BGP problem, BGP's relation with IGPs

From: Mohammed Al-zubi (mohammed@sulafsolutions.com)
Date: Mon Mar 10 2003 - 22:26:00 GMT-3


Hello group,
the issue is related to synchronization, from my understanding (and it
could be wrong) is that if sync is enabled (default) a route would not be
advertised to an EBGP peer unless that route is in the routing table of the
sending router. So here comes the question, I have 2 routers that are not
directly connected that have an IBGP session between them, A and B. B also
has an EBGP session with C. There is a loopback on A with the address
200.200.200.5 /24, and there is a network statement in the BGP process for
that address on A, also, the loopback is part of the OSPF process so router
B sees the route in its routing table as an OSPF route, it also sees the
route in the BGP RIP, but its not synchronized. Why is it not synched if
its in the routing table, I added a static route on router B for
200.200.200.0/24 and pointed it to null zero, and it works, I delete it, the
B router stops advertising that route to its EBGP neighbors and the route
becomes not synchronized. I have attached the different shows I did

   <----IBGP----><-----EBGP---->
A-------X----Y-----B(R2)-------------------C

2#sho ip route 200.200.200.0
Routing entry for 200.200.200.0/24
Known via "ospf 1", distance 110, metric 846, type inter area
Last update from 120.20.234.4 on Serial0/0.234, 00:06:05 ago
Routing Descriptor Blocks:
* 120.20.234.4, from 5.5.5.5, 00:06:05 ago, via Serial0/0.234
Route metric is 846, traffic share count is 1

R2#sho ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
O IA 200.200.200.0/24 [110/846] via 120.20.234.4, 00:06:35, Serial0/0.234
170.70.0.0/24 is subnetted, 10 subnets
D 170.70.13.0 [90/158208] via 120.20.23.7, 00:19:22, FastEthernet0/0
D 170.70.15.0 [90/158208] via 120.20.23.7, 00:19:22, FastEthernet0/0
D 170.70.9.0 [90/158208] via 120.20.23.7, 00:19:22, FastEthernet0/0
D 170.70.11.0 [90/158208] via 120.20.23.7, 00:19:22, FastEthernet0/0
D 170.70.5.0 [90/158208] via 120.20.23.7, 00:19:22, FastEthernet0/0

R2#sho ip bgp
BGP table version is 8, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i200.200.200.0 5.5.5.5 0 100 0 i

R2#sho ip bgp 200.200.200.0
BGP routing table entry for 200.200.200.0/24, version 8
Paths: (1 available, no best path)
Not advertised to any peer
Local
5.5.5.5 (metric 846) from 5.5.5.5 (200.200.200.5)
Origin IGP, metric 0, localpref 100, valid, internal, not synchronized



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