Hi,
Thanks for the lab. Even after i changed the router-id at Sydeny, the route for 1.1.1.0 is still preferred through Newdelhi. Could anyone please tell me the reason please.
BGP Attribute Router ID
Scenario:
You are one of the trainees at a fairly small ISP. Your boss is on vacation and all colleagues are out of the office troubleshooting problems at customers. You receive a phonecall of one of the network engineers in NewDelhi who complains you are sending all traffic meant for Paris through his network and he wants it solved in the next 10 minutes. You google a bit for BGP attributes but it isn't making sense much to you at this moment...you do read something about Router ID so maybe that'll be helpful to you...ignorance is bliss right? Let's go!
Goal:
- All IP addresses have been preconfigured for you as specified in the topology picture.
- Configure EBGP between all ASes.
- Advertise the loopback0 interface on router Barcelona in BGP.
- Router Paris should have two paths to reach network 1.1.1.0 /24.
- Ensure router Paris sends traffic to router Sydney to reach network 1.1.1.0 /24. You are only allowed to make changes on router Sydney.
IOS:
c3640-jk9o3s-mz.124-16.bin
Topology:
Video Solution:
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Comments (4)
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ReneMolenaar 2011-11-03 23:03:00Hi Marshall,
Do you see both paths in the routing table of router Paris? Did you change anything else that could influence the BGP routing decision?
W Weight (Highest)
L Local_Pref (Highest)
O Originate (local originate)
AS As_Path (shortest)
O Origin Code (IGP < EGP < Incomplete)
M MED (lowest)
P Paths (External Paths preferred Over Internal)
R Router ID (lowest)Also keep in mind that if you change the router ID...do a BGP neighbor reset just to make sure.
Next week I'm back to recording videos, I'll do this one as well!
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ReneMolenaar 2012-01-02 21:57:31
Hi Everyone,
There's something else you need to know if you do this lab or watch my video.
Between the "Paths" and "Router ID" attribute there is something else that will determine which route will be used:
1. Prefer the path with the minimum IGP metric to reach the next hop IP address.
2. Prefer the prefix with the longest uptime, this is done to prevent route flapping.So in order to have the Router ID as the tie-breaker both prefixes have to be equally long in the BGP table. Otherwise it will prefer one over the other.
Thanks to Diesel for this tip!
Rene
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Bougrine 2012-03-06 00:21:43The router-ID attribute is working "properly" only in combination with the command:
"bgp bestpath compare-routerid" under BGP config.
If you shutdown the BGP between Paris-Sydney and enable it again, the Paris router will not converge to Sydney path even it has a lower router-ID.
But if you add the command above it will converge always to the lower router-ID.