From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Fri Feb 25 2005 - 14:13:25 GMT-3
Tim,
Configure a route-map inbound from the neighbor that says "set
ip next-hop ....". The reason this is happening is because both of
these routers are on the same broadcast segment. For example support
that R1, R2, and R3 are all on the Ethernet segment 10.0.0.0/8. R2
learns a network from R1 and advertises on to R3. When R3 sends traffic
to R2, R2 will just send an ICMP redirect to tell R3 to send it directly
to R1. BGP avoids this by not modifying the next-hop value when the
originator is on the same segment as the device you're advertising it
to. In other words R2 will advertise the prefix learned from R1 to R3
with a next-hop value of R1.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> ccie2be
> Sent: Thursday, February 24, 2005 11:43 AM
> To: 'Group Study'
> Subject: BGP in IE lab 20
>
> Hi guys,
>
>
>
> I've come across something I've never seen before in a bgp table.
>
>
>
> Here's the config:
>
>
>
> router bgp 300
>
> no synchronization
>
> bgp router-id 150.1.3.3
>
> bgp log-neighbor-changes
>
> neighbor 142.1.34.4 remote-as 300
>
> neighbor 204.12.1.6 remote-as 100
>
> neighbor 204.12.1.254 remote-as 54
>
> neighbor 204.12.1.254 local-as 100
>
> neighbor 204.12.1.254 route-map FROM-BB3 in
>
>
>
>
>
> route-map FROM-BB3 permit 10
>
> set as-path prepend 54 54 54
>
>
>
>
>
> Here's the topology:
>
>
>
> R3 <-----> R4 (AS 300)
>
> | /
>
> |----------------| /
>
> | | /
>
> BB3 R 6 <---------> BB1 (AS 54)
>
> ^ ^
>
> AS54 AS100
>
>
>
>
>
> r3(config-route-map)#do 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
>
> * 28.119.16.0/24 204.12.1.254 0 0
> 54 54 54 100 54 i
>
> * i 142.1.46.6 0 100
> 0
> 100 54 i
>
> *> 204.12.1.254
> 0
> 100 54 i
>
>
>
>
>
> * 28.119.17.0/24 204.12.1.254 0 0
> 54
> 54 54 100 54 i
>
> * i 142.1.46.6 0 100
> 0
> 100 54 i
>
> *> 204.12.1.254
> 0
> 100 54 i
>
>
>
>
>
> * 112.0.0.0 204.12.1.254
0
> 54 54 54 100 54 50 60 i
>
> * i 142.1.46.6 0 100
> 0
> 100 54 50 60 i
>
> *> 204.12.1.254
> 0
> 100 54 50 60 i
>
>
>
>
>
> * 113.0.0.0 204.12.1.254
0
> 54 54 54 100 54 50 60 i
>
> * i 142.1.46.6 0 100
> 0
> 100 54 50 60 i
>
> *> 204.12.1.254
> 0
> 100 54 50 60 i
>
>
>
> (Sorry if the formatting doesn't come out properly.)
>
>
>
> Notice that each of the routes has 2 paths which both show
204.12.1.254 as
> the next hop.
>
> This is misleading since the 2nd 204.12.1.254 entry is really from
> 204.12.1.6
>
>
>
> Is there any way I can configure R3 such that the prefixes learned
from
> 204.12.1.6
>
> show the next-hop as 204.12.1.6 instead of automatically changing it
to
> 204.12.1.254?
>
>
>
> I know that I can add on R6 the next-hop-self command pointing to R3
to
> change this but can the same effect
>
> be achieved by ONLY changing the config on R3?
>
>
>
>
>
> My objective is to have the BGP process on R3 and R4 prefer the path
via
> 142.1.46.6 by using prepend.
>
>
>
> TIA, Tim
>
>
This archive was generated by hypermail 2.1.4 : Thu Mar 03 2005 - 08:51:25 GMT-3