Forum Replies

1. Hi Daniel,

   I might write a short tutorial on it with an example but it is a simple one. A BGP router will prefer routes that it installed into BGP itself over a route that another router installed in BGP.

   Rene

2. http://i66.tinypic.com/33w3x29.jpg

   http://i66.tinypic.com/33w3x29.jpg

   Hello Rene,

   This is the setup I am using to configure BGP multihoming.

   AS 100 has two exit points to reach R5 which is hosting a subnet: 8.8.8.8

   My Question is what difference does it make if I advertise directly connected subnets in BGP.

   For example:

   R1(config-router)#network 192.168.1.0 mask 255.255.255.252

   R2(config-router)#network 192.168.2.0 mask 255.255.255.252

   One thing which I have noticed is if I do not advertise 192.168.1.0 subnet on R1, R2 did not know about it at all so advertisin

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3. Thanks a lot for the explanation Rene.

   I would like you to clarify one more thing.

   For the LAN segment: 192.168.100.0 : R1 is the active router to reach 8.8.8.8 and R2 is the standby as the priority for R1 is 150.

   I am not using interface for tracking as i want the failover to happen on the basis of reachability of 8.8.8.8.

   I have set a tracking object with IP SLA1 for type: echo to ping 8.8.8.8 and mapped it with Standby 1 on R1 to decrement priority by 90 if the ping fails.

   To verify this, I did a “Shut” on the interface of R1 connecting to R3, what happened

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4. Hello Chris

   BGP will choose the router ID based first on if the router-id command is present. It is not in this case. Secondly, it will choose the highest IPv4 IP address of a loopback interface. You are correct, that both R1 and R3 have the same loopback address (seen in the configs), and thus would be advertising the same router-id. So the next step would be to determine the lowest neighbour IP address, which is indeed 192.168.12.1. So you are correct, it does prefer 192.168.12.1 but not because of the router ID but because of the lowest neighbour IP add

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5. Hello Aung

   AS Path Prepending is a feature that is shared only between ASs. It cannot be propogated within an AS using iBGP. There is no notion of AS paths within iBGP so it is information that is not relevant. Secondly, you can apply prepending inbound. What that will do is cause any shared routes from the particular neighbor to increase the number of prepended AS numbers. This is somewhat dangerous unless you are controlling both ASs involved in this route exchange and you can control what is being advertised to you to which you are prepending.

   I hope this has been helpful!

   Laz