From: Ralph Sherry (yodajedi6678@yahoo.com)
Date: Thu May 26 2005 - 00:19:11 GMT-3
I have done some more reading on this tonight and I think I am correct with my thinking. Correct me if I am wrong though. I have constructed the following model. Unfortuantly I don't have enough routers in my lab to test out my theory. My thinking is that in order to go from R1 to R13 the chosen path would be R2, R5, R7, R9, R11. From R13 to R1 It would be R12, R10, R8, R4 and R3. While this makes sense to me logically I would like somebody to confirm my thinking. If all the links were the same type what would be the best way to play with the metrics to fix this asymetric routing problem?
R1
/ \
R2 R3
| |
| R4
Area 2 | |
---------- R5 R6
| |
Area 0 R7--------R8
| |
----------- R9 R10
Area 1 | |
R11 R12
\ /
R13
Ralph Sherry <yodajedi6678@yahoo.com> wrote:
Thanks simon for the BGP explanation. I think you lay that out pretty well. Does OSPF operate in the same way? If there are 2 ABR in a Area does the router send it to the ABR that is closeset to it or does it look at the overall topology and send it to the ABR that makes the over all path through out the OSPF cloud the shortest?
SIMON HART wrote:
Ralph,
BGP and OSPF operate differently. However BGP will be closely related to the underlying routing protocol.
BGP is used to advertise reachability outside of your AS. IGP is used to advertise reachability within your AS.
Let's say for example you are peering with an AS, that is advertising reachability to 10.10.10.1
The eBGP speaker that is peering with the remote AS will install this route into it's BGP table as a reachable network with a next hop address of the remote peer. This route will then be installed into the BGP speakers IP route table as a reachable route. If you have iBGP speakers in your network then the route will be advertised to the iBGP peers with a next hop address of the eBGP speaker (either the local eBGP speaker or the remote eBGP speaker depending on things such as next-hop-self).
Now in order for traffic to determine the correct route to 10.10.10.1, it will first look at the IP route table. This will have an entry for this prefix and a next hop of the eBGP speaker. The eBGP speakers reachability (its ip address) must be present in the ip route table (in fact, if it is not then the BGP route does not get installed.)
A recursive lookup will now be conducted on the IP route table to determine how to reach the eBGP peer. The eBGP peers address would normally be advertised internally via a dynamic routing protocol (OSPF etc), or could be a static route.
From this information the traffic now knows that in order to get to 10.10.10.1 it needs to go through the eBGP speaker (thanks to the internal BGP advertisement), and it also knows how to get to the eBGP courtesy of the IGP.
So in answer to your question, the IGP routing protocol will still determine the quickest route out of the AS. BGP will determine the most appropiate AS to send the traffic too.
HTH
Simon
Ralph Sherry wrote:
I think i have confused myself on the the route decisions of BGP and OSPF. With BGP the router finds the quickest route out of the current AS into the next AS path. The current AS doesn't care if the way it enters the next AS is the best path as long as it is the quickest path out of its current AS.
My questions is if OSPF operates in the same way when dealing with stubs, NSSA, and different areas. Right now I am thinking it does but I may have all my routing protocols confused.
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