From: Ronnie Royston (RonnieR@xxxxxxxxxxxxxxxxx)
Date: Sat Dec 02 2000 - 15:19:16 GMT-3
Hi Robert. I believe that the synchronization feature of BGP is meant to
keep multihomed ASs from advertising routes that it cannot actually route.
An IBGP router will share EBGP destinations with his internal peer with his
EBGP neighbors ID as the next hop. However IGP routers internal to the IBGP
AS (not running BGP) will not get the information that the destination
network (in your case 206.11.20.16) is via the RID (in your case
192.168.4.1) of the EBGP router. So, even though the IGP routers may have
the route to the EBGP address who really does know how to route to the final
destination, they do not know that the final destination is via the RID of
the EBGP router. Therefore, the IGP router would drop the traffic, that's
why BGP will not advertise IGBP learned routes to an EBGP peer unless the
BGP and IGP are in sync (the IGP has the route). Basically, Halabi suggests
to disable synchronization when (1) all IBGP routers are fully meshed (or
route reflectors?) or when (2) the AS is not a transit AS. If I were you,
if these two conditions are met, turn off synch and get it working and move
on to somehting else, ...there's alot to cover!
-----Original Message-----
From: Robert DeVito [mailto:robertdevito@hotmail.com]
Sent: Saturday, December 02, 2000 8:44 AM
To: ccielab@groupstudy.com
Subject: BGP Sync Question
I have a question about BGP Sync...
If I do a "show ip bgp" I see the following:
Network Next Hop Metric LocPrf Weight Path
*>i206.11.20.16/29 192.168.4.1 0 100 0 300 i
*>i206.11.20.64/29 192.168.4.1 0 100 0 300 i
Now if I do a "show ip route" I see the following:
Gateway of last resort is not set
10.0.0.0/16 is subnetted, 4 subnets
R 10.2.0.0 [120/2] via 192.168.1.2, 00:00:19, Ethernet0
R 10.3.0.0 [120/1] via 192.168.1.2, 00:00:19, Ethernet0
C 10.4.0.0 is directly connected, Loopback0
R 10.5.0.0 [120/2] via 192.168.1.2, 00:00:19, Ethernet0
C 192.168.1.0/24 is directly connected, Ethernet0
R 192.168.2.0/24 [120/1] via 192.168.1.2, 00:00:19, Ethernet0
R 192.168.3.0/24 [120/1] via 192.168.1.2, 00:00:19, Ethernet0
R 192.168.4.0/24 [120/2] via 192.168.1.2, 00:00:19, Ethernet0
C 192.19.15.0/24 is directly connected, Serial1
As you can see I know how to get to the next hop of the two BGP learned
route, if I do not turn sync off I do not see the routes in my routing
table, why? I thought if you have a "*>" next to your route, I should see it
in my routing table and it complies with the rules of BGP sync?
Thank you!
Robert DeVito
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