BGP Series 14: BGP Path Attributes and BGP Best Path Algorithm

BGP Path Attributes and BGP Best Path Algorithm:

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• BGP uses different path attributes to calculate the best path to a certain destination

• But, they can also be used for a variety of things other than best path calculation such as AS_PATH attribute for loop-detection, …

• Each path-attribute describes something about the path

• The most common bgp path attributes are:

BGP Series 13: BGP Filtering- Part 2

BGP Filtering- Part2:

• Filter-List Example:

• • Consider the AS-path for a route of Network-X to be 100 200 400 6678
  • One thing to consider is no matter how big the AS_PATH, the beginning should have the ‘^’ symbol and end of every AS_PATH must have the ‘$’ sign

• • This is the simplest AS_PATH regex, but

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