Introduction to BGP. BGP Basics BGP. Border Gateway Protocol. Path Vector Protocol. Path Vector Protocol INET 2000 NTW

Transcription

1 Introduction to GP GP asics INET 2000 NTW quick reminder ISP/IXP Workshops 2000, isco Systems, Inc. 1 ISP/IXP Presentation_I Workshops 2000, 1999, isco Systems, Inc. 2 order Gateway Protocol GP Routing Protocol used to exchange routing information between networks exterior gateway protocol RF1771 work in progress to update draft-ietf-idr-bgp4-10.txt urrently Version 4 Runs over TP 3 Path Vector Protocol Incremental Updates Many options for policy enforcement lassless Inter omain Routing (IR) Widely used for Internet backbone utonomous systems 4 Path Vector Protocol Path Vector Protocol GP is classified as a path vector routing protocol (see RF 1322) S6337 S11268 path vector protocol defines a route as a pairing between a destination and the attributes of the path to that destination. S7018 S / i S6461 S Path S

2 efinitions Peering and Transit example Transit - carrying traffic across a network, usually for a fee Peering - exchanging routing information and traffic efault - where to send traffic when there is no explicit match is in the routing table provider provider IXP- West ackbone Provider and can peer, but need transit arrangements with to get packets to/from IXP-East provider 7 8 utonomous System (S) emarcation Zone (MZ) S 100 MZ S 100 Network S 101 ollection of networks with same routing policy Single routing protocol E S 102 Usually under single ownership, trust and administrative control Shared network between Ses 9 10 GP asics GP General Operation Peering S 100 S 101 GP speakers are called peers E S 102 Learns multiple paths via internal and external GP speakers Picks the best path and installs in the forwarding table Policies applied by influencing the best path selection

3 onstructing the Forwarding Table GP in process receives path information from peers results of GP path selection placed in the GP table best path flagged GP out process announces best path information to peers est paths installed in forwarding table if: prefix and prefix length are unique lowest protocol distance in bgp peer out onstructing the Forwarding Table everything best paths GP in process GP out process GP table discarded accepted forwarding table External GP Peering (egp) onfiguring External GP (isco IOS) S 100 S 101 Router in S100 interface ethernet 5/0 ip address network mask neighbor remote-as 101 neighbor prefix-list Router in neighbor prefix-list Router out etween GP speakers in different S Should be directly connected o not run an IGP between egp peers 15 Router in S101 interface ethernet 1/0/0 ip address router bgp 101 network mask neighbor remote-as 100 neighbor prefix-list Router in neighbor prefix-list Router out 16 Internal GP (igp) Internal GP Peering (igp) GP peer within the same S Not required to be directly connected S 100 igp speakers need to be fully meshed they originate connected networks they do not pass on prefixes learned from other igp speakers 17 Topology independent Each igp speaker must peer with every other igp speaker in the S E 18 3

4 Stable igp Peering Peering to Loop-ack ddress Peer with loop-back address igp session is not dependent on state of a single interface igp session is not dependent on physical topology Loop-back interface does not go down - ever S onfiguring Internal GP (isco IOS) Router interface loopback 0 ip address network neighbor remote-as 100 neighbor update-source loopback0 neighbor remote-as 100 neighbor update-source loopback0 Router interface loopback 0 ip address network neighbor remote-as 100 neighbor update-source loopback0 neighbor remote-as 100 neighbor update-source loopback0 21 Inserting prefixes into GP - network command onfiguration Example router bgp 109 network mask ip route serial0 matching route must exist in the routing table before the network is announced Forces origin to be IGP 22 onfiguration ggregation - Network ommand onfiguring ggregation - aggregate-address command onfiguration Example router bgp 109 network mask ip route null0 250 matching route must exist in the routing table before the network is announced Easiest and best way of generating an aggregate onfiguration Example router bgp 109 network mask aggregate-address [ summary-only ] Requires more specific prefix in routing table before aggregate is announced {summary-only} keyword optional keyword which ensures that only the summary is announced if a more specific prefix exists in the routing table

5 uto Summarisation Synchronisation isc IOS automatically summarises subprefixes to the classful network. Example: /22 --> /8 In isco IOS, GP does not advertise a route before all routers in the S have learned it via an IGP isable synchronisation if: Must be turned off for any Internet connected site using GP. S doesn t pass traffic from one S to another, or ll transit routers in S run GP, or router bgp 109 no auto-summary 25 igp is used across backbone router bgp 109 no synchronization 26 Summary GP4 - distance vector protocol igp versus egp GP ttributes stable igp - peer with loopbacks announcing prefixes & aggregates no synchronization & no auto-summary 27 ISP/IXP Presentation_I Workshops 1999, 2000, isco Systems, Inc What Is an ttribute? S-Path... Next Hop S Path ME escribes the characteristics of prefix Transitive or non-transitive Some are mandatory Sequence of Ses a route has traversed Loop detection pply policy S 500 S 300 S 200 S / / / / S / / / /

6 Next Hop Next Hop S /16 S 300 S /16 egp igp S / / S /16 Next hop to reach a network Usually a local network is the next hop in egp session S / / / Next hop not changed igp Next Hop Next Hop (summary) Loopback / /24 S 300 igp /23 Loopback /32 IGP should carry route to next hops Recursive route look-up Unlinks GP from actual physical topology Next hop is ibgp router loopback address / / llows IGP to make intelligent forwarding decision Recursive route look-up Origin ggregator onveys the origin of the prefix Influence best path selection Three values - IGP, EGP, incomplete IGP - generated from GP network statement EGP - generated from EGP incomplete - generated by redistribute action 35 Useful for debugging purposes onveys the IP address of the router/gp speaker generating the aggregate route oesn t influence path selection 36 6

7 Local Preference Local Preference Local to an S - non-transitive local preference set to 100 when heard from neighbouring S Used to influence GP path selection determines best path for outbound traffic Path with highest local preference wins 37 S / > / S /16 S E S Local Preference Multi-Exit iscriminator onfiguration of Router : router bgp 400 neighbor remote-as 300 neighbor route-map local-pref in route-map local-pref permit 10 match ip address prefix-list MTH set local-preference 800 ip prefix-list MTH permit /16 ip prefix-list MTH deny /0 le 32 Inter-S - non-transitive metric reset to 0 on announcement to next S Used to convey the relative preference of entry points determines best path for inbound traffic omparable if paths are from same S IGP metric can be conveyed as ME Multi-Exit iscriminator (ME) / S /24 S / Multi-Exit iscriminator onfiguration of Router : router bgp 400 neighbor remote-as 200 neighbor route-map set-med out route-map set-med permit 10 match ip address prefix-list MTH set metric 1000 ip prefix-list MTH permit /24 ip prefix-list MTH deny /0 le

8 GP attribute ommunity Used to group destinations Represented as two 16bit integers Each destination could be member of multiple communities ommunity attribute carried across S s Useful in applying policies ISP 2 X / /16 300: /16 300:1 ommunity S 300 S / /16 300: /16 300:1 ISP 1 E /16 300:1 S 200 S / Well-Known ommunities No-Export ommunity internet = all routes are members of this community no-export = do not advertise to egp peers no-advertise = do not advertise to any peer X.X / X.X No-Export S 100 S 200 E G /16 local-s = do not advertise outside local S (used with confederations) F GP Path Selection lgorithm Why is this the best path? GP Path Selection lgorithm o not consider igp path if not synchronised o not consider path if no route to next hop Highest weight (local to router) Highest local preference (global within S) Shortest S path ISP/IXP Presentation_I Workshops 1999, 2000, isco Systems, Inc

9 GP Path Selection lgorithm (continued) Lowest origin code IGP < EGP < incomplete Multi-Exit iscriminator onsidered only if paths are from same S Prefer egp path over igp path Path with shortest next-hop metric wins Lowest router-id pplying Policy with GP The GP Toolkit 49 ISP/IXP Presentation_I Workshops 1999, 2000, isco Systems, Inc pplying Policy with GP Policy ontrol - Prefix List Policy-based on S path, community or the prefix Rejecting/accepting selected routes Set attributes to influence path selection Tools: Prefix-list (filters prefixes) Filter-list (filters S paths) Route-maps and communities 51 Per neighbour prefix filter incremental configuration High performance access-list Inbound or Outbound ased upon network numbers (using familiar IPv4 address/mask format) 52 Prefix Lists - Examples Prefix Lists - Examples eny default route ip prefix-list EG deny /0 Permit the prefix /8 ip prefix-list EG permit /8 eny the prefix /12 ip prefix-list EG deny /12 In 192/8 allow up to /24 ip prefix-list EG permit /8 le 24 This allows all prefix sizes in the /8 address block, apart from /25, /26, /27, /28, /29, /30, In 192/8 deny /25 and above ip prefix-list EG deny /8 ge 25 This denies all prefix sizes /25, /26, /27, /28, /29, /30, /31 and /32 in the address block /8. It has the same effect as the previous example In 192/8 permit prefixes between /12 and /20 ip prefix-list EG permit /8 ge 12 le 20 This denies all prefix sizes /8, /9, /10, /11, /21, /22, and higher in the address block /8. Permit all prefixes ip prefix-list EG permit /0 le 32 ISP/IXP Workshops /31 and / , isco Systems, Inc matches all possible addresses, 0 le 32 9

10 Policy ontrol - Prefix List Example onfiguration router bgp 200 network neighbor remote-as 210 neighbor prefix-list PEER-IN in neighbor prefix-list PEER-OUT out ip prefix-list PEER-IN deny /16 ip prefix-list PEER-IN permit /0 le 32 ip prefix-list PEER-OUT permit /16 ip prefix-list PEER-OUT deny /0 le Policy ontrol - Filter List Filter routes based on S path Inbound or Outbound Example onfiguration: network neighbor filter-list 5 out neighbor filter-list 6 in ip as-path access-list 5 permit ^200$ ip as-path access-list 6 permit ^150$ 56 Policy ontrol - Regular Expressions Like Unix regular expressions. Match one character * Match any number of preceding expression + Match at least one of preceding expression ^ eginning of line $ End of line _ eginning, end, white-space, brace Or () brackets to contain expression 57 Policy ontrol - Regular Expressions Simple Examples.* Match anything.+ Match at least one character ^$ Match routes local to this S _1800$ Originated by 1800 ^1800_ Received from 1800 _1800_ Via 1800 _790_1800_ Passing through 1800 then 790 _(1800_)+ Match at least one of 1800 in sequence _\(65350\)_ Via (confederation S) 58 Policy ontrol - Regular Expressions Not so simple Examples ^[0-9]+$ ^[0-9]+_[0-9]+$ ^[0-9]*_[0-9]+$ ^[0-9]*_[0-9]*$ ^[0-9]+_[0-9]+_[0-9]+$ _( ) 1849(_.+_)12163$ Match S_PTH length of one Match S_PTH length of two Match S_PTH length of one or two Match S_PTH length of one or two Match S_PTH length of three Match anything which has gone through S701 or S1800 Match anything of origin S12163 and passed through S Policy ontrol - Route Maps Example onfiguration - route map and prefix-lists ip prefix-list HIGH-PREF permit /8 ip prefix-list HIGH-PREF deny /0 le 32 ip prefix-list LOW-PREF permit /8 ip prefix-list LOW-PREF deny /0 le 32 route-map infilter permit 10 match ip address prefix-list HIGH-PREF set local-preference 120 route-map infilter permit 20 match ip address prefix-list LOW-PREF set local-preference 80 neighbor route-map infilter in 60 10

11 Policy ontrol - Route Maps Policy ontrol - Route Maps Example onfiguration - route map and filter lists neighbor remote-as 200 neighbor route-map filter-on-as-path in route-map filter-on-as-path permit 10 match as-path 1 set local-preference 80 route-map filter-on-as-path permit 20 match as-path 2 set local-preference 200 ip as-path access-list 1 permit _150$ ip as-path access-list 2 permit _210_ 61 Example configuration of S-PTH prepend router bgp 300 network neighbor remote-as 100 neighbor route-map SETPTH out route-map SETPTH permit 10 set as-path prepend Standard practice implements two occurrences of the SN when prepending 62 Policy ontrol - Matching ommunities Policy ontrol - Setting ommunities Example onfiguration neighbor remote-as 200 neighbor route-map filter-on-community in route-map filter-on-community permit 10 match community 1 set local-preference 50 route-map filter-on-community permit 20 match community 2 exact-match set local-preference 200 ip community-list 1 permit 150:3 200:5 ip community-list 2 permit 88:6 63 Example onfiguration network neighbor remote-as 200 neighbor send-community neighbor route-map set-community out route-map set-community permit 10 match ip address prefix-list NO-NNOUNE set community no-export route-map set-community permit 20 match ip address prefix-list EVERYTHING ip prefix-list NO-NNOUNE permit /16 ge 17 ip prefix-list EVERYTHING permit /0 le GP Summary ttributes Path Selection Process Policy ontrol Tools ny questions? 65 11