In the Junos OS, which attribute is used before the AS path in the BGP selection algorithm?
A.
highest local preference
B.
lowest local preference
C.
lowest MED
D.
weight
Explanation:
Hierarchy Level
[edit logical-systems logical-system-name protocols bgp],
[edit logical-systems logical-system-name protocols bgp group group-name],
[edit logical-systems logical-system-name protocols bgp group group-name neighboraddress],
[edit logical-systems logical-system-name routing-instances routing-instance-name protocols bgp],
[edit logical-systems logical-system-name routing-instances routing-instance-name protocols bgp group group-name],
[edit logical-systems logical-system-name routing-instances routing-instance-name protocols bgp group group-name neighboraddress],
[edit protocols bgp],
[edit protocols bgp group group-name],
[editprotocolsbgpgroupgroup-name neighboraddress],
[edit routing-instances routing-instance-name protocols bgp],
[edit routing-instances routing-instance-name protocols bgp group group-name],
[editrouting-instances routing-instance-name protocols bgp group group-name neighboraddress]Configuring the BGP Local Preference
Internal BGP sessions use a metric called the local preference, which is carried in internal BGP update packets in the path attribute LOCAL_PREF.
This metric indicates the degree of preference for an external route. The route with the highest local preference value is preferred.The LOCAL_PREF path attribute is always advertised to internal BGP peers and to neighboring confederations. It is never advertised to external BGP peers.
The default behavior is to not modify the LOCAL_PREF path attribute if it is present.Note: The LOCAL_PREF path attribute applies at export time only.
By default, if a received route contains a LOCAL_PREF path attribute value, the value is not modified. If a BGP route is received without a LOCAL_PREF attribute, the route is handled locally (that is, it is stored in the routing table and advertised by BGP) as if it were received with a LOCAL_PREF value of 100. A non-BGP route that is advertised by BGP is advertised with a LOCAL_PREF value of 100 by default.
To change the local preference metric advertised in the path attribute, include the local-preference statement, specifying a value from 0 through 4,294,967,295 (2321):
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.