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BGP Deployment & Scalability

Learn about BGP Path Vector Protocol, AS-Path, BGP speakers, eBGP, iBGP peering, BGP Attributes like Next Hop, Local Preference, MED, and Community. Gain insights into BGP deployment guidelines and best practices from a Cisco expert.

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BGP Deployment & Scalability

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  1. BGP Deployment & Scalability Mike Pennington Network Consulting Engineer Cisco Systems, Denver

  2. Basic BGP Review

  3. Border Gateway Protocol • Routing Protocol used to exchange routing information between networks • exterior gateway protocol • RFC1771 • work in progress to update • draft-ietf-idr-bgp4-17.txt • Currently Version 4 • Runs over TCP

  4. Path Vector Protocol Incremental Updates Many options for policy enforcement Classless Inter Domain Routing (CIDR) Widely used for Internet backbone Autonomous systems BGP

  5. Path Vector Protocol • BGP is classified as a path vector routing protocol (see RFC 1322) • A path vector protocol defines a route as a pairing between a destination and the attributes of the path to that destination. 12.6.126.0/24 207.126.96.43 1021 0 6461 7018 6337 11268 i AS Path

  6. AS-Path • Sequence of ASes a route has traversed • Loop detection • Apply policy AS 200 AS 100 170.10.0.0/16 180.10.0.0/16 180.10.0.0/16 300 200 100 170.10.0.0/16 300 200 AS 300 AS 400 150.10.0.0/16 180.10.0.0/16 300 200 100 170.10.0.0/16 300 200 150.10.0.0/16 300 400 AS 500

  7. AS-Path loop detection AS 200 AS 100 170.10.0.0/16 180.10.0.0/16 140.10.0.0/16 500 300 170.10.0.0/16 500 300 200 AS 300 140.10.0.0/16 180.10.0.0/16 is not announced to AS100 as AS500 sees that it is originated from AS100, and that AS100 is the neighbouring AS – loop detection in action AS 500 180.10.0.0/16 300 200 100 170.10.0.0/16 300 200 140.10.0.0/16 300

  8. AS 100 Autonomous System (AS) • Collection of networks with same routing policy • Single routing protocol • Usually under single ownership, trust and administrative control

  9. BGP Basics Peering A C AS 100 AS 101 B D E BGP speakers are called peers AS 102

  10. BGP General Operation • Learns multiple paths via internal and external BGP speakers • Picks the best path and installs in the forwarding table • Policies applied by influencing the best path selection

  11. External BGP Peering (eBGP) • Between BGP speakers in different AS • Should be directly connected • Do not run an IGP between eBGP peers A AS 100 AS 101 C B

  12. Internal BGP Peering (iBGP) • Topology independent • Each iBGP speaker must peer with every other iBGP speaker in the AS AS 100 D A B E

  13. Internal BGP (iBGP) • BGP peer within the same AS • Not required to be directly connected • iBGP speakers need to be fully meshed • they originate connected networks • they do not pass on prefixes learned from other iBGP speakers

  14. BGP Attributes

  15. What Is an Attribute? Next Hop AS Path ... ... ... • Describes the characteristics of prefix • Transitive or non-transitive • Some are mandatory MED

  16. AS-Path • Sequence of ASes a route has traversed • Loop detection • Apply policy AS 200 AS 100 170.10.0.0/16 180.10.0.0/16 180.10.0.0/16 300 200 100 170.10.0.0/16 300 200 AS 300 AS 400 150.10.0.0/16 180.10.0.0/16 300 200 100 170.10.0.0/16 300 200 150.10.0.0/16 300 400 AS 500

  17. Next Hop 150.10.1.1 150.10.1.2 • Next hop to reach a network • Usually a local network is the next hop in eBGP session AS 200 A B AS 300 150.10.0.0/16 150.10.0.0/16 150.10.1.1 160.10.0.0/16 150.10.1.1 AS 100 160.10.0.0/16 20

  18. Next Hop (continued) • IGP should carry route to next hops • Recursive route look-up • Unlinks BGP from actual physical topology • Allows IGP to make intelligent forwarding decision

  19. Local Preference AS 100 160.10.0.0/16 AS 200 AS 300 500 800 D E A B AS 400 160.10.0.0/16 500 > 160.10.0.0/16 800 C

  20. Local Preference • Local to an AS – non-transitive • local preference set to 100 when heard from neighbouring AS • Used to influence BGP path selection • determines best path for outbound traffic • Path with highest local preference wins

  21. Multi-Exit Discriminator (MED) AS 200 C 192.68.1.0/24 2000 192.68.1.0/24 1000 A B 192.68.1.0/24 AS 201

  22. Multi-Exit Discriminator • Inter-AS – non-transitive • metric reset to 0 on announcement to next AS • Used to convey the relative preference of entry points • determines best path for inbound traffic • Comparable if paths are from same AS • IGP metric can be conveyed as MED

  23. MED & IGP Metric • set metric-type internal • enable BGP to advertise a MED which corresponds to the IGP metric values • changes are monitored (and re-advertised if needed) every 600s • bgp dynamic-med-interval <secs>

  24. Community • BGP attribute • Used to group destinations • Represented as two 16bit integers • Each destination could be member of multiple communities • Community attribute carried across AS’s • Useful in applying policies

  25. 160.10.0.0/16 300:1 170.10.0.0/16 300:1 ISP 2 X E 200.10.0.0/16 D A B Community AS 400 200.10.0.0/16 300:9 ISP 1 AS 300 C 160.10.0.0/16 300:1 170.10.0.0/16 300:1 AS 100 AS 200 160.10.0.0/16 170.10.0.0/16

  26. BGP Deployment Guidelines

  27. Recommended BGP commands for everyone • ip bgp-community new-format • no auto-summary • no synchronization • bgp deterministic-med • Whatever you do, use of deterministic-med MUST be consistent in your Autonomous System.

  28. Other serious considerations • For public peering: filter EBGP routes inbound and outbound • Block your own address space inbound • Block RFC 1918 space (inbound and outbound) • Block DSUA space (inbound and outbound): • http://www.ietf.org/internet-drafts/draft-manning-dsua-08.txt • Use prefix-lists for route-filtering when possible (easier to read than ACLs)

  29. Other serious considerations • If you carry a default in the IGP, your BGP next-hops ALWAYS resolve (generally not good) • bgp bestpath compare-routerid • Restores RFC-compliant path selection; OFF by default to reduce update churn, use with discretion • If you have a large BGP network, consider techniques in the next section

  30. BGP Scaling Techniques

  31. BGP Scaling Techniques • How to scale iBGP mesh beyond a few peers? • How to implement new policy without causing flaps and route churning? • How to reduce the overhead on the routers?

  32. BGP Scaling Techniques • Dynamic reconfiguration • Peer groups • Route flap damping • Route reflectors

  33. Soft Reconfiguration Problem: • Hard BGP peer clear required after every policy change because the router does not store prefixes that are denied by a filter • Hard BGP peer clearing consumes CPU and affects connectivity for all networks Solution: • Soft-reconfiguration

  34. Soft Reconfiguration discarded peer normal BGP in process accepted soft BGP in table received and used BGP table received peer BGP out process

  35. Soft Reconfiguration • New policy is activated without tearing down and restarting the peering session • Per-neighbour basis • Use more memory to keep prefixes whose attributes have been changed or have not been accepted

  36. Configuring Soft reconfiguration router bgp 100 neighbor 1.1.1.1 remote-as 101 neighbor 1.1.1.1 route-map infilter in neighbor 1.1.1.1 soft-reconfiguration inbound ! Outbound does not need to be configured ! Then when we change the policy, we issue an exec command clear ip bgp 1.1.1.1 soft [in | out]

  37. Managing Policy Changes • clear ip bgp <addr> [soft] [in|out] • <addr> may be any of the following • x.x.x.x IP address of a peer • * all peers • ASN all peers in an AS • external all external peers • peer-group <name> all peers in a peer-group

  38. Route Refresh Capability • Facilitates non-disruptive policy changes • No configuration is needed • No additional memory is used • Requires peering routers to support “route refresh capability” – RFC2918 • clear ip bgp x.x.x.x in tells peer to resend full BGP announcement

  39. Soft Reconfiguration vs Route Refresh • Use Route Refresh capability if supported • find out from “show ip bgp neighbor” • uses much less memory • Otherwise use Soft Reconfiguration

  40. Peer Groups Without peer groups • iBGP neighbours receive same update • Large iBGP mesh slow to build • Router CPU wasted on repeat calculations Solution – peer groups! • Group peers with same outbound policy • Updates are generated once per group

  41. Peer Groups - Advantages • Makes configuration easier • Makes configuration less prone to error • Makes configuration more readable • Lower router CPU load • iBGP mesh builds more quickly • Members can have different inbound policy • Can be used for eBGP neighbours too!

  42. Configuring Peer Group router bgp 100 neighbor ibgp-peer peer-group neighbor ibgp-peer remote-as 100 neighbor ibgp-peer update-source loopback 0 neighbor ibgp-peer send-community neighbor ibgp-peer route-map outfilter out neighbor 1.1.1.1 peer-group ibgp-peer neighbor 2.2.2.2 peer-group ibgp-peer neighbor 2.2.2.2 route-map infilter in neighbor 3.3.3.3 peer-group ibgp-peer ! note how 2.2.2.2 has different inbound filter from peer-group !

  43. Route Flap Damping • Route flap • Going up and down of path or change in attribute • BGP WITHDRAW followed by UPDATE = 1 flap • eBGP neighbour going down/up is NOT a flap • Ripples through the entire Internet • Wastes CPU • Damping aims to reduce scope of route flap propagation

  44. Route Flap Damping (Continued) • Requirements • Fast convergence for normal route changes • History predicts future behaviour • Suppress oscillating routes • Advertise stable routes • Implementation described in RFC2439

  45. Operation • Add penalty (1000) for each flap • Change in attribute gets penalty of 500 • Exponentially decay penalty half life determines decay rate • Penalty above suppress-limit do not advertise route to BGP peers • Penalty decayed below reuse-limit re-advertise route to BGP peers penalty reset to zero when it is half of reuse-limit

  46. 4000 Suppress limit 3000 Penalty 2000 Reuse limit 1000 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Time Network Announced Network Re-announced Network Not Announced Operation

  47. Operation • Only applied to inbound announcements from eBGP peers • Alternate paths still usable • Controlled by: • Half-life (default 15 minutes) • reuse-limit (default 750) • suppress-limit (default 2000) • maximum suppress time (default 60 minutes)

  48. Configuration Fixed damping router bgp 100 bgp dampening [<half-life> <reuse-value> <suppress-penalty> <maximum suppress time>] Selective and variable damping bgp dampening [route-map <name>] route-map <name> permit 10 match ip address prefix-list FLAP-LIST set dampening [<half-life> <reuse-value> <suppress-penalty> <maximum suppress time>] ip prefix-list FLAP-LIST permit 192.0.2.0/24 le 32

  49. Operation • Care required when setting parameters • Penalty must be less than reuse-limit at the maximum suppress time • Maximum suppress time and half life must allow penalty to be larger than suppress limit

  50. Configuration • Examples -  • bgp dampening 30 750 3000 60 • reuse-limit of 750 means maximum possible penalty is 3000 – no prefixes suppressed as penalty cannot exceed suppress-limit • Examples -  • bgp dampening 30 2000 3000 60 • reuse-limit of 2000 means maximum possible penalty is 8000 – suppress limit is easily reached

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