

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23

197.8.3.0/24

197.8.2.0/23

n2

n4

Q: what happens to packets from n4 to n2 ?A: There are two routing entries in AS4 routers: one for 197.8.2.0/23 and one for 197.8.3.0/24. Longest prefix match in the packet forwarding algorithm ensures that packets to n2 go on the direct route (sent to AS2).

73

( )If AS4 injects only the aggregate route into its local routing tables, what is the answer now ?

There is now only one entry (197.8.2.0/23 ) and all packets to n2 are sent to AS3back

74


AS1 197.8.2.0/24AS1

AS2

AS3 AS4

197.8.3.0/24

197.8.2.0/23 197.8.2.0/23

197.8.3.0/24 AS5 AS6

197.8.3.0/24n2

n4

AS4 receives197.8.2.0/23 AS_PATH: 3 {1 2}197.8.3.0/24 AS_PATH: 6 5 2

Both routes are received by AS4; Q.what happens for packets from n4 to n2?

197.8.3.0/24197.8.3.0/24

74

Both routes are received by AS4; Q.what happens for packets from n4 to n2?A. they go the long wayAssume now that only shortest AS paths routes are injected into routing tables Q: what happens to packets from n4 to n2?A: they go via AS3back

75

Example Without Aggregation

AS1 197.8.2.0/24 197.8.0.0/23 197.8.0.0/23AS1

AS2

AS3 AS4

197.8.3.0/24

197.8.0.0/23

197.8.3.0/24

197.8.2.0/24

197.8.3.0/24

197.8.0.0/23

197.8.2.0/24

197.8.3.0/24

Q: If AS3 does not aggregate, what are the routes announced by AS 4 ? Is there any benefit ?A:

197.8.0.0/23 AS_PATH: 4 3197 8 2 0/24 AS PATH: 4 3 1

75

197.8.2.0/24 AS_PATH: 4 3 1197.8.3.0/24 AS_PATH: 4 2

A: there is no benefit since all routes go via AS 4 anyhow. AS4 should aggregate.back

76

NEXT‐HOP

AS xE-BGP R6

AS z

E-BGP

R1 R2

R5I-BGP

I-BGP

AS y10.1/16 10.2/16

E BGP

R3 R4

76

R3 advertises 10.2/16 to R1, NEXT‐HOP = R4 IP addressR6 advertises 10.2/16 to R5, NEXT‐HOP = R6 IP addressQ. where is such a scenario likely to happen ?A: in interconnection points with many providers interconnected on one LAN back

77

MED Example

Q1: by which mechanisms will R1 and R2 make sure that packets to ASy use the preferred links ?packets to ASy use the preferred links ?A:

R1 and R2 exchange their routes to AX y via I‐BGP

R1 has 2 routes to 10.1/16, one learnt by E‐BGP (MED=10), one learnt by I‐BGP (MED=5=); prefers the former

R1 has 2 routes to 10 2/16 prefers the latter (route via R2)R1 has 2 routes to 10.2/16, prefers the latter (route via R2)

Q2: router R3 crashes; can 10.1/16 still be reached ? explain the sequence of actions.A:

R1 clears routes to ASy learnt from R3 (keep‐alive mechanism)

77

R2 is informed of the route suppression by I‐BGP

R2 has now only 1 route to 10.1/16 and 1 route to 10.2/16;

traffic to 10.1/16 now goes to R2 back

78

MED Question

Q1: Assume now ASx and ASy are peers (ex: both are ISPs). Explain why ASx is not interested in taking MED into account.A: ASx is interested in sending traffic to ASy to the nearest exit, avoiding transit inside ASx as much as possible. Thus ASx will choose the nearest route to ASy, and will ignore MEDsQ2: By which mechanisms can ASx pick the nearest route to ASy ?A: it depends on the IGP. With OSPF: all routes to ASy are injected into OSPF by means type 5 LSAs. These LSAs say: send to router R3 or R4. Every OSPF router inside ASx knows the cost (determined by OSPF weights) of the path from self to R3 and R4. Packets to 10.1/16 and 10.2/16 are routed to the nearest among R3 and R4 (nearest = lowest OSPF cost).

back

78

back

79

LOCAL‐PREF Example

Q1: The link AS2‐AS4 is expensive. How should AS 4 set local‐prefs on routes received from AS 3 and AS 2 in order to route traffic preferably through AS 3 ?A: for example: set LOCAL‐PREF to 100 to all routes received from AS 3 and to 50 toA: for example: set LOCAL PREF to 100 to all routes received from AS 3 and to 50 to all routes received from AS 2

AS 1

3AS 2

AS1: 10.1/1610.1/16AS1: 10.1/16

Q2: Explain the sequence of events for R1, R2 and R3

A: AS 3AS 2

AS 4R1 R2

R3

A: R1 receives the route AS2 AS1 10.1/16

over E-BGP; sets LOCAL-PREF to 50R2 receives the route AS3 AS1 10.1/16 over E-BGP; sets LOCAL-PREF to 100R3 receives AS2 AS1 10.1/16, LOCAL-

PREF=50 from R1 over I-BGP and AS3 AS1 10.1/16, LOCAL-PREF=100

from R1 over I-BGP R3 l t AS3 AS1 10 1/16 LOCAL

AS3 AS1: 10.1/16

79

AS 5R3 selects AS3 AS1 10.1/16, LOCAL-

PREF=100 and installs it into local-RIBR3 announces only AS3 AS1 10.1/16 to

AS 5

back

80

LOCAL‐PREF Question

Q: Compare MED to LOCAL‐PREFA:A:

MED is used between ASs (i.e. over E‐BGP); LOCAL‐PREF is used inside one AS (over I‐BGP)

MED is used to tell one provider AS which entry link to prefer; LOCAL‐PREF is used to tell the rest of the world which AS path we want to use, by not announcing the other ones.g

back

80

81

Example with Re‐Distributionby I‐BGP, F learns from G the route to D2‐D1‐n1

C redistributes the external route D2:n2 into OSPF;

b I BGP D l th t D2 2 b E BGP D l th t DYD2 2 Dby I‐BGP, D learns the route D2:n2; by E‐BGP D learns the route DYD2:n2; D selects D2:n2 and does not redistribute it to OSPF

by I‐BGP, B learns the route D2:n2 from C

by E‐BGP, A learns the route DX:D2:n2

by OSPF, L learns the route to n2 via C

I-BGP - internal BGPE-BGP - external BGP

D1 = EPFL D2 = UCB

n1

D1: n1

n2

A C

FD2 : n2

G

81

DX DYD2 : n2DY D2: n2

D1: n1B

D EL

physical linkTCP connection

back

82

Example: Recursive Table Lookup

At R1, data packet to 10.1.x.y is receivedThe forwarding table at R1 is looked upThe forwarding table at R1 is looked up

Q: what are the next events ?

A: first, the nex‐hop 2.2.2.63 is found; a second lookup for 2.2.2.63 is done; the packet is sent to MAC address x09:F1:6A:33:76:21

T NEXT HOP l 2 dd

R1

To NEXT-HOP layer-2 addr10.1/16 2.2.2.63 N/A

2.2.2.63 2.2.2.33 x09:F1:6A:33:76:21

2.2.2.33 2.2.2.6310.1/16R2 R3

82

R

2.2.2.93R3

back

83

Avoid Redistribution: Combine Recursive Lookup and NEXT‐HOP

AS xE-BGPR5 R6

AS z

Data packet to 18.1.2.3 is received by R2Q: there is a problem at R1: how can we solve it ?

E-BGP

R1

R218.1/16 I-BGP

IGP

IGP(OSPF)

2.2.2.2

AS y

Q: there is a problem at R1: how can we solve it ?A: the problem is: Packet is sent by R2 towards R1, which is the neighbour towards 2.2.2.2. At R1, there is no forwarding entry to 18.1/16 – blackhole !A solution would be to use loose source routing: R2 adds 2.2.2.2 as loose source routing info into packet. Another solution is label switching (see MPLS module) also called tag switching. R2 establishes a label switched

R4

83

g gpath to 2.2.2.2 and forwards the packet on this path.

In practice however, another solution, as shown next.

back

84

Avoid Redistribution: Practical SolutionAS xE-BGP

R2

R5 R6

18.1/16 I-BGPIGP(OSPF)

2.2.20.12.2.2.2

AS z

Q: repeat the sequence of previous slide with this new assumptionR5 advertises 18.1/16, NEXT-HOP = 2.2.2.2 to R6 via E‐BGPR6 transmits 18.1/16, NEXT-HOP = 2.2.2.2 to R1 and R2 via I BGP

E-BGP

R4

R1

R2/

IGP

(OSPF)

AS y

I‐BGPR6 injects 18.1/16, NEXT-HOP = 2.2.2.2 into its local forwarding tableR2 injects 18.1/16, NEXT-HOP = 2.2.2.2 into its local forwarding table

Independently, IGP finds that, at R2, packets to 2.2.10.1 should be sent to R1 Data packet to 18.1.2.3 is received by R2

At R2, recursive table lookup determines that packet should be forwarded to

84

, p pR1At R1, recursive table lookup determines that packet should be forwarded to R6At R6, recursive table lookup determines that packet should be forwarded to 2.2.2.2

back

85

NO—EXPORT

Q: What is the route followed by a packet sent to 2.2.48 received by R4 ?

A: the packet is sent via R3 and R1p

2.2.0/17 NO-EXPORT 2 2/16R1

2.2.0/17

/

2.2/16

2.2/16

R1

R5

R3

R4

85

2.2.128/17

2.2.128/17 NO-EXPORT 2.2/16R2

R5

back

86

Sol. Ex2: Stub Area, Dual Homing to Single ProviderAS x

R2R1

10 1/16 MED=10 10.1/16 MED=50E BGP E BGP

Provider

AS

R4

10.1/16 MED=1010.2/16 MED=50 10.2/16 MED=10

10.1/16 10.2/16

E-BGP E-BGP

R3

Client

Q1: is it possible to avoid distributing BGP routes into Client IGP ?A: yes, for example: configure R3 and R4 as default routers in Client AS; traffic from Client AS is forwarded to nearest of R3 and R4. If R3 or R4 fails, to the remaining oneQ2: is it possible to avoid assigning an AS number to Client ?A: Yes, it is sufficient to assign to Client a private AS number: Provider translates this

AS y

86

, g pnumber to its own.Q3: is it possible to avoid BGP between Client and Provider ?A: Yes, by running a protocol like RIP between Client and Provider and redistributing Client routes into Provider IGP. Thus Provider pretends to the rest of the world that the prefixes of Client are its own. back

87

Sol. Ex3: Stub Area, Dual Homing to Several Providers

AS x

R2R1 P dAS y

AS z

R4

R2R1

/ 2 1/17

E-BGP E-BGP

R3

Cli t

Providers

Client has own address space and AS numberQ: how can routes be announced between AS 100 and AS x ? AS x and AS z ?A: R3 announces 2.0/17 and 2.0/16; traffic from AS x to 2.0/17 will flow via AS x; if

AS 100

2.0/17 2.1/17Client

87

A: R3 announces 2.0/17 and 2.0/16; traffic from AS x to 2.0/17 will flow via AS x; if R3 fails, it will use the longer prefix and flow via Asy.ASx announces 2.0/17 and 2.0/16 to AS zQ: assume Client wants most traffic to prefer AS y. How can that be done ?A: R3 announces an artificially inflated path: 100 100 100 100 : 2.0/17. AS z will favour the path via AS y which has a shorter AS path length back

88

Sol. Ex4: Hot Potato Routing

Customer 1ISP1

R11 R12

ISP2R21 R22

Customer 2

Packets from Customer 2 to Customer 1Both R21 and R22 have a route to Customer 1Shortest path routing favours R21Q1: by which mechanism is that done ?A Ch i f th b t t ( it i 5) i lll t i ISP2 BGP

88

A: « Choice of the best route » (criteria 5), assuming alll routers in ISP2 run BGP

Q2: what is the path followed in the reverse direction ?A: see picture. Note the asymmetric routingback

89

Exercise

What ASs does EPFL receive service from ?from the previous routes we find AS 559 (Switch)from the previous routes, we find AS 559 (Switch)

What ASs does Switch receive service from ?from the previous routes we see that there are at least:

AS 1299

AS 20965

AS 3549AS 3549

Find the names of the networks that have these AS numbersfrom whois on www.ripe.net:

AS 1299: Telianet

AS 20965: Geant

89

AS 3549: Global Crossing

90

Exercise

Lookup http://rpsl.info.ucl.ac.be. to find out the relationships between Switch and other providersbetween Switch and other providersHow does the software on this site decide whether a relationship is client, provider or peer ?

AS X is client of Switch if AS X accepts ANY path and announces only self (AS X)

AS X is provider of Switch if AS X announces ANY path and accepts only AS Switch

AS X is a peer if AS X accepts and announces only a small set of routes

90

91

91

FÉDÉRALE DE LAUSANNE ExternalRouting BGPica · A router that runs BGP is called a BGP speaker At...

Documents

Transcript of FÉDÉRALE DE LAUSANNE ExternalRouting BGPica · A router that runs BGP is called a BGP speaker At...