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Efficient BGP Prefix Filtering for Internet Routing Optimization

Learn about the impact and solutions for managing Internet routing table size growth with minimal customer impact and enhanced global reachability using prefix filtering strategies.

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Efficient BGP Prefix Filtering for Internet Routing Optimization

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  1. BGP filtering André Chapuis, chapuis@ip-plus.net

  2. Motivation: Internet routing table size evolution

  3. Internet routing table size • Do we really need these 120’000 routes ? • Number of contiguous prefixes with same origin/path * 4.0.0.0 209.10.12.125 8204 0 4513 3356 i *> 4.6.0.0/22 66.185.128.48 514 0 1668 3356 10753 i *> 4.6.4.0/23 66.185.128.48 514 0 1668 3356 10753 i … 50 prefixes with same origin… *> 4.6.172.0/22 66.185.128.48 514 0 1668 3356 10753 i *> 4.6.176.0/22 66.185.128.48 514 0 1668 3356 10753 I * 65.37.128.0/18 134.222.85.45 0 0 286 209 3356 4355 i * 65.37.136.0/23 134.222.85.45 0 0 286 209 3356 4355 I .. 20 prefixes with same origin… * 65.37.220.0/23 134.222.85.45 0 0 286 209 3356 4355 i

  4. Impact of Internet routing table size growth • Router memory (with 125’000 routes) • BGP table memory (21MB) • Routing table memory (21MB) • CEF table memory (21MB) • Distributed on every line card (limit=smallest card) • Second BGP feed (+10M – 20M) • Still many Cisco 7206 with NPE-150: 128MB RAM is a maximum • Crash experience with 128MB and two full feeds on a CPE • Router CPU • More updates -> more activity

  5. Requirements • Solution with minimal (no) impact on customers • No routing holes = global reachability is granted • Multihomed customers must keep all BGP resiliency • Minimal manual tuning wanted • No frequent changes

  6. Solution chosen • Prefix-filtering • RIR minimal allocation sizes • Historical classfull addresses (A and B) • Ad-hoc filters based on size / region • Semi-default routes • To guarantee reachability in case of misconfiguration • Exceptions • Customer prefixes • Chosen prefixes (private peerings) • Swiss peerings

  7. Prefix filtering (1) • RIR minalloc: • http://www.apnic.net/db/min-alloc.html • http://www.arin.net/statistics/index.html#cidr • http://www.ripe.net/ripe/docs/smallest-alloc-sizes.html • Ex: /19 within 62/8 • Changes needed only when IANA allocates e new block to a RIR -> not too frequent (every 3-6 month) • Historical ‘Classful’ address-space: • Class B: /22 • Class A: /21

  8. Prefix filtering (2) • Ad-hoc: • 199/8, ARIN region, default /22 with exceptions • 200/7, LACNIC region, default /22 with exceptions • 202/7, APNIC region, default /22 but 202/10 is /24 • 204/6, ARIN region, default /22 with exceptions • Current table size within AS3303: • 60’793 as seen from Oregon-IX • 63’147 as seen internally (customer more-specifics) • 125’000 average for ISPs not filtering

  9. Prefix filtering (3) • Filter example … ip prefix-list martians seq 40000 permit 40.0.0.0/8 le 21 ip prefix-list martians seq 43000 permit 43.0.0.0/8 le 21 ip prefix-list martians seq 44000 permit 44.0.0.0/6 le 21 ip prefix-list martians seq 48000 permit 48.0.0.0/5 le 21 ip prefix-list martians seq 56000 permit 56.0.0.0/7 le 21 ip prefix-list martians seq 60000 permit 60.0.0.0/7 le 20 ip prefix-list martians seq 62000 permit 62.0.0.0/7 le 19 …

  10. Semi-default routes (1): the problem • Some end-users (or ISPs) get an allocated block from a RIR (say /18), but announce only a part of it (say a /23) without aggregate ! • Example: • 62.61.192.0/23 4513 701 6453 i • ALLOCATED PA is 62.61.192.0/18 -> not routed • Network not reachable • The responsible is the owner of the block/source ISP • But there are so many cases like that. • Therefore we use semi-default routes

  11. Semi-default routes (2) • Aggregates created to cover RIR space: • 62/8, 80/7, 212/7, 217/8 routed towards EU transit ISP • ARIN/APNIC/LACNIC space towards US transit • Class A/B • Class B: 128/3, 160/5 and 168/6 towards US transit • No semi-default for class A • Aggregates announced to customers • Tagged with a special community (3303:9999)

  12. Semi-default routes (3) • = Static routes redistributed into BGP ip route 62.0.0.0 255.0.0.0 POS3/1 router bgp 65000 network 62.0.0.0 route-map semi-default • Original idea was to ask our transit ISP to send us them via BGP • Upstream ISP reluctant to the original idea (particularly the USA ones…) • We provide them to our customers

  13. Exceptions. We don’t filter for: • Some private peerings with fair amount of traffic • Google, Yahoo, Hotmail • Customer prefixes • Accept anything from customers (up to /24) • Prefixes with an origin AS included within our as-set must be accepted to guarantee reachability • Swiss routes (= routes received on CH-peerings in CH) • Routes received from CH-peers are not subject to the filters • Because there are few of them • And we are a swiss ISP

  14. Customer prefixes

  15. Customer prefixes (configuration) route-map set-ipp-peer permit 10 match as-path 198 ! route-map set-ipp-peer permit 20 match ip address prefix-list martians ! ip as-path access-list 198 permit _(AS-SWCMGLOBAL)$ ! ip prefix-list martians seq 3000 permit 3.0.0.0/8 le 21 ip prefix-list martians seq 4000 permit 4.0.0.0/8 le 21 ip prefix-list martians seq 6000 permit 6.0.0.0/8 le 21 ip prefix-list martians seq 8000 permit 8.0.0.0/7 le 21

  16. Results (1) • BGP Updates/min before and after the filter

  17. Results (2) • Stability improved • Number of updates/minute reduced by 40% • Last month de-aggregation of Bellsouth • About 1000 more prefixes injected • Transparent for AS3303 • Traffic engineering done by ISPs outside CH with more-specifics from PA blocks is ignored by AS3303 • Forced ‘traffic engineering’ neglectible • Small amount of traffic following the semi-defaults routes • 204.0.0.0/6 has less than 500kb/s average traffic • For a total of 10’000 prefixes

  18. Other ISPs filtering • Verio AS2914 • Class A space (i.e., 0/1), accept /22 and shorter • Class B space (i.e., 128/2), accept /22 and shorter • Class C space (i.e., 192/3), accept /24 and shorter • SWITCH AS559 • RIR minalloc + /19 in ClassA/B • Jippi (Eunet Finland) AS6667 • 192/7 : accept /24 and shorter • Rest: accept /21 and shorter

  19. Conclusions • Less memory needed (and CPU) • No reachability issues with semi-default routes • BGP customers satisfied • …lots of ‘useless’ routes in the Internet… • Need to have at least one transit provider • Method does not work for Tier-1 (transit-free ISPs) • Good solution for (small) ISPs with limited memory budget

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