BGP Series 10: iBGP Full Mesh and Synchronization

iBGP Full Mesh and Synchronization:

• Issues with iBGP route advertisement:

• • iBGP neighbors prevent loops by not advertising iBGP learned routes to one another

  • This can cause problems with bgp route advertisement, if all are not connected to each other .ie. not in full mesh since the routes will not be present in all the routers of iBGP in the AS--->Traffic Black Hole

  • Like in below example,

  • • even if we create a iBGP neighborship between the border devices R4 and R7 to make R7 receive the iBGP update of network-X, when a traffic destined to network-X comes to R7, it will do a recursive lookup and see that for reaching net-X, I should reach R4 and for reaching R4, I should send to R6.
    • But, R6 does not have net-X in its routing table and drops it

BGP Series 9: BGP Rules for advertising of routes, Next-Hop Reachability with iBGP

• BGP Rules for advertising of routes:

• • BGP will only advertise the best route in any BGP table and will not include all the other routes for the same prefix.

  • • BGP has lots of tie-breakers that will help bgp to always choose a single route as the best route when lots of routes exist for the same prefix.

BGP Series 8: eBGP vs iBGP route, Using BGP for Outbound Routing, iBGP BGP Table Entries and Next-Hop issues with iBGP

• eBGP vs iBGP route:

• • If we learn a route via external neighbor (eBGP), the administrative distance is 20
  • If we learn a route via iBGP, the administrative distance is 200

BGP Series 7: Injecting Routes into BGP- network, redistribution, route summarization

Injecting Routes into BGP:

• 3 Primary ways to inject routes into BGP:

• • Using the BGP ‘network’ command
  • Using redistribution
  • By using route summarization

BGP Series 6: BGP AS-Path Attribute

BGP AS-Path Attribute:

• This attribute is used to decide the best-path for a certain prefix.

• By default, if a bgp router gets two or more updates for the same prefix, then, it will decide at the best path by simply looking at the AS-path attribute...ie. which of these routes went through the fewest number of AS and whichever route had the lowest number (quantity, not value) of AS-path, that is the best route