BGP Attribute and Path Selection

Usually, numerous paths connect to the same destination, which BGP routers receive. The optimal path for traffic forwarding and installation in the IP routing table is determined by the BGP best path algorithm.

BGP does not load balance by default; instead, it attempts to focus path selection on a single best path. In order to achieve this, it looks at the following sequence of path properties for routes with an accessible next-hop:

1. Weight:

Choose the route with the highest weight. This is a Cisco proprietary?attribute?that is local to the router. For any routes that are not originated by the local router the weight is considered as 0.?

2. Local Preference

Prefer the path with the highest local preference. This is shared across iBGP routers and utilized inside an autonomous system. The path with the highest local preference is the one we choose. 100 is the default value.

3. Originate

Choose the route originated?by the local router first. You will see next hop 0.0.0.0 in the BGP table for all locally originated routes. A BGP router will give preference to routes that it has added to BGP itself?over those that have been added by another router.

4. AS Path Length

The path with the smallest AS path length should be chosen. In comparison to AS path 1 2 3 4 5, AS path 1 2 3 is preferred.

5. Origin Code

Choose the lowest possible origin code. Three origin codes are available: IGP, EGP, and INCOMPLETE

EGP is lower than INCOMPLETE, while IGP is lower than EGP.

6. MED (Multi-Exit Discriminator)

Choose the route with the lowest MED, if the same autonomous system advertises the possible routes.

7. eBGP path over iBGP path

Choose an eBGP route over an iBGP route.

8. Shortest IGP path to BGP next hop

Prefer the path with the lowest IGP metric to the BGP next hop.

9. Oldest

When both paths are external, prefer the path that was received first (the oldest one).

10. Router ID

Prefer the route that comes from the BGP router with the lowest router ID.

The router ID is the highest IP address on the router, with preference given to loopback addresses. Also, you can use the bgp router-id command in order to manually set the router ID.

11. Neighbor IP address

Prefer the path that comes from the lowest neighbor address. This address is the IP address that is used in the BGP neighbor configuration. The address corresponds to the remote peer that is used in the TCP connection with the local router.

In order to choose the best possible route, we go along this attribute list until we reach a tiebreaker.