Click the Exhibit button. You are asked to exchange routes between R1 and R4 as shown in the exhibit. These two routers use the same AS number. Which two steps will accomplish this task? (Choose two.)
A.
Configure the BGP group with the advertise-peer-as parameter on R2 and R3.
B.
Configure the BGP group with the as-override parameter on R1 and R4.
C.
Configure the BGP group with the advertise-peer-as parameter on R1 and R4.
D.
Configure the BGP group with the as-override parameter on R2 and R3.
D. as-override on R2 and R3:
R1 and R4 are in the same AS (64522), and the intermediate AS (65411) includes R2 and R3.
When R2 receives a route from R1 (AS 64522), the AS_PATH becomes 64522. R2 adds its own AS (65411) when advertising to R3. R3 then sends the route to R4 (AS 64522), resulting in an AS_PATH of 65411 64522. R4 rejects the route because it sees its own AS in the path.
Configuring as-override on R2 and R3 replaces the originating AS (64522) with the local AS (65411) in the AS_PATH. This ensures R1 and R4 do not see their own AS in the path, allowing routes to be accepted.
A. advertise-peer-as on R2 and R3:
The advertise-peer-as parameter forces R2 and R3 to advertise the peer’s AS (64522) instead of their own (65411). This avoids inserting AS 65411 into the AS_PATH entirely.
When R2 sends routes to R3, it advertises AS 64522 (R1’s AS). R3 then advertises these routes to R4 with AS 64522 in the path, avoiding AS_PATH loop detection.
BD
In the scenario where routers R1 and R4 are using the same AS number and you need to exchange routes between them, the correct answers are:
B. Configure the BGP group with the as-override parameter on R1 and R4.
The as-override option allows BGP to bypass the typical loop prevention mechanism that prevents a router from accepting routes that include its own AS number. Since R1 and R4 are in the same AS, this option is necessary to exchange routes successfully.
D. Configure the BGP group with the as-override parameter on R2 and R3.
Similarly, configuring as-override on R2 and R3 ensures that routes from R1 and R4 (which share the same AS number) can be exchanged, bypassing the default behavior that would prevent them from accepting routes originating from their own AS.
These configurations allow routes to be exchanged even when routers belong to the same AS.
as-override - if the as number of a router recving advertisement is included in as path, router drops this prefix. this can be changed by using as-override.
advertise-peer-as - junos do not advertise route learned from ebgp peer back to same ebgp peer. this can be changed by advertise-peer-as. Hence AD.
Both options on R2 and R3 are meant to advertise the routes that have the same AS origin as the peer, only that with as-override the AS is replaced by the one on R2/R3 before advertising
advertise-peer-as is explained in:
https://www.juniper.net/documentation/us/en/software/junos/routing-policy/bgp/topics/example/bgp-advertise-peer-as.html
Both advertise-peer-as and as-override are BGP settings applied on the PE, not the CE, therefore the correct answer is A and D
The advertise-peer-as parameter allows a router to advertise its peer's AS number as part of the AS path attribute when sending BGP updates to other peers. This parameter is useful when two routers in the same AS need to exchange routes through another AS, such as in the case of R1 and R4. By configuring this parameter on R1 and R4, they can advertise each other's AS number to R2 and R3, respectively.
The as-override parameter allows a router to replace the AS number of its peer with its own AS number when receiving BGP updates from that peer. This parameter is useful when two routers in different AS need to exchange routes through another AS that has the same AS number as one of them, such as in the case of R2 and R3. By configuring this parameter on R2 and R3, they can override the AS number of R1 and R4 with their own AS number when sending BGP updates to each other.
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