© 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 medium ISP dialup ISP dedicated access ISP EGP

© SPRINT N. Taft 4 Purpose: to share connectivity information R border router internal router BGP R2R1R3 A AS1 AS2 you can reach net A via me traffic to A table at R1: dest next hop AR2

© 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 Sessions AS1 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 network host ~2 million nets 256 hosts

© SPRINT N. Taft 11 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: /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).

© SPRINT N. Taft 12 Problem #2: Routing Table Size service provider … … Global internet Without CIDR : service provider … Global internet With CIDR : /16

© SPRINT N. Taft 13 CIDR: Classless Inter-Domain Routing Address format. 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

© SPRINT N. Taft 14 Address Arithmetic: Address Blocks The pair defines an address block: Examples: – /16 => [ ] – /13 => [ ] consider 2nd octet in binary: Address block sizes –a /13 address block has addresses (/16 has ) –a /13 address block is 8 times as big as a /16 address block because = * th bit settable

© SPRINT N. Taft 15 CIDR: longest prefix match Because prefixes of arbitrary length allowed, overlapping prefixes can exist. Example: router hears /16 from one neighbor and /24 from another neighbor Router forwards packet according to most specific forwarding information, called longest prefix match –Packet with destination will be forwarded using /24 entry. –Packet w/destination will be forwarded using /16 entry

© SPRINT N. Taft 16 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

© SPRINT N. Taft 17 Four Basic Messages Open: Establishes BGP session (uses TCP port #179) Notification: Report unusual conditions Update: Inform neighbor of new routes that become active Inform neighbor of old routes that become inactive Keepalive: Inform neighbor that connection is still viable

© SPRINT N. Taft 18 OPEN Message During session establishment, two BGP speakers exchange their –AS numbers –BGP identifiers (usually one of the router’s IP addresses) A BGP speaker has option to refuse a session Select the value of the hold timer: – maximum time to wait to hear something from other end before assuming session is down. authentication information (optional)

© SPRINT N. Taft 19 NOTIFICATION and KEEPALIVE Messages NOTIFICATION –Indicates an error –terminates the TCP session –gives receiver an indication of why BGP session terminated –Examples: header errors, hold timer expiry, bad peer AS, bad BGP identifier, malformed attribute list, missing required attribute, AS routing loop, etc. KEEPALIVE –protocol requires some data to be sent periodically. If no UPDATE to send within the specified time period, then send KEEPALIVE message to assure partner that connection still alive

© SPRINT N. Taft 20 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:

© SPRINT N. Taft 21 Advertising a prefix When a router advertises a prefix to one of its BGP neighbors: –information is valid until first router explicitly advertises that the information is no longer valid –BGP does not require routing information to be refreshed –if node A advertises a path for a prefix to node B, then node B can be sure node A is using that path itself to reach the destination.

© SPRINT N. Taft 22 ATTRIBUTES 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

© SPRINT N. Taft 23 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. BGP Table at Router C: IP Routing Table at Router C: B A D C /24 IBGP EBGP /

© SPRINT N. Taft 24 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 B Link A MED=10 MED=50 AS1 AS2 AS4 AS3

© SPRINT N. Taft 25 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 AS /24 BGP table at AS4:

© SPRINT N. Taft 26 Routing Process Overview Input policy engine Decision process Routes used by router Routes received from peers Routes sent to peers Output policy engine BGP tableIP routing table Choose best route accept, deny, set preferences forward, not forward set MEDs

© SPRINT N. Taft 27 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

© SPRINT N. Taft 28 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.

© SPRINT N. Taft 29 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

© SPRINT N. Taft 30 Transit vs. Nontransit AS AS1 ISP1 ISP2 r1 r2 r3 r2 r1 r3 AS1 ISP1 ISP2 r1 r2,r3r2,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 routes learned from other AS’s AS1 r1 r2 r3 r2 AS2 r1 r3 case 2:

© SPRINT N. Taft 31 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

© SPRINT N. Taft 32 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

© SPRINT N. Taft 33 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

© SPRINT N. Taft 34 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 A B C clusters RR

© SPRINT N. Taft 35 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

© SPRINT N. Taft 36 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 AS rule: 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.

© SPRINT N. Taft 37 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 AS /0 next_hop= /0 next_hop= AS2 Local pref=100 Local pref=300

© SPRINT N. Taft 38 Multihoming

© SPRINT N. Taft 39 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

© SPRINT N. Taft 40 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

© SPRINT N. Taft 41 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

© SPRINT N. Taft 42 Multihoming to Multiple Providers Customer ISP 1 ISP 2 ISP 3

© SPRINT N. Taft 43 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?

© SPRINT N. Taft 44 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 / / /16

© SPRINT N. Taft 45 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 advertise customer routes only advertise default route 0/0 traffic blue: announcements red: traffic

© SPRINT N. Taft 46 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)

© SPRINT N. Taft 47 Case 1 & 2: Get address block from one ISP ISP 3 ISP /16 ISP / /24 customer / / / /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

© SPRINT N. Taft 48 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 / / / / / /24

© SPRINT N. Taft 49 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 /24 ISP 3 ISP 1 ISP 2 customer / / / / /16

© SPRINT N. Taft 50 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 / ISP 3 AS 1 AS / customer /24 AS / /

© SPRINT N. Taft 51 Aggregation

© SPRINT N. Taft 52 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 /8 –Its customers have address blocks /15 and /15 –Provider announces its address block only Rule: Prefix p1 contains prefix p2 iff length(p2) > length(p1) AND address(p2) / 2 32-length(p1) = address(p1) / 2 32-length(p1)

© SPRINT N. Taft 53 Address Arithmetic: When is aggregation possible? Case 2 Some pairs of consecutive prefixes Example: routes within the same AS: AS has 2 address blocks: /24 = / /24 = /24 can announce instead /23 Rule: consecutive prefixes p1 and p2 are aggregatable iff length(p1)=length(p2) AND address(p1) / 2 32-length(p1) +1 = address(p2) / 2 32-length(p2) AND address(p1) % 2 33-length(p1) = 0

© SPRINT N. Taft 54 Aggregation and Multihoming ISP 3 AS 1 AS 2 customer / / /24 Most common scenario: customer breaks its address block in 2 for load sharing purposes. YET, also advertises whole block for reliability.

© SPRINT N. Taft 55 “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 /16 ISP /16 NAP /21 Net A [ ] /20 Net B [ ] Net C /21 [ ] / /16 wrong !! /18

© SPRINT N. Taft 56 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

© SPRINT N. Taft 57 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.

© SPRINT N. Taft 58 Routing Instability

© SPRINT N. Taft 59 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

© SPRINT N. Taft 60 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

© SPRINT N. Taft 61 Anecdotes of Route Flap Storms April 25, 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, misconfigured database server forwarded all queries to “.net” to wrong server. Result: loss of connectivity to all.net servers for few hours. Nov. 8, 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.

© SPRINT N. Taft 62 Taxonomy (as per Labovitz et. al.)

© SPRINT N. Taft 63 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

© SPRINT N. Taft 64 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)

© SPRINT N. Taft 65 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 % of instability comes from prefixes that are announced less than 50 times/day.

© SPRINT N. Taft 66 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

© SPRINT N. Taft 67 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

© SPRINT N. Taft 68 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

© SPRINT N. Taft 69 Instability Problem and Cause. Example 3 AS 1 R2R1R3R4R6R5R border router internal router AS2 AS3 R8R7 E-BGP IGP Net X 4 MED=15MED=5 Example: interaction IGP/BGP policy: set MED using IGP metrics, such as shortest path MED=24

© SPRINT N. Taft 70 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)

© SPRINT N. Taft 71 Route Dampening time penalty reuse limit suppress limit

© SPRINT N. Taft 72 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

© SPRINT N. Taft 73 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

© SPRINT N. Taft 74 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 providerTier 2 providercustomer / /16 flapping not flapping

© SPRINT N. Taft 75 Growth of BGP Tables

© SPRINT N. Taft 76 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?

© SPRINT N. Taft 77 BGP Table Growth ( )

© SPRINT N. Taft 78 AS Number Growth

© SPRINT N. Taft 79 Reasons for Exponential Growth Data in last 3 slides from Geoff Huston’s INET publications

© SPRINT N. Taft 80 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)

© SPRINT N. Taft 81 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.

© SPRINT N. Taft 82 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?

© SPRINT N. Taft 83 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

© SPRINT N. Taft 84 Bibliography

© SPRINT N. Taft 85 Bibliography (cont’d)