Sorry to say but you are incorrect. Florian is correct if local preference
is 100 for one route that should be the best path.
How the Best Path Algorithm Works
BGP assigns the first valid path as the current best path. BGP then compares
the best path with the next path in the list, until BGP reaches the end of
the list of valid paths. This list provides the rules that are used to
determine the best path:
1.
Prefer the path with the highest WEIGHT.
*Note: * WEIGHT<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#weight>is
a Cisco-specific parameter. It is local to the router on which it is
configured.
2.
Prefer the path with the highest
LOCAL_PREF<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#localpref>
.
*Note: *A path without LOCAL_PREF is considered to have had the value set
with the *bgp default local-preference* command, or to have a value of
100 by default.
3.
Prefer the path that was locally originated via a *network* or *aggregate
* BGP subcommand or through redistribution from an IGP.
Local paths that are sourced by the *network*
<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#networkcommand>or
*redistribute* commands are preferred over local aggregates that are
sourced by the *aggregate-address*
<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094826.shtml>command.
4.
Prefer the path with the shortest AS_PATH.
*Note: *Be aware of these items:
-
This step is skipped if you have configured the *bgp bestpath as-path
ignore*
<http://www.cisco.com/en/US/docs/ios/12_3t/ip_route/command/reference/ip2_b1gt.html#wp1072077>command.
-
An AS_SET counts as 1, no matter how many ASs are in the set.
-
The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the
AS_PATH length.
5.
Prefer the path with the lowest origin type.
*Note: *IGP is lower than Exterior Gateway Protocol (EGP), and EGP is
lower than INCOMPLETE.
6.
Prefer the path with the lowest multi-exit discriminator
(MED)<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094934.shtml#med>
.
*Note: *Be aware of these items:
-
This comparison only occurs if the first (the neighboring) AS is the
same in the two paths. Any confederation sub-ASs are ignored.
In other words, MEDs are compared only if the first AS in the
AS_SEQUENCE is the same for multiple paths. Any preceding
AS_CONFED_SEQUENCE
is ignored.
-
If *bgp always-compare-med*
<http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1039063>is
enabled, MEDs are compared for all paths.
You must disable this option over the entire AS. Otherwise, routing
loops can occur.
-
If *bgp bestpath med-confed*
<http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1039275>is
enabled, MEDs are compared for all paths that consist only of
AS_CONFED_SEQUENCE.
These paths originated within the local confederation.
-
THE MED of paths that are received from a neighbor with a MED of
4,294,967,295 is changed before insertion into the BGP table. The MED
changes to to 4,294,967,294.
-
Paths received with no MED are assigned a MED of 0, unless you have
enabled *bgp bestpath med missing-as-worst*
<http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1039367>
.
If you have enabled *bgp bestpath med missing-as-worst*, the paths are
assigned a MED of 4,294,967,294.
-
The *bgp deterministic med* command can also influence this step.
Refer to How BGP Routers Use the Multi-Exit Discriminator for Best
Path Selection<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094934.shtml>for
a demonstration.
7.
Prefer eBGP over iBGP paths.
If bestpath is selected, go to Step 9 (multipath).
*Note: *Paths that contain AS_CONFED_SEQUENCE and AS_CONFED_SET are local
to the confederation. Therefore, these paths are treated as internal paths.
There is no distinction between Confederation External and Confederation
Internal.
8.
Prefer the path with the lowest IGP metric to the BGP next hop.
Continue, even if bestpath is already selected.
9.
Determine if multiple paths require installation in the routing table for
BGP Multipath<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml#bgpmpath>
.
Continue, if bestpath is not yet selected.
10.
When both paths are external, prefer the path that was received first
(the oldest one).
This step minimizes route-flap because a newer path does not displace an
older one, even if the newer path would be the preferred route based on the
next decision criteria (Steps 11, 12, and 13).
Skip this step if any of these items is true:
-
You have enabled the *bgp best path compare-routerid* command.
*Note: *Cisco IOS Software Releases 12.0.11S, 12.0.11SC, 12.0.11S3,
12.1.3, 12.1.3AA, 12.1.3.T, and 12.1.3.E introduced this command.
-
The router ID is the same for multiple paths because the routes were
received from the same router.
-
There is no current best path.
The current best path can be lost when, for example, the neighbor that
offers the path goes down.
11.
Prefer the route that comes from the BGP router with the lowest router
ID.
The router ID is the highest IP address on the router, with preference
given to loopback addresses. Also, you can use the *bgp
router-id*command to manually set the router ID.
*Note: *If a path contains route reflector (RR) attributes, the
originator ID is substituted for the router ID in the path selection
process.
12.
If the originator or router ID is the same for multiple paths, prefer the
path with the minimum cluster list length.
This is only present in BGP RR environments. It allows clients to peer
with RRs or clients in other clusters. In this scenario, the client must be
aware of the RR-specific BGP attribute.
13.
Prefer the path that comes from the lowest neighbor address.
This address is the IP address that is used in the BGP
*neighbor*configuration. The address corresponds to the remote peer
that is used in
the TCP connection with the local router.
refer this ::
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml
HTH
CCIEMYLOVE
-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Florian Frotzler
Sent: Tuesday, February 16, 2010 2:04 PM
To: Cisco certification
Subject: Re: BGP Path Selection - eBGP vs iBGP
I don't know what you hacked with 28.8k, but certainly not IP ;-) eBGP
and iBGP are considered different protocols and are therefore selected
by administrative distance.
Cheers,
Florian
2010/2/16 Garth Bryden <hacked.the.planet.on.28.8k.dialup_at_gmail.com>:
> Hello Group!
>
> I have what is probably a fairly simple question, but I am slightly
confused
> with my BGP Labs.
>
> I have a simple topology.
>
> R1 and R3 are in AS 1 and iBGP Peers
>
> R4 and R5 are in AS 2 and iBGP Peers
>
> R1 and R4 are eBGP Peers
>
> R3 and R5 are eBGP Peers
>
> I have the network 10.0.0.0/16 being advertised by R3
>
> On R5 I recieve the NLRI with two paths. One is eBGP via R3 the other iBGP
> via R4... R4 sends the iBGP prefix with a Local Preference of 100 and of
> course since it's external R3 doesn't send a Local Preference attribute...
> Now since the second method of best path selection is "Highest Local
> Preference" why do my routers choose eBGP over iBGP?
>
> Here is the BGP RIB.....
>
> Network Next Hop Metric LocPrf Weight Path
> * i10.0.0.0/16 155.1.45.4 0 100 0 2 i
> *> 155.1.0.3 0 2 i
>
>
> Blogs and organic groups at http://www.ccie.net
>
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Received on Tue Feb 16 2010 - 15:06:21 ART
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