AS Path Prepend Problem

Discussion in 'Cisco' started by amyl@paxemail.com, Apr 13, 2005.

  1. Guest

    I am attempting to learn BGP by simulating various tasks in a lab
    environment.

    *Please note that everything including IP's, AS numbers, etc are made
    up. Yes, I am mocking up a little internet type lab to learn this
    stuff and I was trying to make everything seem as real as it can be.

    Problem: In the lab I am seeing the prepending correctly on the routers
    that I directly peer with. However, on any other router in my lab the
    prepend information is not passing through. From my reading it appears
    as if this information should stay in tact and be sent to all routers
    through BGP.

    Details:
    I have a single router that is multihoming to two different AS'es (7018
    and 209). This router will advertise 3 IP blocks

    12.3.2.0/24
    12.3.1.0/24
    65.200.5.0/27

    I am using the following route map to set the preference on which links
    I would prefer the ip addresses to use.

    access-list 1 permit 12.3.2.0 0.0.0.255
    access-list 2 permit 12.3.1.0 0.0.0.255
    access-list 2 permit 65.200.5.0 0.0.0.31

    route-map PrependToATT permit 10
    match ip address 2
    set as-path prepend 1000 1000 1000
    !
    route-map PrependToATT permit 20
    match ip address 1
    !
    route-map PrependToQwest permit 10
    match ip address 1
    set as-path prepend 1000 1000 1000
    !
    route-map PrependToQwest permit 20
    match ip address 2
    !

    On the routers that AS1000 directly peers with I see the path
    prepending properly. However, any other router (IBGP or EBGP) this
    information seems to be lost.

    I have tried using soft reconfiguration to force the changes out but no
    luck.

    What am I missing?

    Thanks
    Amy.
     
    , Apr 13, 2005
    #1
    1. Advertising

  2. In article <>,
    wrote:

    > I am attempting to learn BGP by simulating various tasks in a lab
    > environment.
    >
    > *Please note that everything including IP's, AS numbers, etc are made
    > up. Yes, I am mocking up a little internet type lab to learn this
    > stuff and I was trying to make everything seem as real as it can be.
    >
    > Problem: In the lab I am seeing the prepending correctly on the routers
    > that I directly peer with. However, on any other router in my lab the
    > prepend information is not passing through. From my reading it appears
    > as if this information should stay in tact and be sent to all routers
    > through BGP.
    >
    > Details:
    > I have a single router that is multihoming to two different AS'es (7018
    > and 209). This router will advertise 3 IP blocks
    >
    > 12.3.2.0/24
    > 12.3.1.0/24
    > 65.200.5.0/27
    >
    > I am using the following route map to set the preference on which links
    > I would prefer the ip addresses to use.
    >
    > access-list 1 permit 12.3.2.0 0.0.0.255
    > access-list 2 permit 12.3.1.0 0.0.0.255
    > access-list 2 permit 65.200.5.0 0.0.0.31
    >
    > route-map PrependToATT permit 10
    > match ip address 2
    > set as-path prepend 1000 1000 1000
    > !
    > route-map PrependToATT permit 20
    > match ip address 1
    > !
    > route-map PrependToQwest permit 10
    > match ip address 1
    > set as-path prepend 1000 1000 1000
    > !
    > route-map PrependToQwest permit 20
    > match ip address 2
    > !
    >
    > On the routers that AS1000 directly peers with I see the path
    > prepending properly. However, any other router (IBGP or EBGP) this
    > information seems to be lost.


    If these routers receive the route from multiple peers, they'll only
    pass on the one with the shortest AS path. So if there's an
    advertisement without the prepending, it will be passed on in favor of
    the one with prepending.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
     
    Barry Margolin, Apr 14, 2005
    #2
    1. Advertising

  3. Amy L. Guest

    Let me see if I understand what you are saying:

    Assume Router A is where I am doing the prepending. Sorry for the ASCII
    art.

    A--B--E--| (A,B, and E = AS209)
    |--C--D---| (C,D = AS7018)

    In Words:
    A Peers with B,C
    B Peers with E
    C Peers With D
    D Peers with E

    Because the routes are advertised to B with no prepending they go through E
    to D. This would explain why on "D" I do not see the routes with prepending
    as it prefers the shortest AS path being advertised in from E.

    If this is the case how do you advertise preferred paths using AS path
    prepending to prevent what is happening above. Or did I completely
    misunderstand what is occurring?

    Thanks
    Amy.

    "Barry Margolin" <> wrote in message
    news:...
    > In article <>,
    > wrote:
    >
    > > I am attempting to learn BGP by simulating various tasks in a lab
    > > environment.
    > >
    > > *Please note that everything including IP's, AS numbers, etc are made
    > > up. Yes, I am mocking up a little internet type lab to learn this
    > > stuff and I was trying to make everything seem as real as it can be.
    > >
    > > Problem: In the lab I am seeing the prepending correctly on the routers
    > > that I directly peer with. However, on any other router in my lab the
    > > prepend information is not passing through. From my reading it appears
    > > as if this information should stay in tact and be sent to all routers
    > > through BGP.
    > >
    > > Details:
    > > I have a single router that is multihoming to two different AS'es (7018
    > > and 209). This router will advertise 3 IP blocks
    > >
    > > 12.3.2.0/24
    > > 12.3.1.0/24
    > > 65.200.5.0/27
    > >
    > > I am using the following route map to set the preference on which links
    > > I would prefer the ip addresses to use.
    > >
    > > access-list 1 permit 12.3.2.0 0.0.0.255
    > > access-list 2 permit 12.3.1.0 0.0.0.255
    > > access-list 2 permit 65.200.5.0 0.0.0.31
    > >
    > > route-map PrependToATT permit 10
    > > match ip address 2
    > > set as-path prepend 1000 1000 1000
    > > !
    > > route-map PrependToATT permit 20
    > > match ip address 1
    > > !
    > > route-map PrependToQwest permit 10
    > > match ip address 1
    > > set as-path prepend 1000 1000 1000
    > > !
    > > route-map PrependToQwest permit 20
    > > match ip address 2
    > > !
    > >
    > > On the routers that AS1000 directly peers with I see the path
    > > prepending properly. However, any other router (IBGP or EBGP) this
    > > information seems to be lost.

    >
    > If these routers receive the route from multiple peers, they'll only
    > pass on the one with the shortest AS path. So if there's an
    > advertisement without the prepending, it will be passed on in favor of
    > the one with prepending.
    >
    > --
    > Barry Margolin,
    > Arlington, MA
    > *** PLEASE post questions in newsgroups, not directly to me ***
     
    Amy L., Apr 14, 2005
    #3
  4. In article <>,
    "Amy L." <> wrote:

    > Let me see if I understand what you are saying:
    >
    > Assume Router A is where I am doing the prepending. Sorry for the ASCII
    > art.
    >
    > A--B--E--| (A,B, and E = AS209)
    > |--C--D---| (C,D = AS7018)
    >
    > In Words:
    > A Peers with B,C
    > B Peers with E
    > C Peers With D
    > D Peers with E
    >
    > Because the routes are advertised to B with no prepending they go through E
    > to D. This would explain why on "D" I do not see the routes with prepending
    > as it prefers the shortest AS path being advertised in from E.


    Right.

    >
    > If this is the case how do you advertise preferred paths using AS path
    > prepending to prevent what is happening above. Or did I completely
    > misunderstand what is occurring?
    >


    Prepending is used to make a path *not* be preferred. So if you want a
    path to be preferred, why are you prepending in the first place?

    In other words, what are you attempting to accomplish?

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
     
    Barry Margolin, Apr 14, 2005
    #4
  5. Amy L. Guest

    What I was trying to accomplish was making the routes

    12.3.2.0/24 - Prefered via AS 7018
    12.3.1.0/24 - Prefered via AS 209
    65.200.5.0/27 - Prefered via AS 209.

    So for example if I was in a different AS like 701 traffic to 12.3.2.0 would
    flow through 7018 and the other routes 12.3.1.0 and 65.200.5.0/27 from AS
    701 would want to flow through AS 209. With the assumption that in my lap
    AS 701 peers with both 7018 and 209.

    I figured that if I prepended to the AS path for the routes down the
    opposite peer where I wanted the routes to flow it would accomplish what I
    wanted to do in the route list above.

    Thanks for your help
    Amy.

    "Barry Margolin" <> wrote in message
    news:...
    > In article <>,
    > "Amy L." <> wrote:
    >
    > > Let me see if I understand what you are saying:
    > >
    > > Assume Router A is where I am doing the prepending. Sorry for the ASCII
    > > art.
    > >
    > > A--B--E--| (A,B, and E = AS209)
    > > |--C--D---| (C,D = AS7018)
    > >
    > > In Words:
    > > A Peers with B,C
    > > B Peers with E
    > > C Peers With D
    > > D Peers with E
    > >
    > > Because the routes are advertised to B with no prepending they go

    through E
    > > to D. This would explain why on "D" I do not see the routes with

    prepending
    > > as it prefers the shortest AS path being advertised in from E.

    >
    > Right.
    >
    > >
    > > If this is the case how do you advertise preferred paths using AS path
    > > prepending to prevent what is happening above. Or did I completely
    > > misunderstand what is occurring?
    > >

    >
    > Prepending is used to make a path *not* be preferred. So if you want a
    > path to be preferred, why are you prepending in the first place?
    >
    > In other words, what are you attempting to accomplish?
    >
    > --
    > Barry Margolin,
    > Arlington, MA
    > *** PLEASE post questions in newsgroups, not directly to me ***
     
    Amy L., Apr 14, 2005
    #5
  6. Midhun p j

    Joined:
    Oct 12, 2008
    Messages:
    1
    Hi,
    For the below senario,

    Please check the below config


    12.3.2.0/24 - Prefered via AS 7018 ATT
    12.3.1.0/24 - Prefered via AS 209 QWEST
    65.200.5.0/27 - Prefered via AS 209.

    Router bgp 701
    neighbor 100.1.1.5 remote as 7018
    neighbor 100.1.1.8 remote as 209
    neighbor 100.1.1.5 route map advATT out
    neighbor 100.1.1.8 route map advQwest out

    access list 1 permit 12.3.2.0 0.0.0.255
    access list 2 permit 12.3.1.0 0.0.0.255
    access list 3 permit 65.200.5.0 0.0.0.31

    route map advATT permit 10 -- Prefered path for 12.3.2.0
    match ip address 1

    route map advQwest permit 20
    match ip address 1
    set as path prepend 701 701 701

    route map advQwest permit 10 --- Prefered path for 12.3.1.0
    match ip address 2

    route map advATT permit 20
    match ip address 2
    set as path prepend 701 701 701

    route map advQwest permit 10 --- Prefered path for 65.200.5.0
    match ip address 3

    route map advATT permit 20
    match ip address 3
    set as path prepend 701 701 701

    Regds,
    Midhun p j
    Bangalore, India
     
    Midhun p j, Oct 13, 2008
    #6
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