Asymmetric Routing using router

Discussion in 'Cisco' started by SarojDey, Mar 31, 2006.

  1. SarojDey

    SarojDey Guest

    Hi,
    Need some info , how to do the asymmetric routing with 2 different ISP
    as upstream provider.
    We have our own IP pool and own ASN from APNIC.

    We have /18 IP pool. Wants to route /19 thru one ISP ( 20 Mbps) and
    other /19 thru another ISP(10 Mbps).

    Both shopuld acts as backup to each others.

    Thanks in advance.

    Kind Regards,
    SD
    SarojDey, Mar 31, 2006
    #1
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  2. SarojDey

    Charlie Root Guest

    Hi,

    "SarojDey" <> wrote in message
    news:...
    > We have our own IP pool and own ASN from APNIC.
    >
    > We have /18 IP pool. Wants to route /19 thru one ISP ( 20 Mbps) and
    > other /19 thru another ISP(10 Mbps).
    >


    For incoming traffic towards your addresses technically it's nothing
    special - just configure BGP to advertise prefixes in such way that given
    prefix gets better preference over selected provider, while other gets lower
    (in simplest scenario AS_PATH, more sophisticated would using communities -
    check with your upstreams what communities they support). However there
    might be serious administrative problem - if default allocation size in
    major /8 block where you got your addresses from is more than /19 you some
    networks may filter your announces at their discretion. Fortunately /19 is
    rather big and I don't recall any default allocation being bigger than this.
    So you should be ok here (but check).

    For outgoing traffic you have choice between:
    - simply using best AS_PATH through whichever ISP it happens to be and hope
    traffic will be about equal on each link
    - adjust local-pref of selected incoming announces in such way that traffic
    moves over whichever link you choose
    - disregard any incoming announce and use policy-based routing (PBR) to
    send traffic over the link which is primary for given /19 of the source IP
    (and backup over second if it fails)

    While last option may look attractive it usually won't produce best
    performance both due to PBR and likely sub-optimum path to destination.
    First option looks ok, but chances to get optimum balance of traffic are not
    very high (depends on your traffic patterns and providers of course). So the
    middle option seems to be the best choice but it requires a lot of work
    analysing traffic patterns, tweaking local-pref, then repeat it until you
    get desired optimum. Quite common to choose routing to/from some part of the
    world via one provider, while to/from another part of the world via another.
    Though this assumes that providers are either about equal or have best
    connectivity to choosen part of the world better than to another.

    As for backup - as long as you advertise you prefixes over both links (just
    with different attributes) and have either full-bgp or partial+default
    routes on both links BGP will do backup automagically.

    Is this what you're looking for?

    Kind regards,
    iLya
    Charlie Root, Mar 31, 2006
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  3. SarojDey

    Merv Guest

    in terms of configuration it would look something like


    router bgp <MY-ASN>
    neighbor <ip-address-2> remote-as 2
    neighbor <ip-address-2> route-map AS-2-OUTBOUND out
    neighbor <ip-address-3> remote-as 3
    neighbor <ip-address-3> route-map AS-3-OUTBOUND out
    exit


    route-map AS-2-OUTBOUND permit 10
    match ip address prefix-list BLOCK-2
    set as-path prepend <MY-ASN> <MY-ASN>


    route-map AS-3-OUTBOUND permit 10
    match ip address prefix-list BLOCK-1
    set as-path prepend <MY-ASN> <MY-ASN>


    ip prefix-list BLOCK-1 description 1st /19 block
    ip prefix-list BLOCK-1 permit x.x.x.x/19


    ip prefix-list BLOCK-2 description 2nd /19 block
    ip prefix-list BLOCK-2 permit y.y.y.y/19
    Merv, Mar 31, 2006
    #3
  4. SarojDey

    SarojDey Guest

    Exactly this is wht i wants to achieve. Main intention is to pass on
    the dedicated bw customer thru one link ( ISP) and shared customer thru
    the other link (ISP).
    So the incomming traffic and out going traffic should take the intended
    route. No load balance is required. Or else shared customer will take
    route of dedicated Link.

    But if one link is down, the other can be used as backup.

    Will appreciate if you can suggest brief config too.

    Thanks !
    Kind Regards,
    SD
    SarojDey, Mar 31, 2006
    #4
  5. In article <>,
    "SarojDey" <> wrote:

    > Exactly this is wht i wants to achieve. Main intention is to pass on
    > the dedicated bw customer thru one link ( ISP) and shared customer thru
    > the other link (ISP).
    > So the incomming traffic and out going traffic should take the intended
    > route.


    That's the exact *opposite* of asymmetric routing. Asymmetric means
    that the inbound and outbound paths are different, but you want them to
    be the same.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
    Barry Margolin, Apr 1, 2006
    #5
  6. SarojDey

    SD Guest

    I understand this is opposite of asymmetric routing. This is the
    initial setup. After this i will have another ISP connected , which
    will only provide incomming traffic to me. And another link will take
    care of outgoing traffic.

    I feel this point it will make more complex. So i wants to acieve the
    following first.

    Can i have some :***advice about the configration in my router ***

    Exactly this is wht i wants to achieve. Main intention is to pass on
    the dedicated bw customer thru one link ( ISP) and shared customer thru

    the other link (ISP).
    So the incomming traffic and out going traffic should take the intended

    route. No load balance is required. Or else shared customer will take
    route of dedicated Link.

    But if one link is down, the other can be used as backup.

    Both the ISP will be connected to 2 different iterface of a single
    Router.

    Will appreciate if you can suggest brief config too.


    Thanks !
    Kind Regards,
    SD
    SD, Apr 1, 2006
    #6
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