From: Shine Joseph (shinepjoseph@iprimus.com.au)
Date: Thu Mar 13 2008 - 08:54:08 ARST
Emil,
See below working configuration as per the requirement you have provided.
I have labbed up as follows.
The loopback prefixes are advertised with a route-map to convert them into
incomplete origin (?). On R1 a new route-map is created to change the origin
codes to IGP just for R1 and R3 loopbacks.
R1
router bgp 123
no synchronization
bgp router-id 150.1.1.1
bgp log-neighbor-changes
network 172.16.1.1 mask 255.255.255.255 route-map ORIGIN_R1
neighbor 172.16.13.3 remote-as 123
neighbor 172.16.14.4 remote-as 456
neighbor 172.16.14.4 route-map ORIGIN out
neighbor 172.16.123.2 remote-as 123
no auto-summary
ip prefix-list NET_3 seq 5 permit 172.16.3.3/32
ip prefix-list NET_3 seq 10 permit 172.16.1.1/32
route-map ORIGIN_R1 permit 10
set origin incomplete
route-map ORIGIN_R4 permit 10
match ip address prefix-list NET_3
set origin incomplete
R2
router bgp 123
no synchronization
bgp router-id 150.1.2.2
bgp log-neighbor-changes
network 172.16.2.2 mask 255.255.255.255
neighbor 172.16.23.3 remote-as 123
neighbor 172.16.23.3 next-hop-self
neighbor 172.16.24.4 remote-as 456
neighbor 172.16.123.1 remote-as 123
no auto-summary
R3
router bgp 123
no synchronization
bgp router-id 150.1.3.3
bgp log-neighbor-changes
network 172.16.3.3 mask 255.255.255.255 route-map ORIGIN
neighbor 172.16.13.1 remote-as 123
neighbor 172.16.23.2 remote-as 123
no auto-summary
route-map ORIGIN permit 10
set origin incomplete
R3
sh ip bgp
BGP table version is 14, local router ID is 150.1.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
r>i172.16.1.1/32 172.16.13.1 0 100 0 ?
r>i172.16.2.2/32 172.16.23.2 0 100 0 i
*> 172.16.3.3/32 0.0.0.0
R4
sh run | s bgp
router bgp 456
no synchronization
bgp router-id 150.1.4.4
bgp log-neighbor-changes
neighbor 172.16.14.1 remote-as 123
neighbor 172.16.24.2 remote-as 123
no auto-summary
R4
sh ip bgp
BGP table version is 10, local router ID is 150.1.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 172.16.1.1/32 172.16.14.1 0 0 123 i
* 172.16.24.2 0 123 ?
*> 172.16.2.2/32 172.16.24.2 0 0 123 i
*> 172.16.3.3/32 172.16.14.1 0 123 i
* 172.16.24.2 0 123 ?
R4(config-router)#do trace 172.16.3.3
Type escape sequence to abort.
Tracing the route to 172.16.3.3
1 172.16.14.1 24 msec 24 msec 24 msec
2 172.16.13.3 52 msec * 48 msec
R4(config-router)#
R4(config-router)#do trace 172.16.1.1
Type escape sequence to abort.
Tracing the route to 172.16.1.1
1 172.16.14.1 28 msec * 24 msec
R4(config-router)#
R4(config-router)#
R4(config-router)#do trace 172.16.2.2
Type escape sequence to abort.
Tracing the route to 172.16.2.2
1 172.16.24.2 28 msec * 24 msec
HTH,
Shine
_____
From: YourPal [mailto:dearprudence28@gmail.com]
Sent: Thursday, 13 March 2008 9:36 PM
To: Shine Joseph
Cc: Cisco certification
Subject: Re: Question on Influencing BGP Inbound Path Selection
Hi All,
Those 3 prefixes are loopback addresses of the respective router. They are
not advertised in IGP.
An earlier task asks to advertise those loopbacks into BGP with no
restriction. Then comes this task that restricts configuration on R1 only.
Is it possible at all? I still think that we need to configure something on
R2 (whether setting MED, community, origin code, etc) in order to satisfy
this task, if that's allowed at all.
Thank you.
BR,
Emil
On 3/13/08, Shine Joseph <shinepjoseph@iprimus.com.au> wrote:
Emil,
The fact that origin code is ahead of MED in best path calculation, you can
change the origin code to accomplish this task.
If there is no restriction in how you advertise the networks in AS123, I can
think of advertising the network with origin code of igp to R4 from R1. If
you are restricted with no-redistribution, then apply a route-map in the
network statement to change the origin code to incomplete.
I have labbed up your scenario and it works.
HTH,
Shine
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
YourPal
Sent: Thursday, 13 March 2008 5:19 PM
To: Cisco certification
Subject: Question on Influencing BGP Inbound Path Selection
Hi Group,
I came across the following BGP scenario:
R1, R2, and R3 are in AS 123. They are fully-meshed. R4 is in AS 456. R4
peers with R1 and R2.
R1 advertises 172.16.1.1/32 into BGP.
R2 advertises 172.16.2.2/32 into BGP.
R3 advertises 172.16.3.3/32 into BGP.
Configure R1 so that R4 prefers R1 to reach 172.16.1.1/32 and 172.16.3.3/32.
Route filtering and AS path manipulation are prohibited.
The way I see it, I'm left with the option of configuring MED. However since
the task restricts the config to be done on only R1, AFAIK there's no way to
influence MED because both R1 and R2 will advertise the prefixes to R4 with
MED=0 (unless I'm allowed to configure a higher MED value on R2).
Can't think of a solution. I may be missing something. Appreciate any help.
Thank you.
BR,
Emil
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