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