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BGP and metric configuration ????/

Jul 7, 2010 7:58 AM

Mohammed 114 posts since
May 27, 2010

from the pic. the two ebgp links between AS100 and AS200 have an average inbound load of 65% and 20 % traffic to 10.10.1.16/28 accounts for  45%  and other networks as from the pic account for 20% of the inbounf load  how would u configure As100 to influence AS200 to use eBGP links more evenly

choose 2 options

1-

neighbour 192.168.30.2 route-map as_50 out

 

2-

neighbour 192.168.20.2 route-map as_50 out

 

3-

route-map as_50 permit 10

match ip address 50

set metric 150

access-list 50 permit 10.10.1.16 0.0.0.240

 

4-

route-map as_50 permit 10

match ip address 50

set metric  150

access-list 50 permit 10.10.1.32 0.0.0.240

first of all i think the access-list in option 3 and 4 is wrong this is if didn't forget access-list so lets assume they mean(255-240=15) okay

 

where i got the question from they say the correct answer is option 2 &3 but ithink it is 1 & 4

what is the correct answer????????///

Attachments:
  • Conwyn 7,907 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Jul 7, 2010 8:31 AM (in response to Mohammed)
    Re: BGP and metric configuration ????/
  • jneiberger 465 posts since
    Jun 10, 2010
    Currently Being Moderated
    2. Jul 7, 2010 8:33 AM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    I believe that 2 and 3 are correct. Those statements raise the metric on that route that is being advertised to Router E. That will cause AS 200 to prefer Router B as the entry point into AS 100 for the network 10.10.1.16, which will shift some of the total traffic from the top link down to the bottom link.

  • jneiberger 465 posts since
    Jun 10, 2010
    Currently Being Moderated
    4. Jul 7, 2010 9:41 AM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    Yep, I think you're right. If I understand the question, what you'd really want to do is advertise 10.10.1.32/28 with a higher metric to Router E. That would shift 20% more down to the bottom link.

  • Sal 217 posts since
    Nov 23, 2009
    Currently Being Moderated
    5. Jul 11, 2010 4:27 PM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    all output in zip file in last thread

     

    Message was edited by: Sal

  • Sal 217 posts since
    Nov 23, 2009
    Currently Being Moderated
    6. Jul 7, 2010 10:49 PM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    Guys , I think this question was only based on assumptions, its inaccurate in real-time. the evidance is that in IOS the keyword neighbour is without a letter "u"  !!?? . This is plus that every-one knows that a route-map and/or and ACL ends with an implicit deny.

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    7. Jul 8, 2010 11:22 AM (in response to Sal)
    Re: BGP and metric configuration ????/

    All,

     

    Nothing more is needed, only statements 2 and 3 on router A.  This tells AS200 (Router E) to only allow 10.10.1.16/28 subnet through the top path (45% load).  The remaining two subnets (40% load total) would come through the bottom path.  This is due to the explicit deny at the end of the ACL 50 that denies 10.10.1.32/28 and 10.10.1.48/28 from the top path.

     

    HTH

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    9. Jul 8, 2010 3:20 PM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    Mohammed,

     

    Please note in the diagram E and F are nor connected ( at least not directly ).  While Router F will receive the routes for all three subnets and pass traffic for all three.  Router E only receives the 10.10.1.16/28 route and therefore, only traffic for this subnet will pass through the top path.  Based on the load percentages identified in the diagram this will result in roughly a 45% load on the top path and a 40% load on the bottom path.

     

    Sorry, I cannot lab this up with real equipment, but I can try my simulator.  I will give it a try later today and post the results for you.

     

    HTH

  • Sal 217 posts since
    Nov 23, 2009
    Currently Being Moderated
    10. Jul 8, 2010 9:12 PM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    Thats right , All traffic for 10.10.1.16/24 will move to the bottom as the the top path has become less favourable by increasing the metric .. have alook at the detailed lab results I have attached previously. At this point we have not achieved anything yet. So we need to move traffic for the other to networks and make sure the top path is more favourable for it then we have the 40% : 45% .

     

    Regards

  • Sal 217 posts since
    Nov 23, 2009
    Currently Being Moderated
    11. Jul 8, 2010 10:11 PM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    just like to add that by implicitly denying all other traffic (except for 10.10.1.16/24/0)  Router E will not  learn from A about the other networks so we end up with a single point of failure  when the bottom link goes down we have no other path to AS 100 except for the permitted network. Which raises a question : Whats the point of Multi-homing or multiple redundant AS Paths ?!

     

    ************************************************************************************************************************************************
    This how the routing table an internal router in AS 200 look like if after using all four answers on approriate Top and Bottom

    routers in AS100 in addition to a permit all additional route-map seq. 20 that overidesthe default implicit deny
    **************************************************************************************************************************************************

    G#sh ip bgp
    BGP table version is 23, local router ID is 10.200.3.3
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

       Network             Next Hop            Metric LocPrf Weight Path
    * i10.10.1.16/28     10.200.2.1             150    100      0 100 i
    *>i                        10.200.3.1               2    100      0 100 i
    *>i10.10.1.32/28    10.200.2.1              11    100      0 100 i
    * i                         10.200.3.1             150    100      0 100 i
    *>i10.10.1.48/28    10.200.2.1               2    100      0 100 i
    * i                         10.200.3.1             150    100      0 100 i
    *>i20.20.20.0/28    10.200.2.1               0    100      0 i
    *>i21.21.21.0/28    10.200.2.1               0    100      0 i
    *>i22.22.22.0/28    10.200.2.1               0    100      0 i
    *>i30.30.30.0/28    10.200.3.1               0    100      0 i
    *>i31.31.31.0/28    10.200.3.1               0    100      0 i
    *>i32.32.32.0/28    10.200.3.1               0    100      0 i

    ******************************************************************************************************************
      This how the routing table an internal router in AS 200 look like if the bottom path is down

      ( serial link shutdown between B and F) and the route-maps donot have a permit any statement
    *****************************************************************************************************************
    G#sh ip bgp
    BGP table version is 17, local router ID is 50.50.50.1
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

       Network          Next Hop            Metric LocPrf Weight Path
    *>i10.10.1.16/28    10.200.2.1             150    100      0 100 i
    *>i20.20.20.0/28    10.200.2.1               0    100      0 i
    *>i21.21.21.0/28    10.200.2.1               0    100      0 i
    *>i22.22.22.0/28    10.200.2.1               0    100      0 i
    *>i30.30.30.0/28    10.200.3.1               0    100      0 i
    *>i31.31.31.0/28    10.200.3.1               0    100      0 i
    *>i32.32.32.0/28    10.200.3.1               0    100      0 i

  • Sal 217 posts since
    Nov 23, 2009
    Currently Being Moderated
    12. Jul 8, 2010 9:33 PM (in response to Brian)
    Re: BGP and metric configuration ????/

    I appreciate your opinion but the purpose of the design is Redundency , High- Availabily and Efficiency . We are not trying to filter routes for security purposes here. This way we end up with a non-resilient results. And its an implict deny that is not explicitly mentioned (configured) but rather inheritted.

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    14. Jul 9, 2010 12:31 PM (in response to Mohammed)
    Re: BGP and metric configuration ????/

    All,

     

    After researching a bit more regarding MED and BGP path selection.  I will change my answer.  Based my new found knowledge (cross your fingers)  in the network as designed, the 10.10.1.16/28 subnet will traverse the top path based on BGP best path rule selection.  Likewise the 10.10.1.48/28 subnet will choose the bottom path based on BGP best path rule selection.  It is the third subnet off of router C, the 10.10.1.32/28 will choose the top path based on the BGP best path rule selection.  All things equal, it will choose the top path based on the lowest Router ID.  Therefore, my new answer is #2 and #4 that are correct.

     

    Reasoning, #2 is correct because you want the MED to be large on the top path for 10.10.1.32/28 subnet so it will choose the bottom path.  Now, regarding the route-map and the ACL.  The ACL only allows the 10.10.1.32/28 subnet through which means it is the only subnet tagged with its MED set to 150.  Since the others do not "match" they go through with a MED of 0.  There is no expilcit deny with this route-map as it is not used with the "redistribute" command.  So now, on router E it sees the route to 10.10.1.32/28 with a MED = 150, while router F sees the route to 10.10.1.32/28 with a MED = 0.  Therefore, the top path carries the 10.10.1.16/28 subnet (45%) and the bottom path carries both the 10.10.1.32/28 and 10.10.1.48/28 subnets (40% total).

     

     

    Hope this is better explaination.  Thanks for the question Mohammed.  I too am studying for ROUTE exam, so this was very rewarding.  Below are some documents that help me in my re-evaluation.

     

    How BGP uses MED for best path selection
    http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094934.shtml 4451 - BGP MED Considerations
    http://tools.ietf.org/html/rfc4451

    RFC

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