BGP multihoming single site, two isp

Discussion in 'Cisco' started by jan.rockstedt, Apr 4, 2007.

  1. Hi all,

    We are soon starting up bgp and multihoming.
    Two ISPs and we have one site.
    We have already PI number and ASN assign to us.
    A Cisco 3825 with 3 ethernet interface with IOS advanced feature pack
    is order.
    No full routing table.

    Any sugestion for a config?

    Regards Jan Rockstedt
    jan.rockstedt, Apr 4, 2007
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  2. Hi Jan

    You could do it two ways:

    1. Use one ISP as primary and one as secondary
    2. Use both ISPs and recieve default routes from both ISP along with
    their "own" IPs

    For both ISPs you setup standard BGP settings as described in the
    documentation from Cisco (take a look at CCO and do a search on BGP).

    With option 1 you tag the inbound routes from both ISPs, setting the
    local-preference for 200 on the primary routes and leave the secondary
    ISP with default local-preference (that'll be 100). When you advertise
    your own routes, do a prepend to the routes you advertise to the
    secondary ISP.

    With option 2 you simply accept any routes advertised by the ISP and
    advertise your own routes with default settings.

    If you need further information / help, please respond to this group.

    Lars Christensen
    Lars L. Christensen, Apr 4, 2007
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  3. jan.rockstedt

    Bod43 Guest

    Thanks, that is interesting to me too.

    Is it possible to find out how many AS numbers to
    "prepend" to effectively guarantee that one path
    will become primary for all ISPs?

    I have made a stab at 4 relating to 2 connections within a single
    How many would be needed to guarantee this globally?

    Is there a limit to the number of ASs in a path and
    if so what is a reasonable number to limit our prepends

    Bod43, Apr 4, 2007
  4. Hi Lars,

    Thanks for your respond.

    I have found so many doc on the google.
    Many are old quit .....

    Our primary isp are the link 50 Mbit and our second isp we have a 10
    So two default routes, one with static with default
    100 to the 50 Mbit and one with with 200 to the 10
    Redistribute static to bgp?

    router bgp our-ASN

    neighbor remote-as ISP1-ASN
    neighbor route-map localonly out
    neighbor remote-as IPS2-ASN
    neighbor route-map localonly out

    jan.rockstedt, Apr 5, 2007
  5. A few comments...

    Don't worry about the age of the docs on the web! Not much has changed with
    BGP since version 4 was released last century. If you can track down a copy
    of my book, read the chapter on connecting to an ISP, it still applies.

    As for your proposal, I do not recommend it if your goal is to minimize
    potential for black holes. Rather than using static default routes, have
    your ISPs advertise a default route to you (that way, you only get a
    default route if your link to the ISP is up). There are even better ways to
    protect against problems in your ISP or upstream, but they take more
    effort, as does setting up reasonable load balancing.

    Good luck and have fun!
    Vincent C Jones, Apr 10, 2007
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