How to configure EBGP (External BGP)

This topic is to discuss the following lesson:

Hi,

I would like to know how I can be able to see the ebgp prefixes from my OSPF lan segment without using redistribution of BGP to OSPF?

I would send you the configs and topology I have made if you are interested?

Thanks,

Hi Rane ,
I am a big fan of your blog. I find your lessons very helpful . Can you please publish an lesson " how BGP works " Explanation of the process.

Hi Romeo,

There are many things to learn about BGP. If you are completely new to it then you might like some of my blog posts that I wrote a long time ago:


I’ll revise these sometime in the future.

Rene

Hi,

If you need the prefixes that you learned through EBGP on your internal routers then you have two options:

- Redistribute them into your IGP (not a good idea if you have a LOT of prefixes).
- Run IBGP on your internal routers.

You also need to consider if you really need all EBGP prefixes on your internal routers, maybe a default route in OSPF also does the job…

Rene

Hi Rene,
1st of all thank you for all of your precious writings.

I started reading your BGP blog. But im struggling to find an easy way to start with BGP. Because topics are scattered all over the place. Can’t figure out which topics I should read first. Can you please help and show me the correct order that i should follow to become a BGP guru.

Thank you

Hi Mohammad,

I ordered them in the CCIE R&S overview, I’ll probably create one for BGP as well:

Rene

Thank you Rene. Great efforts from you…

Hi Rene,

Nice Explanation.

Can you also describe the operations behind the start-up of a BGP session

Thank you

BR
Taslim

Hi Taslim,

For sure, I’ll add this soon.

Rene

Hi Taslim,

I just added this:

Rene

Thank you Rene. You are a great teacher !!!

Taslim

Hi Rene,
Nice lessons on BGP !!
Three questions.

  1. What is the significance of 32768 in the o/p given below for the local route?

  2. When will metric be non-zero for a given route in bgp?

  3. Is there a way to change the default admin distance of e BGP from 20 to a different value ?
    I believe admin distance is used to compare alternate paths for a given prefix from multiple sources ( directly connected, static, ospf etc…)
    Will I be able to change the order of priority for these sources for a given prefix?

    Emma#show ip bgp
    BGP table version is 3, local router ID is 1.1.1.1
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
    r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

    Network          Next Hop            Metric LocPrf Weight Path
    

    *> 1.1.1.0/24 0.0.0.0 0 32768 i
    *> 2.2.2.0/24 192.168.12.2 0 0 2 i

Hi Ganesh,

The 32768 is the weight, which is the maximum value. Weight is a Cisco proprietary BGP attribute that is on top of the list. The 1.1.1.0/24 prefix is advertised on this router so that’s why you see the maximum weight value and a next hop of 0.0.0.0.

The metric can be changed with a route-map, here’s an example:

The administrative distance can be changed yes, we use the AD when you have multiple sources advertising the exact same prefix:

BGP allows fine control of choosing which prefixes you want to use, take a look at the “BGP Attributes” on this page:

Rene

i know it sounds so weird but when I try to advertise loopback addresses in BGP I was not able to see it the way you do it

Emma(config)#router bgp 1
Emma(config-router)#network 1.1.1.0 mask 255.255.255.0

but when put network 1.1.1.1 mask 255.255.255.255 it works fine .I tried different version of gns3 but it didnt change anything .
did you ever have this problem before Rene ?

Georgi,
Do you have the loopback defined as a /24 or /32?

If you have it defined as a /24, when you issue

#show ip route

do you see an entry for 1.1.1.0/24 in the routing table? BGP will only advertise “network” statement route when there is already an exact match in the main routing table.

Hi rene, it’s always necessary run other IGP routing protocol like OSPF? If I run BGP and bgp IGP -do I need to redistribute routes between them?

Happens that when I configure an eBGP between two iBGPs of more than 2 router each one, I can´t see the routes from one iBGP to the other iBGP… Please help me :slight_smile:

Thank you Rene!!
Regards!
Jimmy
Chile

Jimmy,
Technically, you don’t need an IGP at all. It comes down to what is the best design decision. In most cases, it is much preferred to use an IGP for internal routing as opposed to using iBGP exclusively. iBGP has very slow convergence, and there are a number of rules (gotchas) you should be aware of when using it. Think about it this way: Why do you suppose the Administrative Distance of iBGP is so high (200)? It is Cisco’s way of saying the information that iBGP shares with the network should be less trustworthy than any true IGP.

The specific reason why you can’t see the routes you mentioned could be varied. The two most common ones, however, are:

  1. iBGP neighbors don’t advertise routes learned from an eBGP neighbor to other iBGP neighbors by default. Using a BGP route reflector will fix this.
  2. BGP will not install a route if it doesn’t know about the “next hop” to use that route. The fix for this is usually to have the BGP router on the edge of your network invoke a property called “next-hop-self”

I recommend you read through the following Lessons to get a better understanding:
Troubleshooting BGP Route Advertisements

Internal BGP Explained

BGP Next Hop Self

BGP Route Reflector

Hey Rene,

I was trying to use the loopback address 1.1.1.0/24 on R1 to built IBGP with R2 and EBGP with R3 also. I tried but it is not working. Can we do like this??

Deepak,
Please be more specific than “it is not working.” With the iBGP and EBGP connections, did you use the “neighbor … update-source” option? With the EBGP neighbor, did you use the “neighbor … ebgp multi-hop” or “neighbor … disable-connected-check” command?