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help with BGP routing

Discussion in 'Routing & Switching' started by shadds, Mar 23, 2009.

  1. shadds

    shadds Bit Poster

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    Hi Im trying to get my head round this issue regarding the next hop to an bgp AS with two identical routes to take. am i right in thinking that the router would go to the next hop with the lowest ip address? I havent added any varibles as of yet but the router chooses the best hop as the one with the highest ip? and i thought it was the lowest it would go to?

    ive included the running configs and bgp route of the routers below?

    router ISP is conencted to sanjose 1 & 2 though serial connections and is on another AS 200 whereas sanjose 1 & 2 are on AS 64512

    If i am totally wrong please tell me as i dont want to learn the stuff the wrong way around

    many thanks


     
    WIP: CCNA
  2. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Have you reviewed the BGP algorithm order of operation found here?
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
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  3. shadds

    shadds Bit Poster

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    Hi thanks for the link I think its provided my answer yet im still unsure.

    To determine the best path BGP checks several parameters in order starting with 'weight' (cisco only) , local_pref , AS_Path etc. I think i understand this but havent had chance to implement this.

    If all the above are the same then..

    This basically means the first route that the router has learned ?
    if this is the case then the best path is 192.168.1.6 as comes up on 'show ip bgp' in my previous example.




    although the algorithm states as well albeit one step after the above

    This would make the 192.168.1.2 route the best path as it has a lower loopback id?


    so finally if i wanted to make 192.168.1.2 the best path without using parameters such as weight, local_pref, AS_path etc could i just remove the 192.168.1.6 link and clear the bgp processes and then install the route again after the bgp process has picked up the 192.168.1.2 link and made this best path?

    Sorry if this is long winded im just trying to rack my brains around the basics before attempting attempting to use perameters to adjust the best path!

    Thanks for your help
     
    WIP: CCNA
  4. kammodo

    kammodo Nibble Poster

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    As per your previous:
    "When both paths are external, prefer the path that was received first (the oldest one).
    This step minimizes route-flap because a newer path does not displace an older one, even if the newer path would be the preferred route
    This basically means the first route that the router has learned ?
    if this is the case then the best path is 192.168.1.6 as comes up on 'show ip bgp' in my previous example."

    Answer yes. You've more or less answered your own question. When a router that is peering IBGP or EBGP learns a new route, and I dont mean when it is first powered on.The router will retain the old route, because it is a known stable route. To have the bgp re-run the algorithm for that specific peer, conduct a soft reset on that neighbour what ever the ip is: sanjose1#clear ip bgp 192.168.X.X. But I dont suppose much will be trafficking accross the s0/0/1 untill you put clocking on the interface. Step 10 of the BGP path selection will occur on: router reboot, soft bgp reset, hard bgp reset, providing all conditions are equal up untill this point.
    Note: Hard reset only if you are local to the router (clear ip bgp*) as the first time I tried this on a live network, I lost visibility of the router in Brazil for 15 long and painfull minutes.
     
    Certifications: CCNA R+S, CCDA , CCNP r+s , CCDP
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  5. r.h.lee

    r.h.lee Gigabyte Poster

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    shadds,

    Are you taking the CCNP: Building Scalable Internetworks v5.0 course at a Cisco Networking Academy?

     
    Certifications: MCSE, MCP+I, MCP, CCNA, A+
    WIP: CCDA
  6. shadds

    shadds Bit Poster

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    r.h.lee yes i am currently doing CCNP modules 1 & 2 as part of my university degree. why are you intrested?
     
    WIP: CCNA
  7. r.h.lee

    r.h.lee Gigabyte Poster

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    shadds,

    I've taken the CCNP: Building Scalable Internetworks v5.0 course through my local Cisco Networking Academy. When you mentioned "BGP", "ISP", "SanJose1", and "SanJose2", I recognized that lab. Are you having problems with Lab 6-3?
     
    Certifications: MCSE, MCP+I, MCP, CCNA, A+
    WIP: CCDA

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