Click on the Exhibit button.
Based on the exhibit, which step in the BGP route-selection process causes the router to choose the selected path?
A.
MED
B.
metric to next hop
C.
local preference
D.
neighbor ID
Explanation:
Routing devices send hello packets at a fixed interval on all interfaces to establish and maintain neighbor relationships. This interval is advertised in the hello interval field in the hello packet. By default, a designated intermediate system (DIS) router sends hello packets every 3 seconds, and a non-DIS router sends hello packets every 9 seconds.Understanding BGP Path Selection
For each prefix in the routing table, the routing protocol process selects a single best path. After the best path is selected, the route is installed in the routing table. The best path becomes the active route if the same prefix is not learned by a protocol with a lower (more preferred) global preference value, also known as the administrative distance. The algorithm for determining the active route is as follows:1. Verify that the next hop can be resolved.
2. Choose the path with the lowest preference value (routing protocol process preference).
Routes that are not eligible to be used for forwarding (for example, because they were rejected by routing policy or because a next hop is inaccessible) have a preference of 1 and are never chosen.
3. Prefer the path with higher local preference.
For non-BGP paths, choose the path with the lowest preference2 value.
4. If the accumulated interior gateway protocol (AIGP) attribute is enabled, prefer the path with the lower AIGP attribute.
5. Prefer the path with the shortest autonomous system (AS) path value (skipped if the as-path-ignore statement is configured).
A confederation segment (sequence or set) has a path length of 0. An AS set has a path length of 1.
6. Prefer the route with the lower origin code.
Routes learned from an IGP have a lower origin code than those learned from an exterior gateway protocol (EGP), and both have lower origin codes than incomplete routes (routes whose origin is unknown).
7. Prefer the path with the lowest multiple exit discriminator (MED) metric.
Depending on whether nondeterministic routing table path selection behavior is configured, there are two possible cases:If nondeterministic routing table path selection behavior is not configured (that is, if the path-selectioncisco-nondeterministic statement is not included in the BGP configuration), for paths with the same neighboring AS numbers at the front of the AS path, prefer the path with the lowest MED metric. To always compare MEDs whether or not the peer ASs of the compared routes are the same, include the path-selectionalways-compare-med statement.
If nondeterministic routing table path selection behavior is configured (that is, the path-selectioncisco-nondeterministic statement is included in the BGP configuration), prefer the path with the lowest MED metric.Confederations are not considered when determining neighboring ASs. A missing MED metric is treated as if a MED were present but zero.
Note: MED comparison works for single path selection within an AS (when the route does not include an AS path), though this usage Is uncommon.
8. Prefer strictly internal paths, which include IGP routes and locally generated routes (static, direct, local, and so forth).
9. Prefer strictly external BGP (EBGP) paths over external paths learned through internal BGP (IBGP) sessions.
10. Prefer the path whose next hop is resolved through the IGP route with the lowest metric.Note: A path is considered a BGP equal-cost path (and will be used for forwarding) if a tie-break is performed after the previous step. All paths with the same neighboring AS, learned by a multipath-enabled BGP neighbor, are considered.
BGP multipath does not apply to paths that share the same MED-plus-IGP cost yet differ in IGP cost. Multipath path selection is based on the IGP cost metric, even if two paths have the same MED-plus-IGP cost.11. If both paths are external, prefer the currently active path to minimize route-flapping. This rule is not used if:
* path-selection external-router-id is configured.
* Both peers have the same router ID.
* Either peer is a confederation peer.
* Neither path is the current active path.
12. Prefer the path from the peer with the lowest router ID. For any path with an originator ID attribute, substitute the originator ID for the router ID during router ID comparison.
13. Prefer the path with the shortest cluster list length. The length is0 for no list.
14. Prefer the path from the peer with the lowest peer IP address.By default, only the multiple exit discriminators (MEDs) of routes that have the same peer autonomous systems (ASs) are compared. You can configure routing table path selection options to obtain different behaviors.
The third step of the algorithm, by default, evaluates the length of the AS path and determines the active path. You can configure an option that enables Junos OS to skip this third step of the algorithm by including the as-path-ignore option.
Note: The as-path-ignore option is not supported for routing instances.
To configure routing table path selection behavior, include the path-selection statement:
path-selection {(always-compare-med | cisco-non-deterministic | external-router-id);as-path-ignore;med-plus-igp {igp-multiplier number;med-multiplier number;}}
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Routing table path selection can be configured in one of the following ways:
* Using the same nondeterministic behavior as does the Cisco IOS software (cisco-non-deterministic). This behavior has two effects:
* The active path is always first. All nonactive but eligible paths follow the active path and are maintained in the order in which they were received, with the most recent path first. Ineligible paths remain at the end of the list.
* When a new path is added to the routing table, path comparisons are made without removing from consideration those paths that should never be selected because those paths lose the MED tie-breaking rule.Note: The result of these two effects is that the system only sometimes compares the MED values between paths that it should otherwise compare. Because of this, we recommend that you not configure nondeterministic behavior.
* Always comparing MEDs whether or not the peer ASs of the compared routes are the same (always-compare-med).
* Comparing the router ID between external BGP paths to determine the active path (external-router-id). By default, router ID comparison is not performed if one of the external paths is active. You can force the router ID comparison by restarting the routing process with the restart routing operational-mode command.
* Adding the IGP cost to the next-hop destination to the MED value before comparing MED values for path selection.BGP multipath does not apply to paths that share the same MED-plus-IGP cost, yet differ in IGP cost. Multipath path selection is based on the IGP cost metric, even if two paths have the same MED-plus-IGP cost.