BGP Always compare MED

thanks Rene. I just finished this lab. took me a while but I understand a few things as a result. this is my final Spiderman config.

router bgp 100
no synchronization
bgp always-compare-med
bgp log-neighbor-changes
network 192.168.12.0
network 192.168.13.0
network 192.168.14.0
neighbor 192.168.12.2 remote-as 200
neighbor 192.168.13.3 remote-as 300
neighbor 192.168.14.4 remote-as 300
no auto-summary
!

and this is my final Batman config.
router bgp 200
no synchronization
bgp log-neighbor-changes
network 1.2.3.4 mask 255.255.255.255
neighbor 192.168.12.1 remote-as 100
neighbor 192.168.12.1 route-map policy1 out
no auto-summary
!
access-list 1 permit 1.2.3.4 0.0.0.0
!
route-map policy1 permit 10
match ip address 1
set metric 150
!

Rene, are you able to help me explain why it prefers the path through Batman for the 1.2.3.4 network (if "bgp always-compare-med" is not added to Spiderman)?

Still can't figure out.

thanks.

Let me help you a bit...first of all these are the BGP attributes:

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)

By default MED is only compared when you have multiple links to the SAME AS. When you configure the "always compare MED" it'll compare MED even when it's from different AS'es.

Walk through the attribute list and try to see why it prefers one route over another

thanks Rene. I think I got it which one =) cheers. on to the next lab...

Very good Just walk down the BGP attribute list whenever you are in doubt and you'll find the answer. Keep in mind that this list can change...an example of this is when you enable BGP Load-balancing. By default BGP only uses a single path but you can enable load balancing which means the list will change/shift

Rene, would it benefit to set the 'bgp determistic-med' bgp subcommand? Your labs are great.

Very nice, I was stopped at
Network Next Hop Metric LocPrf Weight Path
* 1.2.3.4/32 192.168.13.3 50 0 300 ?
* 192.168.14.4 50 0 300 ?
*> 192.168.12.2 150 0 200 ?

WOW, better med and no best path :O
These kind of think you won't think it happen until you face them

Sorry I cannot edit my comment so I asked to delete it, but I think, lower med is better . In my lab is going through Superman
Superman - 50
Batman - 150
Ironman - 50

After that I used "bgp always-compare-med" under router bgp 100 and nothing changed.

And I guess that this "Configure router Spiderman so traffic towards 1.2.3.4 is sent towards AS 300, the only attribute you are allowed to change is MED." is done with an incoming route map that enforces to change the MED received from Superman to be lower than 50 and all is done.

Very interesting lab.

First let me explain the difference between BGP deterministic-med and BGP always-compare med.

BGP deterministic-med will make sure that MED variable will be compared when a route is advertised by different peers in the same AS.

BGP always-compare will make sure that you compare MED for peers in different AS'es.

If you want to influence the MED the best thing to do is create a route-map and match/set the prefix

Glad you guys like the labs!

I did it like that, with set metric in the route-map (metric = med)

I wouldn't have got the #bgp always-compare-med without the hint, in fact it did switch to AS300 when I reset Batman, but not Spiderman, I suppose in 30 mins time that would of changed.

Nice route-map practice.

glad you liked it!

Thanks, I remember glancing over the bgp always-compare-med and bgp deterministic-med in my studies. This lab definitely clears it up for me.