From: Bill Lijewski (bill@eccie.com)
Date: Thu Apr 01 2004 - 18:44:57 GMT-3
Synchronization only comes into play for iBGP routes. With
synchronization on the router will check its IGP table for all iBGP
routes it learns to make sure that it has a matching entry before it
will ever put the route into its BGP table as a best route.
Synchronization does not check for a match in the IGP table for eBGP
routes. If no match is found in the IGP table for the iBGP routes they
will not be marked as best, and the routes will not be passed to any
other BGP neighbors.
There is one extra step that BGP goes through for iBGP routes when
synchronization is on and OSPF is the underlying IGP. BGP will check
for a matching route in the IGP table, if one is found it will check to
see which IGP protocol is running. If the router is running OSPF as its
IGP, BGP will check and make sure that the origin of the routes for both
OSPF and BGP are the same. In other words BGP will make sure that the
OSPF and BGP router IDs of the source of the route match before it will
put it into the BGP table.
You can check these Router ID origins with the following two commands:
For the OSPF route: show ip route x.x.x.x
For an example we can see what the origin of the 210.10.10.0/24 network:
Routing entry for 210.10.10.0/24
Known via "ospf 1", distance 110, metric 1, type extern 2, forward
metric 10
Last update from 172.168.1.2 on Ethernet0, 1d03h ago
Routing Descriptor Blocks:
* 172.168.1.2, from 5.5.5.5, 1d03h ago, via Ethernet0
Route metric is 1, traffic share count is 1
The second line up from the bottom is the important one. It shows 'from
5.5.5.5', 5.5.5.5 is the OSPF origin ID for this route.
For the BGP route: show ip bgp x.x.x.x
Again we can take a look at the origin of the 210.10.10.0/24 network:
BGP routing table entry for 210.10.10.0/24, version 2
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
192.168.2.2
Local
172.168.1.2 from 5.5.5.5
Origin IGP, metric 0, localpref 100, valid, internal, synchronized,
best
Again the second line up from the bottom shows 'from 5.5.5.5', 5.5.5.5
is the BGP origin ID for this route.
In this case both the BGP and the OSPF origin Router ID's match so this
route could be placed into the BGP table as a best route.
- Bill Lijewski
CCIE#8642
Network Learning Inc
5 Day R&S CCIE Bootcamp Instructor
http://www.ccbootcamp.com
bill@eccie.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
gladston@br.ibm.com
Sent: Thursday, April 01, 2004 1:25 PM
To: ccielab@groupstudy.com
Subject: BGP and OSPF RID [bcc][faked-from][bayes]
Importance: Low
I remember to see a problem related to OSPF and BGP RID, redistribution
and Route Refletor.
Any help on where it is on the DOCCD or cisco.com?
This archive was generated by hypermail 2.1.4 : Mon May 03 2004 - 19:48:41 GMT-3