From: Connary, Julie Ann (jconnary@xxxxxxxxx)
Date: Wed Dec 13 2000 - 12:55:51 GMT-3
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/1c
prt1/1cbgp.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/5cbook
/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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