BGP Series 12: BGP Filtering- Part1

BGP Filtering- Part1:

• Filtering can be done for inbound (filtering before installing them in bgp table) or outbound routes (before advertising it to our neighbors)

• After filtering is applied, BGP neighborships MUST be reset or cleared for the filter to take effect

• To reset a bgp neighborship, cli command is:

• • ‘Clear ip bgp *’ → clears or all neighbors
  • ‘Clear ip bgp ’ → clears neighborship only for that particular neighbor

• But, resetting the bgp neighborship for implementing a filter is destructive esp.

BGP Series 11: BGP Peer Groups

BGP Peer-Groups:

• Why do we need bgp peer groups?

• • In a network, we may have a couple of eBgp neighbors and dozens of iBGP neighbors

  • With each Bgp neighbor, there is a lot of other policies such as filter-list/ route-map/ distribute-list. If each neighbor had different policy, no other way than applying to them individually, but, Many neighbors might require the same BGP policies like:

  • • Apply same inbound RouteMaps to a group of neighbors

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