From: OhioHondo (ohiohondo@columbus.rr.com)
Date: Fri Mar 07 2003 - 19:07:47 GMT-3
Hi Everyone
I thought I understood this BUT...
In the scenario below how can Local_Preference be configured to have an
affect if BGP Synchronization is applied?
I have an AS with 3 fully meshed routers (A, B and C) running iBGP. All 3
routers receive a prefix for 75.0.0.0/8 from external AS's. On router A I
have set the Local_Preference for this route to 400. The default of 100 was
accepted for routers B and C. In other words, I want router A to be the
preferred egress point from my AS to 75.0.0.0/8.
When I look at router B or router C's BGP table there is an entry from each
of the routers for the 75.0.0.0/8 network. The EBGP learned route is the *<
(best) route not the route with the highest Local_Preference which would be
A. (The Local_Preference for A does show as 400 while the others are 100.)
In the IP routing table of router B, the EBGP learned route for 75.0.0.0/8
is there with its' admin distance of 20. In the BGP table the 75.0.0.0/8
routes learned from routers A and C show they are unsynchronized. This must
be because there is no IGP route for 75.0.0.0/8 in router B's IP routing
table.
So in this environment, my setting of Local_Preference has no effect on the
selection of the egress point for the 75.0.0.0/8 network. Is this working
properly? How do I configure A, B and C so router A is the preferred egress
point without using synchronization? (Note: If synchronization is applied
the Best Route is chosen by the higest Local Preference.) Some output from
router B is given below.
(3640-1_R1 is router B)
3640-1_R1#sho ip bgp 75.0.0.0
BGP routing table entry for 75.0.0.0/8, version 8
Paths: (3 available, best #2, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
139.7.67.254 139.7.161.1
301, (aggregated by 301 198.18.18.1)
139.7.14.1 (metric 3333) from 139.7.67.254 (3.0.0.0)
Origin IGP, localpref 400, valid, internal, not synchronized,
atomic-aggr
201, (aggregated by 201 198.18.18.1)
170.1.1.1 from 170.1.1.1 (198.18.18.1)
Origin IGP, localpref 100, valid, external, atomic-aggregate, best
101 201, (aggregated by 201 198.18.18.1)
201.1.1.1 (metric 74) from 139.7.161.1 (6.0.0.0)
Origin IGP, localpref 100, valid, internal, not synchronized,
atomic-aggr
3640-1_R1#sho ip route | include B
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
B 69.0.0.0/8 [20/0] via 170.1.1.1, 00:00:07
B 139.7.0.0/16 [200/0] via 0.0.0.0, 00:00:07, Null0
B 21.0.0.0/8 [20/0] via 170.1.1.1, 00:00:07
B 126.0.0.0/8 [20/0] via 170.1.1.1, 00:00:07
B 95.0.0.0/8 [20/0] via 170.1.1.1, 00:00:07
B 75.0.0.0/8 [20/0] via 170.1.1.1, 00:00:07 --- > the route
3640-1_R1#sho ip route 75.0.0.0
Routing entry for 75.0.0.0/8
Known via "bgp 501", distance 20, metric 0
Tag 201, type external
Redistributing via ospf 100
Advertised by ospf 100 metric 11111 route-map RedisBGP
Last update from 170.1.1.1 00:07:13 ago
Routing Descriptor Blocks:
* 170.1.1.1, from 170.1.1.1, 00:07:13 ago
Route metric is 0, traffic share count is 1
AS Hops 1
with synchronization applied to router B (3640-1_R1), Loc_Pref of 400 is
chosen.
3640-1_R1#sho ip bgp 75.0.0.0
BGP routing table entry for 75.0.0.0/8, version 8
Paths: (3 available, best #3, table Default-IP-Routing-Table)
Flag: 0x208
Not advertised to any peer
201, (aggregated by 201 198.18.18.1)
170.1.1.1 from 170.1.1.1 (198.18.18.1)
Origin IGP, localpref 100, valid, external, atomic-aggregate
101 201, (aggregated by 201 198.18.18.1)
201.1.1.1 (metric 74) from 139.7.161.1 (6.0.0.0)
Origin IGP, localpref 100, valid, internal, atomic-aggregate
301, (aggregated by 301 198.18.18.1)
139.7.14.1 (metric 3333) from 139.7.67.254 (3.0.0.0)
Origin IGP, localpref 400, valid, internal, atomic-aggregate, best
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