From: Sam Munzani (sam@xxxxxxxxxxx)
Date: Fri Dec 15 2000 - 11:57:42 GMT-3
I totally agree and that's what I was trying to say. You don't need it for
EBGP since it always replaces next-hop with it's neighbor address for all
learned subnets.
Sam
----- Original Message -----
From: "Justin Menga" <Justin.Menga@computerland.co.nz>
To: "'Connary, Julie Ann'" <jconnary@cisco.com>; "Sam Munzani"
<sam@munzani.com>
Cc: <SherefMohamed@cdh.org>; "Lev Terebizh" <lter@rmconsulting.com>;
<ccielab@groupstudy.com>; <nobody@groupstudy.com>
Sent: Friday, December 15, 2000 3:48 AM
Subject: RE: BGP synchronization in IOS 11.3 and 12.0
> Sam - you need next-hop-self in IBGP if your AS doesn't have a route to
the
> standard next hop of an external BGP route.
>
> E.g.
>
> R1
> |
> |
> |
> R2---------R3
>
> Say R2 and R3 are in the same AS 2, and R1 is in AS 1.
>
> R1 will send EBGP routes to R2. These will have a next-hop of R1.
>
> R2 will then send these to R3 as IBGP routes. By default, R2 will NOT
alter
> the next-hop. Thus R3 would receive a route with next-hop of R1.
>
> So, if R3's IGP has a route to the next-hop of R1, that's fine. If it
> doesn't you either:
>
> 1. Add the route to IGP
> 2. Set next-hop-self on R2 for neighbor R3. This way R2 will receive the
> routes from R1, but next-hop will be R2.
>
>
> -----Original Message-----
> From: Connary, Julie Ann [mailto:jconnary@cisco.com]
> Sent: Thursday, December 14, 2000 4:56 AM
> To: Sam Munzani
> Cc: SherefMohamed@cdh.org; Lev Terebizh; ccielab@groupstudy.com;
> nobody@groupstudy.com
> Subject: Re: BGP synchronization in IOS 11.3 and 12.0
>
>
> Sam,
>
>
> Check the BGP documents on CCO for some differences between how the 11.0
> code and 12.0 code
> deal with synchronization. Note the highlighted one in the 12.0 code (#2)
> that talks about
> synchronization and Internal routes. I struggled with this in a similar
lab
> and then found this out.
>
> Julie Ann
>
>
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_c
> /1cprt1/1cbgp.htm
>
> How BGP Selects Paths
>
> A router running Cisco IOS Release 12.0 or later does not select or use an
> IBGP route unless both of the following are true:
>
> the router has a route available to the next-hop router
>
> the router has received synchronization via an IGP (unless IGP
> synchronization has been disabled)
>
> BGP bases its decision process on the attribute values. When faced with
> multiple routes to the same destination, BGP chooses the best route for
> routing traffic
> toward the destination. The following process summarizes how BGP chooses
> the best route.
>
> 1. If the next hop is inaccessible, do not consider it.
>
> This is why it is important to have an IGP route to the next hop.
>
> 2. If the path is internal, synchronization is enabled, and the
route
> is not in the IGP, do not consider the route.
>
> 3. Prefer the path with the largest weight (weight is a Cisco
> proprietary parameter).
>
> 4. If the routes have the same weight, prefer the route with the
> largest local preference.
>
> 5. If the routes have the same local preference, prefer the route
> that was originated by the local router.
>
> For example, a route might be originated by the local router using
> the network bgp command, or through redistribution from an IGP.
>
> 6. If the local preference is the same, or if no route was
originated
> by the local router, prefer the route with the shortest autonomous system
> path.
>
> 7. If the autonomous system path length is the same, prefer the
route
> with the lowest origin code (IGP < EGP < INCOMPLETE).
>
> 8. If the origin codes are the same, prefer the route with the
lowest
> Multi Exit Discriminator (MED) metric attribute.
>
> This comparison is only done if the neighboring autonomous system
> is the same for all routes considered, unless bgp always-compare-med is
> enabled.
>
>
> Note The most recent IETF decision regarding BGP MED assigns a value
> of infinity to the missing MED, making the route lacking the MED variable
> the least
> preferred. The default behavior of BGP routers running Cisco IOS
> software is to treat routes without the MED attribute as having a MED of
0,
> making the
> route lacking the MED variable the most preferred. To configure the
> router to conform to the IETF standard, use the bgp bestpath
> missing-as-worst
> command.
>
>
> 9. Prefer the external (EBGP) path over the internal (IBGP) path.
>
> All confederation paths are considered internal paths.
>
> 10. Prefer the route that can be reached through the closest IGP
> neighbor (the lowest IGP metric).
>
> This means the router will prefer the shortest internal path
within
> the autonomous system to reach the destination (the shortest path to the
> BGP next-hop).
>
> 11. If the following conditions are all true, insert the route for
> this path into the IP routing table:
>
> Both the best route and this route are external.
>
> Both the best route and this route are from the same
neighboring
> autonomous system.
>
> maximum-paths is enabled.
>
>
> Note EBGP load sharing can occur at this point, which means that
> multiple paths can be installed in the forwarding table.
>
>
> 12. If multipath is not enabled, prefer the route with the lowest IP
> address value for the BGP router ID.
>
> The router ID is usually the highest IP address on the router or
> the loopback (virtual) address, but might be implementation-specific.
>
>
>
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios112/112cg_cr/5cb
> ook/5ciprout.htm
>
> How BGP Selects Paths
>
> The BGP process selects a single autonomous system path to use and to pass
> along to other BGP-speaking routers. Cisco's BGP implementation has a
> reasonable
> set of factory defaults that can be overridden by administrative weights.
> The algorithm for path selection is as follows:
>
> If the next hop is inaccessible, do not consider it.
>
> Consider larger BGP administrative weights first.
>
> If the routers have the same weight, consider the route with higher
> local preference.
>
> If the routes have the same local preference, prefer the route that
> the local router originated.
>
> If no route was originated, prefer the shorter autonomous system
path.
>
> If all paths are of the same autonomous system path length, prefer
> the lowest origin code (IGP < EGP < INCOMPLETE).
>
> If origin codes are the same and all the paths are from the same
> autonomous system, prefer the path with the lowest Multi Exit
Discriminator
> (MED) metric. A
> missing metric is treated as zero.
>
> Prefer external paths over internal paths.
>
> If IGP synchronization is disabled and only internal paths remain,
> prefer the path through the closest neighbor.
>
> Prefer the route with the lowest IP address value for the BGP router
> ID.
>
> When a BGP speaker learns two identical EBGP paths for a prefix from a
> neighboring AS, it will pick the path with the least route-id as the best
> path. This best path
> with the least router-id will be installed in the IP routing table. If BGP
> multipath support is enabled, instead of picking one best path, if the
EBGP
> paths are learned
> from the same neighboring AS, multiple paths will be installed in the IP
> routing table.
>
> At 03:31 PM 12/12/2000 -0600, Sam Munzani wrote:
> >There is no limitation on when to use that command. If your IGP doesn't
> know
> >about DMZ network, you have to use this command.
> >
> >
> > >
> > > Why you are using Next-hop-self in IBGP, I think you only need it if
you
> > > have EBGP !
> > >
> > >
> > >
> > >
> > > "Lev
> > > Terebizh" To: "GroupStudy"
> ><ccielab@groupstudy.com>
> > > <lter@rmconsu cc:
> > > lting.com> Subject: BGP
> synchronization
> >in IOS 11.3 and 12.0
> > > Sent by:
> > > nobody@groups
> > > tudy.com
> > >
> > >
> > > 12/08/2000
> > > 04:50 AM
> > > Please
> > > respond to
> > > "Lev
> > > Terebizh"
> > >
> > >
> > >
> > >
> > >
> > >
> > > Hi.
> > > I found some interesting in BGP synchronization.
> > >
> > > ------------r3
> > > |
> > > r1--------r2-----------r4
> > > |
> > > -------------r5
> > >
> > > r1-r2 - ebgp, r2/r3/r4/r5 - ibgp. r2 is route-reflect server.
r3,r4,r5 -
> > > route-reflect clients.
> > > synch is turned on on all routers
> > > r3 - IOS 11.3
> > > r4,r5 - IOS 12.0.
> > > r2 have next hop self to r3-5. r2-5 run ospf. r1-r2 have no routing
> > > protocol.
> > > r1 is source of bgp routes.
> > >
> > > Now question:
> > >
> > > r2 have routes from r1 marked as "*>".
> > > r4,5 have the same routes marked as "*" - it is looking as I had
> >expect...
> > >
> > > BUT on r3 I see that routes as "*>".
> > > And if I change IOS on 11.3 to 12.0 then all router do not show that
bgp
> > > routes marked as "*>"
> > >
> > > What is the issue? Can anybody explain this..?
> > >
> > >
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