BGP multihoming with uneven links - sample config and Qs

Discussion in 'Cisco' started by papi, Apr 5, 2005.

  1. papi

    papi Guest

    Hi, everyone,

    I was hoping for some input from some of the BGP gurus here, in regards
    to a suggested configuration of a dual ISP + 1 router, with uneven links
    (DS3 and T1). I have asked earlier about this, and I have been advised to
    upgrade the T1 to DS3, but we do not have that financial capability, and
    would rather eliminate some Internet services, if the DS3 goes down ...
    but this is just logistics. The issue I would like to raise is a
    configuration suggested to us by the ISP with the T1 (I'll call them
    ISP-T1), involving something along these lines:

    router bgp <my-ASN>
    no synchronization
    no bgp log-neighbor-changes
    bgp dampening
    network <my-public-net-class-C> --> this one comes from the ISP-T1
    neighbor ISP-T1 peer-group
    neighbor ISP-T1 remote-as XXXX --> ISP-T1 AS number
    neighbor ISP-T1 version 4
    neighbor ISP-T1 send-community
    neighbor ISP-DS3 peer-group
    neighbor ISP-DS3 remote-as YYYY --> ISP-T1 AS number
    neighbor ISP-DS3 version 4
    neighbor ISP-DS3 route-map localpref in
    neighbor ISP-DS3 route-map ISP-DS3 out
    no auto-summary

    ..... various ip route ...

    ip bgp-community new-format
    ip as-path access-list 10 deny _XXXX_
    ip as-path access-list 10 permit .*
    ip as-path access-list 20 deny _YYYY_
    ip as-path access-list 20 permit .*
    ip flow-export version 5
    ip flow-export destination <my-flow-collector>

    ..... other access lists ...

    route-map ISP-T1 permit 10
    match as-path 10
    set community ZZZZ:WW
    route-map localpref permit 10
    set local preference 300
    route-map ISP-DS3 permit 10
    match as-path 20


    So - looking at the above: ISP-T1 implies that he could help us control
    the majority of the traffic through ISP-DS3, because of a community
    ZZZZ:WW used at his end, also, and the local preference. My questions are:
    1. I do not understand (yet) all this, so I have no idea what the
    community woullld do (how does it get defined at the ISP end, top allow -
    presumably - more routes to come the other way (through ISP-DS3)?
    2. What would happen in case of ISP-T1 failure (under two scenarios: my
    link to them, or their end)?

    NOTE: all of the above has been typed just now, as we scribbled the stuff
    over the phone, on a notepad, so if there are typos, please do not
    consider them fundamental mistakes.

    I would appreciate any comments,
    papi, Apr 5, 2005
    1. Advertisements

  2. I assume there's supposed to be:

    neighbor ISP-T1 route-map ISP-T1 out
    You don't need to deny XXXX and YYYY in these ACLs. BGP automatically
    excludes the routes that were received from an ASN when sending to that

    Unless you're providing transit service for other ASes, you should just

    ip as-path access-list ## permit ^$

    to advertise only the routes that you originate.
    Should this be:

    set community XXXX:WW

    ? The convention for communities is that the prefix is the ASN of the
    AS that uses the community. But I suppose ZZZZ could be their upstream
    provider's ASN.

    What this *probably* does is tell them to change the local-pref of the
    routes they receive from you. Normally, an ISP will set the local-pref
    of their customer routes so that they're preferred over routes received
    from peers. But in your case, you only want the T1 to be used when the
    DS3 is down, so this drops the local-pref down below the local-pref for
    the peer routes.

    Another possibility is that it tells them to pad the AS Path that they
    send to their peers. So their own customers will use the T1, but
    everyone else will prefer to the DS3.
    The meaning of communities is established by the AS that you're
    advertising to. They should be able to provide detailed documentation
    of the various communities that they use.
    They'll stop receiving routes from you, and their traffic to you will go
    through ISP-DS3.
    Barry Margolin, Apr 5, 2005
    1. Advertisements

  3. papi

    papi Guest

    Thank you so much for your comments. A little more to the subject:

    In fact we were hoping to send absolutely everything out the DS3, out of
    fear that we may not be able to properly control the "out" between the two
    unbalanced lines.
    Good to know - thank you!
    You are right - I got carried away with abstractization, out of fear of
    not making public info from providers that may or may not agree to have
    their opinion "google-ized" without consent :) ZZZZ is, in fact, XXXX.

    Thank you again for all comments.
    papi, Apr 5, 2005
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.