Introduction to the Border Gateway Protocol (BGP) Part-3

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iBGP

Scaling the iBGP mesh

  • Two methods:

BGP Confederations 

BGP Route Reflectors

BGP Confederations

  • Method to subdivide a single AS into multiple, internal sub-AS’s, yet still advertise a single AS to external peers
  • Reduces iBGP mesh

BGP Confederation

  • AS10
  • AS20
  • AS30
  • AS300

BGP Route Reflectors (RR)

  • Another method to reduce iBGP mesh
  • iBGP re-advertisement restrictions are relaxed
  • Single iBGP peer advertises (reflects) routes to subordinate iBGP peers
  • Clients must not peer with RR’s outside of cluster

Route Reflectors

  • (RR)
  • AS300
  • (RR)
  • (RR)

Asymmetric Routing Paradigm

  • In the Internet, it is impossible to control end-to-end routing and paths traversed
  • Packets may not always traverse same downstream path as they do when forwarded upstream

Asymmetric Routing

Achieving stability

  • Using loopbacks
  • BGP/IGP Interaction
  • Peer groups
  • Route-flap dampening
  • BGP soft reconfiguration

Stable iBGP Peering

  • Peer with loop-back addresses
  • iBGP session is not dependent on a single interface
  • Loop-back interface does not go down

Peering to Loop-Back Address

  • AS 100

Relationship with IGP

  • BGP carries full Internet routing table
  • IGP’s are used to carry next hop and interior network information
  • Routes are never redistributed from BGP into IGP (and vice versa)
  • Recursive route lookup

Peer Groups

  •  iBGP neighbors receive same update
  • Group peers with same outbound policy
  • Updates are generated once per group
  • Makes configuration easier
  • Members could have different inbound policy
  • eBGP neighbors

Route Flap Dampening

  • Route flap
  • Going up and down of path Change in attribute
  • Ripples through the entire Internet
  • Consumes CPU
  • Reduce scope of route flap propagation

Route Flap Dampening (Cont.)

  • Fast convergence for normal route changes
  • History predicts future behavior
  •  Suppress oscillating routes
  • Advertise stable suppressed routes

Flap Dampening: Terminology

  • Penalty
  • Decay
  • Half-life time
  • Suppress-limit
  • Reuse-limit
  • Suppress

Route Flap Dampening

Flap Dampening: Operation

  • Add penalty for each flap
  • Exponentially decay penalty
  • Penalty above suppress-limit—do not advertise up route
  • Penalty decayed below reuse-limit—advertise route
  • History path

Flap Dampening: Operation (Cont.)

  • Done only for external path
  • Alternate paths still usable
  • Suppress-limit, reuse-limit and half-life time give control
  • Less overhead

BGP soft reconfig

  • Soft reconfig allows BGP policies to be configured & activated without clearing the BGP session
  • Does not invalidate forwarding cache, hence no short-term interruptions
  • Outbound preferable over inbound reconfig

Cisco IOS™ Version Information

  • Peer-groups—10.2
  • Communities—10.3
  • Route-reflectors—10.3
  • Confederation—10.3
  • Route flap dampening—11.0
  • BGP Multipath, Soft Reconfig—11.2

Summary

  • Use BGP only when needed!
  • IGP and conventional BGP network problems
  • BGP gives flexibility and control
  • Route reflectors and confederations helps iBGP mesh scale

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