Introduction to BGP

Thanks Rene

Hello Rene,

Will a BGP Neighbor adjacency form if keepalives are set to 0?

Thanks in advance


Yes they will.

BGP neighbor is,  remote AS 100, external link
  BGP version 4, remote router ID
  BGP state = Established, up for 00:00:58
  Last read 00:00:58, last write 00:00:58, hold time is 0, keepalive interval is 0 seconds
  Configured hold time is 0, keepalive interval is 0 seconds
  Minimum holdtime from neighbor is 0 seconds
  Neighbor sessions:
    1 active, is not multisession capable (disabled)

Andrew P - Thanks for the response. So I image from the output with all timers set to 0, additional traffic will be generated between the peers and any instability in the link shared between the peers will cause the adjacency to reset causing unnecessary periods of no usable bgp routes. Paul

you said router decides to use default route to ISP 1 rather than ISP 2, how does it decide ISP 1 over ISP 2?

Hi Rene

can you explain if the customer has two ISP1 and ISP2 connections and the customer has his own /24 and ASN.
how does the customer advertise /24 to ISP1 and ISP2, and choose ISP1 as primary for incoming traffic and ISP2 as

Thank you


The most common way of doing this is by using a route-map that prepends the customer’s own ASN several times to the as-path. You would then use this route-map towards ISP2 (the LESS desirable source of traffic). For example, for ASN 65000 for double pre-pending, the route-map statement to do this would be:
set as-path prepend 65000 65000

Do you have a lesson on BGP MD5 authentication? Is it the same for EIGRP - make a keychain, make a key etc etc?

Hi Jason,

It’s much easier. You only need one command:

R1(config)#router bgp 1
R1(config-router)#neighbor password MYPASS

You can see an example here:

EBGP configuration


19 posts were merged into an existing topic: Introduction to BGP

Great lesson so far its simple and straight forward. I have read further in the book though and I will have questions but first trying to catch up on the web pages to see if that helps push some ideas that I stalled a little bit on. Its more along the lines of using Internal with external and some differences with BGP not seeing networks like the others. I don’t want to ask that question here though as I want to put the question under the right lesson if I still have confusion after reading.

great stuff!

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i use ospf route to advertised loopbacks and afterwards i config BGP but if i give sh ip route means they send packets based on ospf only not BGP why ? AD VALUE of ospf 120,BGP 20 but it prefer only ospf…not BGP … second time i try static and BGP means it prefer only static …not BGP why ?? explain this topology

Hi Shiva,

Do you have internal or external BGP?

- External BGP has an AD of 20
- Internal BGP has an AD of 200

Also, make sure the exact same network is advertised. OSPF will advertise a loopback interface with a /32 subnet mask, no matter what subnet mask you have configured on the loopback. You can change this behavior by changing the OSPF network type:

interface loopback 0
ip ospf network point-to-point

Hey Rene, can you explain why we need a single AS in BGP?

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Hello Pinki

BGP architecture is designed in such a way so that each router participating in BGP is assigned to a single AS. According to Cisco

“BGP prepends the AS number from each BGP network that a route traverses in order to maintain network reachability information and to prevent routing loops.”

There are however a couple of things you should keep in mind as far as this issue is concerned. First of all, BGP allows you to define only a single AS, but you can still peer to multiple ASs. This simply means that you can handle multiple BGP connections (eBGP), but it is just simply that all the information will be kept in the same BGP table.

Secondly, Cisco does support dual AS configurations for BGP but only as a tool for AS migrations. This is not a permanent configuration option but one to facilitate migrations. This is because multiple ASs can easily result in BGP routing loops. More about this can be found in the following Cisco documentation.

I hope this has been helpful!


Hello folks

I’m a little confused about what networks we usually advertise in BGP.
For example when I want to establish a connection between customers trough a service providers using Bgp, the networks that I advertise should be private network address or public IP address?

Hello Rodrigo

As with many issues dealing with networks, the answer is: it depends. There is no hard and fast rule about what addresses you can advertise with BGP. It all depends on what you actually want to achieve. In most cases for enterprise networks, BGP is used to advertise the public IP addresses of the organization to the Internet at large. This way, everyone knows how to reach your internal servers. However, BGP can also be used to advertise private addresses between private networks if your network architecture is such. That all depends on what you mean when you say “I want to establish a connection between customers…” There are various ways to do this, and one that uses BGP substantially is MPLS VPNs. You can find out more about that here.

What you can’t do of course is advertise private addresses to the Internet, as these would be blocked immediately by the first Internet router they reach.

I hope this has been helpful!


Thanks for your answering. Cool to know there are people that can explain things better.

Best regards



I am new to BGP. We have established BGP neighborship with 2 uplink providers advertising different networks and receiving default routes from both using same AS number. We couldn’t establish iBGP as we have a requirement to use both ISP A and ISP B for specific networks to reach to internet.

  • Network A should prefer ISP A for incoming and outgoing
  • Network B should prefer ISP B for incoming and outgoing
  • Network C should prefer ISP B but in case ISP B goes down should prefer ISP A
    Please advise

Hello Sravani

Concerning your requirements, if this is for outgoing traffic, then you don’t have to involve BGP at all. You can achieve this by either configuring your dynamic routing appropriately, or by using a redundant gateway protocol such as HSRP or VRRP, or you can use some combination of both. This of course depends heavily on the infrastructure you have at the edge of your network and how the network itself is designed. If you do have BGP running on the edge of your network, and you want to use it to direct traffic to particular routes, you can use various BGP attributes including Weight, Local Preference and AS Path Length to name a few.

On the network edge, BGP is primarily involved when you want to regulate which ISP will be used for incoming traffic , and it deals with which public prefixes you choose to advertise to which ISP.

Now you can use BGP

I hope this has been helpful!