Modifying the BGP Path Selection Process on IOS

The implementation of Border Gateway Protocol (BGP) on Cisco IOS provides a number of ways to manipulate the default best path selection process. This article does not discuss ways to influence the selection of one BGP route over another (for example, by AS prepending), but rather examines how the path selection process itself can be modified.

Default BGP Path Selection

Before we dig into modifications, let's review the default BGP path selection process. An in-depth analysis of the BGP path selection mechanism on Cisco IOS can be found here. A simplified summary of the process follows.

1. Prefer the route with the highest weight (a local, Cisco-proprietary parameter).
2. Prefer the route with the highest local preference.
3. Prefer locally-originated routes over those learned from a neighbor.
4. Prefer the route with the shortest AS path.
5. Prefer IGP-learned routes over those with an unknown origin.
6. Prefer the route with the lowest MED (per AS).
7. Prefer EBGP-learned routes over IBGP-learned routes.
8. Prefer the route whose next-hop address has the lowest IGP metric.
9. Multipath determination
10. If a best path exists, prefer it over newer routes.
11. Prefer the route from the router with the lowest router ID.
12. If the router IDs match, prefer the lowest neighbor IP address.

Modifying BGP Path Selection Logic

Ignoring BGP Cost Communities

BGP cost communities are a non-standardized type of non-transitive extended BGP community that allow for arbitrary customization of the BGP path selection process. Cost communities can be ignored with the command bgp bestpath cost-community ignore.

Ignoring AS Path Length

Evaluation of AS path length is the fourth step in BGP path selection. Routes with shorter AS paths are preferred over those with longer AS paths. (An AS_SET counts as 1, regardless of how many ASNs it contains.) Although not typically desired, the AS path evaluation step of BGP path selection can be disabled with the command bgp bestpath as-path ignore.

In the example below, R1 is receiving a route for 172.16.0.0/16 from each of three EBGP neighbors. The path through 10.0.14.4 is preferred because it has the shortest AS path.

R1# show ip bgp
BGP table version is 2, local router ID is 10.0.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0       10.0.14.4                0             0 65004 ?
*                   10.0.12.2                0             0 65002 65003 65004 ?
*                   10.0.13.3                0             0 65003 65004 ?

After disabling AS path length evaluation, and assuming all other attributes are equal among the routes, the router defaults to preferring the route from the EBGP neighbor with the lowest router ID. (Note that step 10 in the process does not apply in this case because the previous best path was removed when the BGP neighbor adjacencies were reset with clear ip bgp.)

R1(config)# router bgp 65001
R1(config-router)# bgp bestpath as-path ignore
R1# clear ip bgp *
R1# show ip bgp
BGP table version is 2, local router ID is 10.0.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0       10.0.12.2                0             0 65002 65003 65004 ?
*                   10.0.14.4                0             0 65004 ?
*                   10.0.13.3                0             0 65003 65004 ?

Force MED Comparison Across All Neighboring ASes

By default, multi-exit discriminator (MED) values are only compared among routes advertised by the same neighboring autonomous system. The BGP configuration command bgp always-compare-med forces evaluation of the MED of all routes for a destination, regardless of the AS from which they were learned. When enabling this feature, it is advisable to enable deterministic MED computation as well, using the command bgp deterministic-med.

R1# show ip bgp
BGP table version is 2, local router ID is 10.0.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  172.16.0.0       10.0.13.3              100             0 65003 ?
*                   10.0.14.4              200             0 65004 ?
*>                  10.0.12.2              300             0 65002 ?

Because the above routes are all learned from different autonomous systems, their MED (metric) values are not compared. After enabling the command bgp always-compare-med, we can see that the route with the lowest MED is now preferred.

R1(config)# router bgp 65001
R1(config-router)# bgp always-compare-med
R1(config-router)# bgp deterministic-med
R1# clear ip bgp *
R1# show ip bgp
BGP table version is 2, local router ID is 10.0.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  172.16.0.0       10.0.14.4              200             0 65004 ?
*>                  10.0.13.3              100             0 65003 ?
*                   10.0.12.2              300             0 65002 ?

Force MED Comparison Inside a Confederation

By default, MED values are not compared among routes from BGP confederation peers. The command bgp bestpath med confed can be used to force evaluation of the MED attribute so long as no external (non-confederation) ASNs are present in the AS path.

Punish Routes Without a MED

BGP routes without a MED attribute are considered to have a MED of zero by default. The command bgp bestpath med missing-as-worst can be used to reverse this behavior, assigning routes with no MED a MED of "infinity" (the highest possible and therefore least favorable MED value). However, testing shows that most types of BGP routes are assigned a MED of zero before being advertised.

Disable Step 10

Step 10 in the path selection process instructs the BGP router to prefer a current best path, if it has one, over a newer route with equal attributes. This behavior can be disabled by issuing the command bgp bestpath compare-routerid, which will force comparison of advertising router IDs (and, if they are equal, neighbor IP addresses) even if a current best route exists (steps 11 and 12).