© SPRINT N. Taft 1 The Basics of BGP Routing and its Performance in Today’s Internet Nina Taft Sprint, Advanced Technology Labs California, USA May 2001.

© SPRINT N. Taft 1

The Basics of BGP Routing and its Performance in Today’s Internet

Nina Taft

Sprint, Advanced Technology Labs

California, USA

May 2001

© SPRINT N. Taft 2

Outline

1. Highlights

2. Addressing and CIDR

3. BGP Messages and Prefix Attributes

4. BGP Decision and Filtering Processes

5. I-BGP

6. Route Reflectors

7. Multihoming

8. Aggregation

9. Routing Instability

10. BGP Table Growth

© SPRINT N. Taft 3

Internet Topology

• AS (Autonomous System) - a collection of routers under the same technical and administrative domain.• EGP (External Gateway Protocol) - used between two AS’s to allow them to exchange routing information so that traffic can be forwarded across AS borders. Example: BGP

large ISP

large ISP

medium ISP

dialupISP

dedicatedaccess ISP

EGP

© SPRINT N. Taft 4

Purpose: to share connectivity information

R border router

internal router

BGPR2

R1

R3

A

AS1

AS2

you can reachnet A via me

traffic to A

table at R1:dest next hopA R2

© SPRINT N. Taft 5

BGP Sessions

• One router can participate in many BGP sessions.• Initially … node advertises ALL routes it wants

neighbor to know (could be >50K routes)• Ongoing … only inform neighbor of changes

BGP SessionsAS1

AS2

AS3

© SPRINT N. Taft 6

Routing Protocols

R border router

internal router

R1

AS1

R4

R5

B

AS3

E-BGP

R2R3

A

AS2 announce B

IGP: Interior Gateway Protocol.Examples: IS-IS, OSPF

I-BGP

© SPRINT N. Taft 7

Configuration and Policy

• A BGP node has a notion of which routes to share with its neighbor. It may only advertise a portion of its routing table to a neighbor.

• A BGP node does not have to accept every route that it learns from its neighbor. It can selectively accept and reject messages.

• What to share with neighbors and what to accept from neighbors is determined by the routing policy, that is specified in a router’s configuration file.

© SPRINT N. Taft 8

History

• Popularity: – until a few years ago BGP fairly unknown. Used by small

number of large ISPs.

– in 1995 (beginning of web popularity) number of organizations using BGP grew tremendously.

• Two reasons for growth of usage and interest: – significant growth in number of ISPs;

– organizations were born that had mission-critical dependence upon their connectivity

• CIDR (Classless Inter-Domain Routing) introduced in 1995

© SPRINT N. Taft 9

Assigning IP address and AS numbers (Ideally)

• A host gets its IP address from the IP address block of its organization

• An organization gets an IP address block from its ISP’s address block

• An ISP gets its address block from its own provider OR from one of the 3 routing registries:– ARIN: American Registry for Internet Numbers

– RIPE: Reseaux IP Europeens

– APNIC: Asia Pacific Network Information Center

• Each AS is assigned a 16-bit number (65536 total) Currently 10,000 AS’s in use

© SPRINT N. Taft 10

Addressing Schemes

• Original addressing schemes (class-based):

– 32 bits divided into 2 parts:

– Class A

– Class B

– Class C

• CIDR introduced to solve 2 problems:– exhaustion of IP address space– size and growth rate of routing table

network host 0

80

network host 0

160

network host 0

240~2 million nets256 hosts


Problem #1: Lifetime of Address Space

• Example: an organization needs 500 addresses. A single class C address not enough (256 hosts). Instead a class B address is allocated. (~64K hosts) That’s overkill -a huge waste.

• CIDR allows networks to be assigned on arbitrary bit boundaries.– permits arbitrary sized masks: 178.24.14.0/23 is valid

– requires explicit masks to be passed in routing protocols

• CIDR solution for example above: organization is allocated a single /23 address (equivalent of 2 class C’s).


Problem #2: Routing Table Size

serviceprovider

232.71.0.0232.71.1.0232.71.2.0…..232.71.255.0

232.71.0.0232.71.1.0232.71.2.0…..232.71.255.0

Globalinternet

Without CIDR:

serviceprovider

232.71.0.0232.71.1.0232.71.2.0…..232.71.255.0

Globalinternet

With CIDR:

232.71.0.0/16


CIDR: Classless Inter-Domain Routing

• Address format <IP address/prefix P>. The prefix denotes the upper P bits of the IP address.

• Idea - use aggregation - provide routing for a large number of customers by advertising one common prefix.– This is possible because nature of addressing is hierarchical

• Summarizing routing information reduces the size of routing tables, but allows to maintain connectivity.

• Aggregation is critical to the scalability and survivability of the Internet


Address Arithmetic: Address Blocks

• The <address/prefix> pair defines an address block:• Examples:

– 128.15.0.0/16 => [ 128.15.0.0 - 128.15.255.255 ]

– 188.24.0.0/13 => [ 188.24.0.0 - 188.31.255.255 ]consider 2nd octet in binary:

• Address block sizes– a /13 address block has 232-13 addresses (/16 has 232-16)

– a /13 address block is 8 times as big as a /16 address blockbecause 232-13 = 232-16 * 23

13th bit

198.00011000.0.0

settable


CIDR: longest prefix match

• Because prefixes of arbitrary length allowed, overlapping prefixes can exist.

• Example: router hears 124.39.0.0/16 from one neighborand 124.39.11.0/24 from another neighbor

• Router forwards packet according to most specific forwarding information, called longest prefix match– Packet with destination 124.39.11.32 will be forwarded

using /24 entry.

– Packet w/destination 124.39.22.45 will be forwarded using /16 entry


Will CIDR work ?

• For CIDR to be successful need:– address registries must assign addresses using CIDR

strategy

– providers and subscribers should configure their networks, and allocate addresses to allow for a maximum amount of aggregation

– BGP must be configured to do aggregation as much as possible

• Factors that complicate achieving aggregation– multihoming, proxy aggregation, changing providers


Four Basic Messages

• Open: Establishes BGP session (uses TCP port #179)

• Notification: Report unusual conditions

• Update:Inform neighbor of new routes that become activeInform neighbor of old routes that become inactive

• Keepalive: Inform neighbor that connection is still viable


UPDATE Message

• used to either advertise and/or withdraw prefixes• path attributes: list of attributes that pertain to

ALL the prefixes in the Reachability Info field

Withdrawn routes length (2 octets)

Withdrawn routes (variable length)

Total path attributes length (2 octets)

Path Attributes (variable length)

Reachability Information (variable length)

FORMAT:


ATTRIBUTES

Prefix Next hop AS Path

128.73.4.21/21 232.14.63.4 1239 701 3985 631

• ORIGIN: – Who originated the announcement? Where was a prefix injected

into BGP?

– IGP, EGP or Incomplete (often used for static routes)

• AS-PATH:– a list of AS’s through which the announcement for a prefix has

passed

– each AS prepends its AS # to the AS-PATH attribute when forwarding an announcement

– useful to detect and prevent loops


Attribute: NEXT HOP

• For EBGP session, NEXT HOP = IP address of neighbor that announced the route.

• For IBGP sessions, if route originated inside AS, NEXT HOP = IP address of neighbor that announced the route

• For routes originated outside AS, NEXT HOP of EBGP node that learned of route, is carried unaltered into IBGP.

Destination Next Hop

140.20.1.0/24 2.2.2.2

209.15.1.0/24 1.1.1.1

Destination Next Hop

140.20.1.0/24 2.2.2.2

2.2.2.0/24 3.3.3.3

3.3.3.0/24 Connected

209.15.1.0/24 1.1.1.1

1.1.1.0/24 3.3.3.3

BGP Table at Router C:

IP Routing Table at Router C:

B

A

D

C 2.2.2.2

3.3.3.3

140.20.1.0/24IBGP

EBGP

209.15.1.0/24

1.1.1.1


Attribute: Multi-Exit Discriminator (MED)

• when AS’s interconnected via 2 or more links

• AS announcing prefix sets MED

• enables AS2 to indicate its preference

• AS receiving prefix uses MED to select link

• a way to specify how close a prefix is to the link it is announced on

Link BLink A

MED=10MED=50

AS1

AS2

AS4 AS3


Attribute: Local Preference

• Used to indicate preference among multiple paths for the same prefix anywhere in the internet.

• The higher the value the more preferred

• Exchanged between IBGP peers only. Local to the AS.

• Often used to select a specific exit point for a particular destination

AS4

AS2 AS3

AS1

140.20.1.0/24

Destination AS Path Local Pref

140.20.1.0/24 AS3 AS1 300

140.20.1.0/24 AS2 AS1 100

BGP table at AS4:


Routing Process Overview

Inputpolicyengine

Decisionprocess

Routesused byrouter

Routesreceived from peers

Routessent topeersOutput

policyengine

BGP table IP routingtable

Choosebest route

accept,deny, set preferences

forward,not forwardset MEDs


Input Policy Engine

• Inbound filtering controls outbound traffic– filters route updates received from other peers

– filtering based on IP prefixes, AS_PATH, community

– denying a prefix means BGP does not want to reach that prefix via the peer that sent the announcement

– accepting a prefix means traffic towards that prefix may be forwarded to the peer that sent the announcement

• Attribute Manipulation– sets attributes on accepted routes

– example: specify LOCAL_PREF to set priorities among multiple peers that can reach a given destination


BGP Decision Process

1. Choose route with highest LOCAL-PREF

2. If have more than 1 route, select route with shortest AS-PATH

3. If have more than 1 route, select according to lowest ORIGIN type where IGP < EGP < INCOMPLETE

4. If have more than 1 route, select route with lowest MED value

5. Select min cost path to NEXT HOP using IGP metrics

6. If have multiple internal paths, use BGP Router ID to break tie.


Output Policy Engine

• Outbound Filtering controls inbound traffic– forwarding a route means others may choose to reach

the prefix through you

– not forwarding a route means others must use another router to reach the prefix

– may depend upon whether E-BGP or I-BGP peer

– example: if ORIGIN=EGP and you are a non-transit AS and BGP peer is non-customer, then don’t forward

• Attribute Manipulation– sets attributes such as AS_PATH and MEDs


Transit vs. Nontransit AS

AS1

ISP1 ISP2

r1r2 r2

r3

r2

r1 r3

AS1

ISP1 ISP2

r1r2,r3 r2,r1

r3

r2

r1 r3

Transit traffic = traffic whose source and destination are outside the AS

Nontransit AS: does not carry transit traffic Transit AS: does carry transit traffic• Advertise own routes only• Do not propagate routes learned from other AS’s• case 1:

• Advertises its own routes PLUS routeslearned from other AS’s

AS1

r1r2 r2r3

r2

AS2r1 r3• case 2:


Clients, Providers and Peers

• AS has customers, providers and peers

• Relationships between AS pairs:– customer-provider

– peer-to-peer

• Type of relationship influences policies

• Exporting to provider:AS exports its routes & its customer’s routes, but not routes learned from other providers or peers

• Exporting to peer: (same as above)

• Exporting to customer:AS exports its routes plus routes learned from its providers and peers


Internal BGP (I-BGP)

• Used to distribute routes learned via EBGP to all the routers within an AS

• I-BGP and E-BGP are same protocol in that– same message types used– same attributes used– same state machine– BUT use different rules for readvertising prefixes

• Rule #1: prefixes learned from an E-BGP neighbor can be readvertised to an I-BGP neighbor, and vice versa

• Rule #2: prefixes learned from an I-BGP neighbor cannot be readvertised to another I-BGP neighbor


I-BGP: Preventing Loops and Setting Attributes

• Why rule #2? To prevent announcements from looping. – In E-BGP can detect via AS-PATH.

– AS-PATH not changed in I-BGP

• Implication of rule: a full mesh of I-BGP sessions between each pair of routers in an AS is required

• Setting Attributes: The router that injects the route into the I-BGP mesh is responsible for – setting the LOCAL-PREF attribute

– prepending AS # to AS-PATH


Route Reflectors

• Problem: requiring a full mesh of I-BGP sessions between all pairs of routers is hard to manage for large AS’s.

• Solution: – group routers into clusters.

– Assign a leader to each cluster, called a route reflector (RR).

– Members of a cluster are called clients of the RR

• I-BGP Peering

– clients peer only with their RR

– RR’s must be fully meshed

clients

RR

AB

C clusters

RR

RR


Route Reflectors: Rule on Announcements

• If received from RR, reflect to clients• If received from a client, reflect to RRs and clients• If received from E-BGP, reflect to all - RRs and

clients• RR’s reflect only the best route to a given prefix,

not all announcements they receive. – helps size of routing table

– sometimes clients don’t need to carry full table


Avoiding Loops with Route Reflectors

• Loops cannot be detected by traditional approach using AS-PATH because AS-PATH not modified within an AS.

• Announcements could leave a cluster and re-enter it.• Two new attributes introduced:

– ORIGINATOR_ID: router id of route’s originator in ASrule: announcement discarded if returns to originator

– CLUSTER_LIST: a sequence of cluster id’s. set by RRs.rule: if an RR receives an update and the cluster list contains its cluster id, then update is discarded.


Default Routes

• If you don’t have a routing entry in the table for a destination, send it along the default route

• Can be statically configured

• Can be learned via BGP

• Can have multiple defaults– use LOCAL_PREF to

distinguish primary and backup default routes

AS1

0.0.0.0/0 next_hop=1.1.1.1

0.0.0.0/0 next_hop=2.2.2.2

AS2

Local pref=100Local pref=300


Multihoming


Single-homed vs. Multi-homed subscribers

• A single-homed network has one connection to the Internet (i.e., to networks outside its domain)

• A multi-homed network has multiple connections to the Internet. Two scenarios:– can be multi-homed to a single provider

– can be multi-homed to multiple providers

• Why multi-home?– Reliability

– Performance• a site’s bandwidth to internet is sum of bandwidth on all links


Single-homed AS

• Subscriber called a “stub AS”

• Provider-Subscriber communication for route advertisement:– static configuration. most common.

• Provider’s router is configured with subscriber’s prefix.

• good if customer’s routes can be represented by small set of aggregate routes

• bad if customer has many noncontiguous subnets

– can use BGP between provider and stub AS

R2

Subscriber

R1

Provider


Multihoming to a Single Provider: 4 scenarios

R2

Customer

R1

ISP

R2

Customer

R1

ISP

R2

Customer

R3

R1

ISP

R3

Customer

R1

ISP

R2

R4R3


Multihoming to Multiple Providers

Customer

ISP 1 ISP 2

ISP 3


Multihoming Issues

• Load sharing– how distribute the traffic over the multiple links?

• Reliability– if load sharing leads to preferencing certain links for

certain subnets, is reliability reduced?

• Address/Aggregation– which subnet addresses should the multihomed

customer use?

– how will this affect its provider’s ability to aggregate routes?


Load sharing from ISP to Customer using attributes

• Goal: provider splits traffic across 2 links according to prefix

• Implement this strategy using attributes– customer sets MEDs

– provider sets LOCAL_PREF

R2

Customer

R3

R1

ISP

140.35/16

208.22/16

208.22/16140.35/16


Load sharing from Customer to ISP using policy

• Goal: send traffic to ISP’s customers on one link; send traffic to the rest of the Internet on 2nd link

• Implement using policy to control announcements

R3

Customer

R1

ISP

R2

advertisecustomerroutes only

advertisedefaultroute 0/0

traffic

blue: announcementsred: traffic


Address/Aggregation Issue

• Where should the customer get its address block from?– 1. From ISP1

– 2. From ISP2

– 3. From both ISP1 and ISP2

– 4. Independently from address registry

ISP 3

ISP 1 ISP 2

customer(cases 1 and 2 are equivalent)


Case 1 & 2: Get address block from one ISP

ISP 3

ISP 1

140.20/16

ISP 2

200.50/16

140.20.6/24

customer

140.20.6/24140.20.6/24

140.20/16200.50/16140.20.6/24

• example: customer gets address from ISP 1

• ISP 1’s aggregation is not broken

• customer’s prefix not aggregatable at ISP 2

• longer prefix becomes a traffic magnet

• How good is load sharing?If all ISP’s generate same amount of traffic for customer, then ISP2-customer link twice as loaded as ISP1-customer link


Case 3: Get address block from both ISPs

• announcement policy: announce prefix only to its “parent”

• advantage: both ISP’s can aggregate the prefix they receive

• disadvantage: lose reliability

• load balancing good? depends upon how much traffic sent to each prefix

ISP 3

ISP 1 ISP 2

customer

140.20/16 200.50/16

140.20.1/24 200.50.1/24

140.20.1/24 200.50.1/24


Case 4: obtain address block from registry

• no aggregation possible

• no traffic magnets created

• good reliability

• how achieve load sharing?– customer breaks address

block into 2 /25 blocks, and announce one per link (but may lose reliability)

– OR use method of AS-PATH manipulation

150.55.10/24

150.55.10/24

ISP 3

ISP 1 ISP 2

customer

100.20/16 200.50/16

150.55.10/24

150.55.10/24

150.55.10/24

200.50/16100.20/16


AS-PATH manipulation

• Idea: prepend your AS number in AS-PATH multiple times to discourage use of a link

• makes AS-PATH seem longer than it is

• recall BGP decision process uses shortest AS-PATH length as a criteria for selecting best path

• Example: ISP 3 will choose path through AS 2 because its AS-PATH appears shorter

150.55.10/24 - 33

ISP 3

AS 1 AS 2

150.55.10/24 - 33 33

customer

150.55.10/24

AS 33

150.55.10/24 - 1 33 33 150.55.10/24 - 2 33


Aggregation


Address Arithmetic: When is aggregation possible? Case 1

• Possible when one prefix contained in another. • Example: Two AS’s having customer-provider

relationship. Provider does the aggregation. – Provider has address block 18.0.0.0/8

– Its customers have address blocks 18.6.0.0/15 and 18.9.0.0/15

– Provider announces its address block only

• Rule: Prefix p1 contains prefix p2 iff length(p2) > length(p1) ANDaddress(p2) / 232-length(p1) = address(p1) / 232-length(p1)


Address Arithmetic: When is aggregation possible? Case 2

• Some pairs of consecutive prefixes• Example: routes within the same AS:

AS has 2 address blocks: 1.2.2.0/24 = 0000001.00000010.00000010.00000000/241.2.3.0/24 = 0000001.00000010.00000011.00000000/24can announce instead 1.2.2.0/23

• Rule: consecutive prefixes p1 and p2 are aggregatable iff length(p1)=length(p2) AND

address(p1) / 232-length(p1) +1 = address(p2) / 232-length(p2) AND address(p1) % 233-length(p1) = 0


• “The cardinal sin of BGP routing is advertising routes that you don't know how to get to; called "black-holing" someone”

• If you announce part of IP space owned by someone else, using a more-specific prefix, all their traffic flows to you. Makes that address block disconnected from the Internet.

• Example: black holes can happen inadvertently by non-careful aggregation

Black holes and cardinal sins

ISP 1100.24/16

ISP 2222.2/16

NAP

100.24.16.0/21

Net A

[100.24.16.0-100.24.23.255]

100.24.0.0/20

Net B[100.24.0.0-100.24.15.255]

Net C

100.24.56.0/21

[100.24.56.0-100.24.63.255]

100.24/16

222.2/16

wrong !!100.24.0.0/18


Limitations to Aggregation

• Lack of hierarchical allocation of address space prior to CIDR (before 1995)

• A single AS can have noncontiguous address blocks• Customer AS and provider AS can have non-

contiguous address blocks• Reluctance of customers to renumber their address

space when they switch providers• Multi-homing

– multi-homed prefixes require global visibility

– may choose not to: load sharing


Rules of Thumb for Aggregation

• To avoid black holes: an ISP is not allowed to aggregate routes unless it is a supernet of the address block it wants to aggregate

• In other words, an ISP has to specifically announce each IP address range of its downstream customers that are not part of its own address space.


Routing Instability


Route Stability

• Routing instability: rapid fluctuation of network reachability information

• route flapping: when a route is withdrawn and re-announced repeatedly in a short period of time– happens via UPDATE messages

• because messages propagate to global Internet, route flapping behavior can cascade and deteriorate routing performance in many places

• Effects: increased packet loss, increased network latency, CPU overhead, loss of connectivity

• Route Stability Studies by C. Labovitz, R. Malan & F. Jahanian


Types of Routing Updates

• Forwarding instability– reflects legitimate topology changes – e.g., changes in Prefix, NEXT_HOP and/or ASPATH– affects forwarding paths used

• Policy fluctuation– reflects changes in policy – e.g., changes in MED, LOCAL_PREF, etc.– may not necessarily affect forwarding paths used

• Pathological– redundant messages– reflect neither topology nor policy changes


Anecdotes of Route Flap Storms

• April 25, 1997 - small Virginia ISP injected incorrect map into global Internet. Map said Virginia ISP had optimal connectivity to all destinations. Everyone sent their traffic to this ISP. Result: shutdown of Tier-1 ISPs for 2 hours.

• August 14, 1998 - misconfigured database server forwarded all queries to “.net” to wrong server. Result: loss of connectivity to all .net servers for few hours.

• Nov. 8, 1998 - router software bug led to malformed routing control message. Caused interoperability problem between Tier-1 ISPs. Result: persistent pathological oscillations and connectivity loss for several hours.


Taxonomy (as per Labovitz et. al.)

Name Type Character

WADiff Explicit withdrawal followedby announcement. Replaceroute with different path

Legitimate

AADiff Announced twice (implicitwithdrawal). Replace routewith different path.

Legitimate

WADup Explicit withdrawal followedby announcement. Replaceroute with same path.

Legitimate orpathological

AADup Announced twice (implicitwithdrawal). Replace routewith same path.

Policy change orpathological

WWDup Repeated duplicatewithdrawals

Pathological


General Statistics

• 1996: 3-5 million updates per day in Internet core• 1998: 300K-700K updates per day in Internet core• 1996: average number of announcements per day

was ~275K.• 1998: average number of announcements per day

was ~400K• Correlation of instability and usage

– instability highest during business hours

– instability lowest during nights, on weekends and in summer


Per Event Type Statistics

• 1996 relative impact (approximately): WWDup (96%), AADup (2%), WADup (1%), AADiff(1/2%), WADiff (1/2%)

• 1998 relative impact (approximately): AADup (30-40%), WWDup(25-30%), AADiff (~20%) other (rest)


Who’s Responsible?

• AS’s– No single AS dominates instability statistics

– No correlation between the size of an AS and its share of updates generated.

• Prefixes– Instability is evenly distributed across routes.

– Example of measurements: • 75% of AADiff events come from prefixes change less than 10

times a day.

• 80-90% of instability comes from prefixes that are announced less than 50 times/day.


Sources of Instabilities in General

• router configuration errors• transient physical and data link problems• software bugs• problems with leased lines (electrical timing issues

that cause false alarms of disconnect)• router failures• network upgrades and maintenance


Instability Problem and Cause. Example 1.

• Problem: 3-5 million duplicate withdrawals• Cause: stateless BGP implementation

– time-space tradeoff: no state maintained on what advertised to peers

– when receive any change, transmit withdrawal to all peers regardless of whether previously notified or not

– sent updates for both explicit and implicit withdrawals

• By 1998, most vendors had BGP implementations with partial state.

• Result: number of WWDups reduced by an order of magnitude


Instability Problem and Cause. Example 2

• Problem: duplicate announcements• Cause: min-advertisement timer & stateless BGP

– min-adv timer: wait 30 seconds. Combine all received updates in last 30 seconds into single outbound update message (if possible).

– within 30 seconds route can be withdrawn and re-announced so that there is no net change to original announcement

• Solution: Have BGP keep some state about recently sent messages to peers. Avoid sending duplicate messages


Instability Problem and Cause. Example 3

AS 1R2

R1 R3

R4

R6

R5

R border router

internal router

AS2

AS3

R8

R7

E-BGP

10

23

5 6

IGP

Net X

4

MED=15

MED=5

Example: interaction IGP/BGPpolicy: set MED using IGP metrics, such as shortest path

MED=24


Controlling route instability: Route Dampening

• track number of times a route has flapped over a period of time

• routes that exhibit a high level of instability in a short period of time should be suppressed (not advertised)

• penalize ill behaved routes proportionally to their expected future stability

• if a suppressed route stops flapping for a long enough period of time, unsuppress it (readvertise)


Route Dampening

time

pena

lty

reuse limit

suppress limit


Route Dampening Algorithm

• Each time a route flaps, increase the penalty• If the route has not flapped in the last ‘half-life’

time units, then cut penalty in half.• If the penalty > suppress limit, then suppress the

route• If the penalty < reuse limit, then free a suppressed

route


Side Effect of Route Dampening

• A legitimate update may arrive and it will be ignored because that route has been suppressed and not yet released.

• The modification needed for the legitimate announcement is delayed


Aggregation can help route flapping

• If a more-specific route is flapping, but provider only announces aggregated prefix, then other networks don’t see route flap.

• Hence aggregation can mask route flapping.

• Aggregation helps combat instability because it reduces the number of networks visible in the core Internet.

Tier 1 provider

Tier 2 provider

customer

140.40.4/24

140.40/16

flapping

not flapping


Growth of BGP Tables


Long Term Growth Trends in Internet Routing

• Will this routing system be able to scale and meet the growth of the Internet and its ever-expanding level of demands?– Are there any inherent limitations?

– As more devices connect to Internet and consume addresses, the need to maintain reachability to these addresses implies larger routing tables

• What is the ability of the system to produce a stable view of the overall network topology?


BGP Table Growth (1989-2001)


Reasons for Exponential Growth

Measures Growth

Number of AS’s Growing exponentially - 50%yearly

Range of addresses spanned bytable

Growing 7% yearly

Average span of route tableentry

Growing 1 bit per 29 months

Holes in the table Currently at 37%

Number of announcements Growing 42% yearly

Data in last 3 slides from Geoff Huston’s INET publications


Increasing fine levels of routing details in BGP table

• AS space growth at 50% &

addresses spanned by the table growing at 7%

=> each AS advertising smaller address ranges

• /24 is fastest growth prefix in table (in absolute terms). /24 - /31 is area with fastest relative growth

• 1999: average span of prefix 16,000 addresses (mean prefix length 18.03)2000: average span of prefix 12,000 addresses (mean prefix length 18.44)


Holes

• When both aggregated prefix and a more-specific prefix exist in the table, the more-specific prefix is called a “hole”.

• Why are holes created?– Punch hole in policy of larger aggregated

announcement to create a different policy for finer announcement.

– Load sharing in multihoming scenario

– reliability via multihoming

• 37% of BGP table is holes.


Multihoming vs. Resiliency

• Multiple peering relationships can be cheaper than using a single upstream provider– implies: multihoming is seen as a substitute for upstream

service resiliency

• Impact– providers can’t command a price for reliability, and thus

don’t spend money to engineer it into network.

– resiliency is becoming responsibility of customer not provider

• Can BGP scale adequately to continue to undertake this role?


Conclusions

• Things are getting better (stability)– router software and configuration management are

maturing

– increased emphasis on aggregation and route dampening are helping

• Things are getting worse (scalability)– multihoming is still growing

– internet topology growing less hierarchical

– ‘noise’ in BGP table is growing


Bibliography

Papers

M. Faloutsos, P. Faloutsos and C. Faloutsos. “On Power-Law Relationships of the Internet Topology.”Proceedings of ACM SIGCOMM. August 1999.

L. Gao and J. Rexford. “Stable Internet Routing without Global Coordination”. ACM Sigmetrics. June2000.

R. Govindan and A. Reddy. “An Analysis of Internet Inter-domain Topology and Route Stability”. Proc.IEEE Infocom. April 1997.

T. G. Griffin and G. Wilfong. “An Analysis of BGP Convergence Properties” Proceedings of ACMSIGCOMM. August 1999.

Geoff Huston. “Interconnection, Peering and Settlements.” Proceedings of INET, June 1999.

Geoff Huston. “Analyzing the Internet BGP Routing Table.” Proceedings of INET. March 2000.

C. Labovitz, G. R. Malan and F. Jahanian. “Origins of Internet Routing Instability”. Proceedings of IEEEInfocom. March 1999.

C. Labovitz, A. Ahuja and F. Jahanian. “Experimental Study of Internet Stability and Wide-Area BackboneFailures” Proceddings of Fault-Tolerant Ccomputing Symposium. 1999.


Bibliography (cont’d)

C. Labovitz, R. Wattenhofer, S. Venkatachary and A. Ahuja. “The Impact of Internet Policy and Topologyon Delayed Routing Convergence.” Proc. IEEE Infocom 2001.

C. Labovitz, G. R. Malan and F. Jahanian. “Internet Routing Instability”. ACM Sigcomm. 1997.

Vern Paxon, “End-to-End Routing Behavior in the Internet”. IEEE/ACM Transactions on Networking. Vol.5. No. 5. pp. 601-615. October 1997.

Books

Sam Halabi. “Internet Routing Architectures. 2nd Edition”. Cisco Systems Press. 2000

John W. Stewart. BGP4: “Inter-Domain Routing in the Internet.” Addison-Wesley. 1999.

RFCs

E. Chen and T. Bates. “An Application of the BGP Community Attribute in Multi-homing Routing.” RFC1998. August 1996.

Y. Rekher and T. Li. “A Border Gateway Protocol.” RFC 1771 (BGPv4)

C. Villamizar, R. Chandra and R. Govindan. “BGP Route Flap Damping” RFC 2439. 1998

© SPRINT N. Taft 1 The Basics of BGP Routing and its Performance in Today’s Internet Nina Taft Sprint, Advanced Technology Labs California, USA May 2001.

Documents

bgp slide

bgp node

bgp messages

bgp table growth slide

basics of bgp routing

bgp decision

ip address block

routing information