user@router# run show route advertising-protocol bgp 192.168.12.1
user@router# run show route
inet.0: 11 destinations, 12 routes (11 active, 0 holddown, 0 hidden)
+ = Active Route, – = Last Active, * = Both
2.2.2.2/32 *[Direct/0] 3w6d 03:57:51
> via lo0.0
192.168.12.0/24 *[Direct/0] 01:07:34
> via xe-0/0/0.0
192.168.12.2/32 *[Local/0] 01:07:34
Local via xe-0/0/0.0
200.1.0.0/16 *[Aggregate/130] 00:00:58
Reject
[IS-IS/165] 00:10:57, metric 10
> to 200.1.1.2 via xe-0/0/3.0
200.1.1.0/24 *[Direct/0] 00:29:21
> via xe-0/0/3.0
200.1.1.1/32 *[Local/0] 00:29:21
Local via xe-0/0/3.0
iso.0: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, – = Last Active, * = Both
49.0000.0020.0200.2002/72
*[Direct/0] 3w4d 21:07:32
> via lo0.0
inet6.0: 3 destinations, 4 routes (3 active, 0 holddown, 0 hidden)
+ = Active Route, – = Last Active, * = Both
2:2:2::2/128 *[Direct/0] 3w4d 21:22:24
> via lo0.0
[edit]
user@router# show policy-options
policy-statement adv-route {
term t1 {
from {
protocol isis;
route-filter 200.1.0.0/16 exact;
}
then accept;
}
term t2 {
then reject;
}
}
[edit]
user@router# show protocols bgp
group ebgp {
type external;
export adv-route;
neighbor 192.168.12.1 {
peer-as 65000;
}
}
Click the Exhibit button.
Referring to the exhibit, why is the 200.1.0.0/16 prefix failing to be advertised in BGP?
A.
BGP needs a next-hop self policy.
B.
The aggregate route is set to reject.
C.
The policy works for internal BGP only.
D.
The IS-IS route is less preferred than the aggregate route.
Explanation:
Correct Answer is D.
By default, aggregate routes have a preference value of 130. If the routing table contains a dynamic route to a destination that has a better (lower) preference value than this, the dynamic route is chosen as the active route and is installed in the forwarding table.
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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 foremost thing that bgp considers in path selection is route preference.
In this case the Aggregate route has a lower preference of 130 compared to that of isis which is 160 for Level 1 external routes and 165 for Level 2 external routes, so bgp have chosen the prefix advertised by the aggregate route, and the default next-hop for an aggregate route is “reject” , it is never advertised unless there is a contributing route.
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Shortly, inactive (not best) routes does not get advertised (by BGP as well).
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