BGP Attributes and Policy Control. BGP Attributes. BGP Attributes. Agenda. What Is an Attribute? AS-Path. ISP/IXP Workshops.

Transcription

1 genda GP ttributes and Policy ontrol ISP/IXP Workshops GP ttributes GP Path Selection pplying Policy ISP/IXP Workshops 2000, isco Systems, Inc. 1 2 GP ttributes GP ttributes What is an attribute? S path Next hop Local preference Multi-Exit Discriminator (MED) ommunity Others include Origin and ggregator ISP/IXP Presentation_ID Workshops 2000, 1999, isco Systems, Inc What Is an ttribute? Next Hop S Path MED Describes 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-Path S 300 S / / / / S / / / /

2 S-Path loop detection Next Hop S / / / / S / /16 S / / /16 is not announced to S100 as S500 sees that it is originated from S100, and that S100 is the neighbouring S - loop detection in action /16 S /16 S / / Next hop to reach a network Usually a local network is the next hop in egp session Next Hop igp Next Hop / egp igp S 300 Loopback / /24 S 300 igp /23 Loopback /32 D S / / / Next hop not changed 9 Next hop is ibgp router loopback address Recursive route look-up / / Third Party Next Hop Next Hop (summary) S / / egp between Router and Router igp between Router and Router /24 prefix has next hop address of this is passed on to Router instead of More efficient No extra config needed 11 IGP should carry route to next hops Recursive route look-up Unlinks GP from actual physical topology llows IGP to make intelligent forwarding decision 12 2

3 Origin 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 13 ggregator Useful for debugging purposes onveys the IP address of the router/gp speaker generating the aggregate route Doesn t influence path selection 14 Local Preference Local Preference / > / S /16 S 400 D E S 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 16 Local Preference Multi-Exit Discriminator (MED) 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 / /24 S /

4 Multi-Exit Discriminator MED & IGP Metric 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 MED set metric-type internal enable GP to advertise a MED which corresponds to the IGP metric values changes are monitored (and readvertised if needed) every 600s bgp dynamic-med-interval <secs> Multi-Exit Discriminator Weight 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 32 Not really an attribute - local to router Highest weight wins pplied to all routes from a neighbour neighbor weight 100 Weight assigned to routes based on filter neighbor filter-list 3 weight Weight ommunity GP attribute /24 Traffic from or passing through Router D to /24 in S202 should go via S201 S 202 Traffic from the rest of S200 to /24 should go via Router (normal selection rules) Use high weight on RouterD E S 201 F D 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

5 ommunity Well-Known ommunities ISP 2 X / /16 300: /16 300:1 D S 300 S / /16 300: /16 300:1 ISP 1 E /16 300:1 S /16 internet = all routes are members of this community no-export = do not advertise to egp peers no-advertise = do not advertise to any peer local-s = do not advertise outside local S (used with confederations) No-Export ommunity No-Export ommunity X.X / X.X No-Export S 100 E D G /16 S100 announces aggregate and subprefixes aim is to improve loadsharing between S100 and S200 by leaking subprefixes Subprefixes marked with no-export community F Router G in S200 strips out all prefixes with no-export community set GP Path Selection lgorithm Why is this the best route? GP Path Selection lgorithm Do not consider path if no route to next hop Do not consider igp path if not synchronised Highest weight (local to router) Highest local preference (global within S) Prefer locally originated route Shortest S path ISP/IXP Presentation_ID Workshops 1999, 2000, isco Systems, Inc

6 GP Path Selection lgorithm (continued) Lowest origin code IGP < EGP < incomplete Lowest Multi-Exit Discriminator (MED) If bgp deterministic-med, order the paths before comparing If bgp always-compare-med, then compare for all paths otherwise MED only considered if paths are from the same S (default) GP Path Selection lgorithm (continued) Prefer egp path over igp path Path with lowest IGP metric to next-hop For egp paths if multipath enabled, install N parallel paths in routing table if router-id is the same, go to next step if router-id not the same, select oldest GP Path Selection lgorithm (continued) Lowest router-id (originator-id for reflected routes) Shortest luster-list pplying Policy with GP lient must be aware of Route Reflector attributes Lowest neighbor IP address 33 ISP/IXP Presentation_ID Workshops 1999, 2000, isco Systems, Inc pplying Policy with GP Policy-based on S path, community or the prefix Rejecting/accepting selected routes Set attributes to influence path selection Tools: Distribute-list (access-list) or prefix-list Filter-list (as-path access-list) Route-maps and communities 35 Policy ontrol - Distribute List Per neighbour access-list Inbound or Outbound ased upon network numbers (e.g. through the use of access-lists) 36 6

7 Policy ontrol - Distribute List Example onfiguration router bgp 200 network neighbor remote-as 210 neighbor distribute-list 5 in neighbor distribute-list 6 out access-list 5 deny access-list 5 permit any access-list 6 permit access-list 6 deny any 37 Policy ontrol - Prefix List Per neighbour prefix filter incremental configuration High performance access-list Inbound or Outbound ased upon network numbers (using familiar IPv4 address/mask format) 38 Prefix-list ommand Prefix Lists - Examples [no] ip prefix-list <list-name> [seq <seq-value>] deny permit <network>/<len> [ge <ge-value>] [le <levalue>] <network>/<len>: The prefix and its length ge <ge-value>: "greater than or equal to" le <le-value>: "less than or equal to" oth "ge" and "le" are optional. Used to specify the range of the prefix length to be matched for prefixes that are more specific than <network>/<len> 39 Deny default route ip prefix-list EG deny /0 Permit the prefix /8 ip prefix-list EG permit /8 Deny 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, /31 and / Prefix Lists - Examples Policy ontrol - Prefix List 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 193/8 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 matches all possible addresses, 0 le 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

8 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$ 43 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 44 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) 45 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 Policy ontrol - Route Maps Example onfiguration - route map and prefix-lists Example onfiguration - route map and filter 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 47 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_ 48 8

9 Policy ontrol - Route Maps Policy ontrol - Route Maps 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 Route Map MTH rticles as-path clns address clns next-hop clns route-source community ip next-hop ip route-source length metric nlri Standard practice implements two occurrences of the SN when prepending 49 interface ip address route-type tag 50 Policy ontrol - Route Maps Policy ontrol - Route Maps Route map SET rticles as-path dampening automatic-tag default interface Route map SET rticles ip precedence ip qos-group ip tos next-hop nlri multicast nlri unicast clns comm-list community interface ip default next-hop ip next-hop level local preference metric metric-type origin tag traffic-index weight 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 53 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

10 ggregation Policies Suppress Map Used to suppress selected morespecific prefixes (e.g. defined through a route-map) in the absence of the summary-only keyword. Unsuppress Map Used to unsuppress selected morespecific prefixes per GP peering when the summary-only keyword is in use. Example ggregation Policies Suppress Map network network network network network aggregate-address suppress-map block-net neighbor remote-as 200 route-map block-net permit 10 match ip address prefix-list SUPPRESS ip prefix-list SUPPRESS permit /21 le 32 ip prefix-list SUPPRESS deny /0 le ggregation Policies Suppress Map ggregation Policies Suppress Map show ip bgp on the local router router1#sh ip bgp GP table version is 11, local router ID is Status codes: s suppressed, d damped, h history, * valid, > best, i -internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path *> / i s> i s> i s> i *> i *> i show ip bgp on the remote router router2#sh ip bgp GP table version is 90, local router ID is Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path *> / i *> i *> i ggregation Policies Unsuppress Map Example network network network network network aggregate-address summary-only neighbor remote-as 200 neighbor unsuppress-map leak-net route-map leak-net permit 10 match ip address prefix-list LEK ip prefix-list LEK permit /21 le 32 ip prefix-list LEK deny /0 le ggregation Policies Unsuppress Map show ip bgp on the local router router1#sh ip bgp GP table version is 11, local router ID is Status codes: s suppressed, d damped, h history, * valid, > best, i -internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path *> / i s> i s> i s> i s> i s> i 60 10

11 ggregation Policies Unsuppress Map ggregation Policies ggregate ddress show ip bgp on the remote router router2#sh ip bgp GP table version is 90, local router ID is Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path *> / i *> i *> i *> i Summary-only used all subprefixes suppressed unsuppress-map to selectively leak subprefixes bgp per neighbour configuration bsence of summary-only no subprefixes suppressed suppress-map to selectively suppress subprefixes bgp global configuration