Cisco vs Juniper: different eBGP behavior

Last week I had to troubleshoot a problem about eBGP peering with an external provider and I think my findings will be interesting for some of you out there.

Let me start with some background information. I have two locations, same ASN, both connected to the same provider network using eBGP as routing protocol. Due to the looping prevention mechanism, default behavior in eBGP peering between Cisco equipment, is not to accept in BGP table prefixes that have in the AS-Path the same ASN as the local BGP router. You can still accept these prefixes, if you use the “allowas-in” trick on Cisco routers:

The above command will tell BGP protocol on Cisco routers to ignore the presence of its own ASN in the AS-Path and to accept the prefixes. If all other attributes are in good standing, the routes to those prefixes will be installed in the routing table.

As a short example, take the following tolopogy:

I have a typical eBGP peering configuration. On one of the edge Cisco routers I have a Lo0 interface with IP address which I advertise into BGP. Checking the middle Cisco router (provider) I can see that the BGP table has the prefix installed:

Next I check if the provider router does advertise this prefix to the next neighbor:

From this perspective everything is fine. On the edge Cisco router, I cannot see this prefix in the BGP table, which is normal due to the reasons I explained above:

As soon as I add the “allowas-in” configuration on the edge Cisco router, the BGP table will contain the prefix

Now let take the example when the provider is using Juniper technology. The eBGP peering is now between Cisco and Juniper devices:

I have configured the eBGP in the same way and I have advertised from one Cisco router the prefix. 

On Juniper the BGP configuration looks like this:

Checking on the Juniper device I can see the prefix in the BGP table and routing table:

On the second Cisco router, I did not change anything. The “allowas-in” command is still there:

Still the prefix is not in the BGP table:

Here is a big difference on the way eBGP protocol behave on Cisco vs. Juniper. On Cisco devices, the eBGP will send the prefixes, no matter of the peer ASN and it’s the task of the peer to apply the default policy when its own ASN is present in the AS-Path and deny the prefixes to be installed in the BGP table.

With Juniper, eBGP protocol will not even announce the prefixes if the ASN of its peer is already present in the AS-Path.

According to

“The JUNOS software does not advertise the routes learned from one external BGP (EBGP) peer back to the same EBGP peer. In addition, the software does not advertise those routes back to any EBGP peers that are in the same AS as the originating peer, regardless of the routing instance.”

There is a solution to overcome this behavior and it’s pretty simple. You can set this configuration on global basis or per group in Junos BGP configuration:

Now the BGP configuration looks like this:

If I check now the Cisco BGP table, the prefix will be there:

Don’t forget to check my Juniper, first steps after power-on the device if you are new with Juniper products. You may be interested also in my older posts Cisco: BGP path selection for outgoing traffic and Cisco: BGP path selection for inbound traffic if you just start to work with external providers.

Juniper, first steps after power-on the device

As you know from my previous posts, I’m trying to find time to gain some Juniper knowledge. During this “quest” I will add here some basic things about how to start working with Juniper devices. For now I know only the basics of Juniper configuration, but I hope that soon you’ll find here some more challenging scenarios.

I have a basic topology that you’ll find below. The scenario is already prepare to have some tasks which suppose integration between Juniper and Cisco environment.

Let’s assume that I did power on the two boxes J1 and J2 and now I’m connected to J1 through a console cable. After the boot sequence I’m left with something like this:

All platforms running the Junos OS have only the root user configured by default, without any password. Let’s introduce that username and see what’s happening:

What I have now in front is actually the shell of the FreeBSD OS. JunOS is based on the FreeBSD OS. If you ever interacted with a Linux based system, then you can run here specific linux commands. For example:

OK, you got my point. To get from the FreeBSD shell to JunOS CLI, you need to enter the following:

What you see now is the Operational Mode. In this mode the user can run basic and troubleshooting commands (like traceroute, ping…). You can get a list of commands using the ? (question mark):

If you want to compare the Operational Mode is somehow like Privilege level 1 under Cisco CLI. Still I have the feeling that Operational Mode offer a wider area of commands and more powerful than Cisco CLI Privilege level 1. I may be mistaken.

All platforms running the Junos OS come with a factory-default configuration. All factory-default configurations
allow access using the root account without any password. Nevertheless to activate a configuration you have first to set the password root password.Factory-default configurations can vary from one platform family to another or even between the different models
within the same platform family.
My default configuration looks like:

I this first post my target is to set the hostname of the Juniper devices. To accomplish this step, I need to go first into configuration mode:

and then set the hostname:

If I look at the system prompt, it still shows root#, so it doesn’t quite seems to work. This is because I have to commit to activate the configuration:

Well, this didn’t work as expected. The most important thing that I learned when I started with Juniper is that before I can activate any configuration (commit) I need to set the password for the root user:

Let me try to commit one more time, after setting the root password:

You can see that the prompt did change into [email protected]# (in my case this is [email protected]#). If you look again to the system configuration. I will exist the Configuration Mode and have another look at my config file:

The host-name and root password appears now in the active configuration.

That’s it for today. Until next post, I will add the basic configuration for J2 and the Cisco, so I can go to basic interface configuration and connectivity check.