BGP Bestpath Selection

Hi All,

 

 

I have question regarding that part of task

Configure AS 200 so that all traffic from AS 100 destined to this prefix
traverses the Ethernet segment between SW4 and R5.
• In the case that the link between SW4 and R5 is down traffic destined for
the 150.X.11.0/24 prefix should transit the Frame Relay link between R2
and R3.
• Do not use AS-PATH prepending to accomplish this.

 

Acording to the solution there is configuration for MED on the SWR as below

 

ip prefix-list R1_BGP_LOOPBACK seq 5 permit 150.1.11.0/24
!!
SW4 sets R1’s new loopback metric to 100, which is better
! than R2’s setting of 200. This will make AS 100 prefer to go
! via R5 to reach R1’s new loopback
!
route-map MED permit 10
match ip address prefix-list R1_BGP_LOOPBACK
set metric 100
!
route-map MED permit 1000
!
router bgp 200
neighbor 183.1.105.5 route-map MED out

 



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I was wondering if this really
necessary. If I configure higher MED on the R2 and then R2 sends it out to the
R3 it will be enough to influence route decision process for the AS100.

Please clarify this for me why INE put additional configuration on the BGP session between SW4 and R5

Comments

  • I was wondering if this really
    necessary. If I configure higher MED on the R2 and then R2 sends it out to the
    R3 it will be enough to influence route decision process for the AS100

    Yes I take your point in what you are saying.  However you need to very careful with MED as it can be handled differently depending on configuration.  For example by default Cisco see a missing MED as best i.e. 0 whereas other routers implement a missing med with the highest metric the worst.  On Cisco you can change this behaviour by bgp bestpath med missing-as-worst

    So the configuration applied in the this case is belt and braces - in other words ensuring that you set a value - and thus send this attribute.

    HTH



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    I know what you meant however there is nothing like bgp bestpah on
    the R5 so by default R5 gets MED with value 0.

    I was wondering how it would be assess on the real exam if you for
    example miss such a config or it's always better add something more
    to be sure we met expectation?


    Any suggestion?

     

    Regards,

     

     

  • I was wondering how it would be assess on the real exam if you for
    example miss such a config or it's always better add something more
    to be sure we met expectation?

     

    The key in the lab is to meet the task requirements and not to violate any global or specific task requirements - if these are met and you have a verifiable solution then you are cooking on gas. Be aware also that tasks do have dependencies so a particular solution cause you problems down the line.

     

    Try playing around with how MED is handled as you will see different results for bestpath - especially if you don't apply outbound policy on SW4 to R5 peering :-)

     

  • Configure AS 200 so that all traffic from AS 100 destined to this prefix
    traverses the Ethernet segment between SW4 and R5.
    • In the case that the link between SW4 and R5 is down traffic destined for
    the 150.X.11.0/24 prefix should transit the Frame Relay link between R2
    and R3.
    • Do not use AS-PATH prepending to accomplish this.

     

    what i found strange about this is all i did was change the Local preference for the the specific prefix (150.1.11.0) inbound on R5.

    the ONLY other path is via R3 so if the link between R5 and SW4 , it would only be able to go viaa R3 ...

    again, maybe its me ... but it worked when i shut the link between r5-sw4 down !.

    cheers 

  • R5 is in AS 100.  The task says. " Configure AS 200 so that...."

    Sent from my iPhone

    On Oct 22, 2012, at 4:49 AM, secondsight <[email protected]> wrote:

    Configure AS 200 so that all traffic from AS 100 destined to this prefix
    traverses the Ethernet segment between SW4 and R5.
    • In the case that the link between SW4 and R5 is down traffic destined for
    the 150.X.11.0/24 prefix should transit the Frame Relay link between R2
    and R3.
    • Do not use AS-PATH prepending to accomplish this.

     

    what i found strange about this is all i did was change the Local preference for the the specific prefix (150.1.11.0) inbound on R5.

    the ONLY other path is via R3 so if the link between R5 and SW4 , it would only be able to go viaa R3 ...

    again, maybe its me ... but it worked when i shut the link between r5-sw4 down !.

    cheers 




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  • hahaha ,

    Cheers ,

    i just spotted that Today  ... DOH !!!

  • Configure AS 200 so that all traffic from AS 100 destined to this prefix
    traverses the Ethernet segment between SW4 and R5.
    • In the case that the link between SW4 and R5 is down traffic destined for
    the 150.X.11.0/24 prefix should transit the Frame Relay link between R2
    and R3.
    • Do not use AS-PATH prepending to accomplish this.

     

    what i found strange about this is all i did was change the Local preference for the the specific prefix (150.1.11.0) inbound on R5.

    the ONLY other path is via R3 so if the link between R5 and SW4 , it would only be able to go viaa R3 ...

    again, maybe its me ... but it worked when i shut the link between r5-sw4 down !.

    cheers 

    You could also solve the task by changing the weight of the route 150.1.11.0/24 on R3 or R5. However, this is not correct because the task requires to configure AS 200 not AS 100.

     

    Just my 2 cents.

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