Troubleshooting guide

BGP Overview

BGP Path Selection

When BGP receives advertisements for the same route from multiple sources, one path is selected as the

best path and stored in the routing table. The decision logic used by BGP to determine the best path is

fairly extensive. The following BGP criteria are used in AOS to select the best path to a destination:

1. Prefer the path with the higher LOCAL_PREF value.

2. If the LOCAL_PREF value is identical, compare local-origination status. Prefer a route injected into

BGP via the network <ipv4 address> mask <subnet mask> command issued from BGP configuration

mode over a redistributed route.

3. If the local origination status is identical, prefer the shortest AS_PATH distance.

4. If the AS_PATH distance is identical, prefer lower origin type (where routes originally injected via the

network <ipv4 address> mask <subnet mask> command issued from BGP configuration mode or

aggregation (IGP) are lower in origin than routes learned from a neighbor using eBGP. Routes originally

injected by redistribution into BGP (incomplete) have the highest origin value).

5. If the origin type is identical, prefer the route with the lowest MULTI_EXIT_DISC value.

6. If the MULTI_EXIT_DISC value is identical, prefer eBGP paths over iBGP paths.

7. If the paths are still identical, prefer the path through the closest IGP neighbor.

8. Compare and prefer lower value for any other metrics on the route.

9. Compare and prefer the route from the router with the lowest router ID.

10. Compare and prefer the route that came from the lowest neighbor IPv4 address.

VRF and MPLS

The following information is provided to enhance the understanding of how service providers are able to

maintain separation of private routes that belong to different customers. All BGP applications discussed in

this configuration guide simply require the AOS device to be configured for eBGP when connecting to a

service provider. AOS devices support an implementation of multi-VRF, but this functionality is not used

or needed in the BGP applications discussed in this configuration guide.

A service provider uses virtual routing and forwarding (VRF) to separate one customer's routes from

another's and MPLS to ensure that the routes reach only the authorized remote sites. Without VRF,

customers could not transmit private network routes between remote sites; the service provider’s routers

would have no way of knowing which route belonged to which customer.

As of AOS firmware release 18.3, Multi-VRF BGP support is included in BGP. For more

information about this feature, refer to the configuration guide Configuring BGP in AOS,

(for AOS data products using AOS firmware 18.03.00 or later, and AOS voice products

using AOS firmware R10.1.0 or later). The guide is available online at http://kb.adtran.com

(article number 3524).