Which role does the router ID (RID) have in the BGP route selection process?
A.
The router ID is evaluated immediately after MED to determine route preference
B.
The router ID is evaluated last in the route selection process
C.
The router ID is evaluated after the cluster length and a lower router ID is chosen
D.
The router ID is evaluated after analyzing the IGP metric cost
Explanation:
Reference:http://www.juniper.net/techpubs/en_US/junos10.0/information-products/topic-collections/nog- baseline/bgp-routes-and-selection-introduction.html
router id is prefernce lowest #9 in BGP path selection process
1.local prerance, self-originated, as path, origin, med, external, igp cost ebgp peering and router-id
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The selection process proceeds in the following order:
1. Prefer the highest local-preference value
2. Prefer the shortest AS-path length
3. Prefer the lowest origin value
4. Prefer the lowest MED value
5. Prefer routes learned from an EBGP peer over an IBGP peer
6. Prefer best exit from AS (lowest IBGP cost)
7. For EBGP-received routes, prefer the current active route; otherwise, prefer routes from the peer with the lowest RID
8. Prefer paths with the shortest cluster length
9. Prefer routes from the peer with the lowest peer ID
So it is clearly option D
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I agree with Rafa.
Option D
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no it should be C, based on this: http://www.juniper.net/techpubs/en_US/junos9.6/information-products/topic-collections/nog-baseline/bgp-routes-and-selection-introduction.html
Before the router installs a BGP route, it must make sure that the BGP next-hop attribute is reachable. If the BGP next hop cannot be resolved, the route is not installed. When a BGP route is installed in the routing table, it must go through a path selection process if multiple routes exist to the same destination prefix. The BGP path selection process proceeds in the following order:
Route preference in the routing table is compared. For example, if an OSPF and a BGP route exist for a particular destination, the OSPF route is selected as the active route because the OSPF route has a default preference of 10, while the BGP route has a default preference of 170.
Routes are compared for local preference. The route with the highest local preference is preferred. For example, see Examine the Local Preference Selection.
The AS path attribute is evaluated. The shorter AS path is preferred.
The origin code is evaluated. The lowest origin code is preferred ( I (IGP) < E (EGP) >>The router ID is evaluated. The route from the peer with the lowest router ID is preferred (usually the loopback address).
The peer address value is examined. The peer with the lowest peer IP address is preferred.
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but have to say is controversial since course book instigates for D and above link puts Router ID eval after Cluster ID.
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the explanation from mp and the reference is the only source listing teh cluster length before the RID. All other references are the other way around:
http://www.juniper.net/techpubs/en_US/junos11.4/topics/reference/general/routing-ptotocols-address-representation.html
ON that page I checked for different JUNOS but cluster lenght comes always after the RID.
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I guess:
D. The router ID is evaluated after analyzing the IGP metric cost
is correct.
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The correct answer is C.
Folks take notice that if you choose D, then why not option A, because those came before the router ID.
The question was explicit on the role of router ID, that is to say if bgp have considered all the options, what will be the last option to be considered before looking the way of the router ID.
The last attribute that bgp will check before looking at router ID attribute is the Route Reflection Cluster List. So the right answer is C.
1. Route preference in the routing table is compared.
2. Routes are compared for local preference.
3. The AS path attribute is evaluated. The shorter AS path is preferred.
4. The origin code is evaluated.
5. The MED value is evaluated.
6. The route is evaluated as to whether it is learned through EBGP or IBGP. E
7. If the route is learned from IBGP, the route with the lowest IGP cost is preferred.
8. The route reflection cluster list attribute is evaluated.
9. The router ID is evaluated.
10. The peer address value is examined. The peer with the lowest peer IP address is preferred.
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The answer A uses the word “immediately” after MED> So cant be valid.
Should be D –
All techpubs for relevant Juniper codes 12.3 – 15.1 reflect the same context as well as my revision 12a JIR student guide.
https://www.juniper.net/documentation/en_US/junos12.3/topics/topic-map/bgp-path-selection.html
Router id comes before Cluster List Length which makes C an invalid correct answer. Not sure if that has changed from older JUNOS versions to closer match other vendors best path Algorithm, but this would be the most up to date link.
10) Prefer the path whose next hop is resolved through the IGP route with the lowest metric.
13) 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.
Prefer the path with the shortest cluster list length. The length is 0 for no list.
14)Prefer the path with the shortest cluster list length. The length is 0 for no list.
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who are the fucking retarded dumbasses writing these questions? the answer is completely incorrect.
this is straight from junipers motherfucking website (https://www.juniper.net/techpubs/en_US/junos12.1×46/topics/reference/general/routing-ptotocols-address-representation.html):
10. Prefer the path whose next hop is resolved through the IGP route with the lowest metric.
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 is 0 for no list.
14. Prefer the path from the peer with the lowest peer IP address.
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