Category: BGP

Last week, I explained the differences between FRRouting and more traditional networking operating systems in scenarios where OSPF and IBGP advertise the same prefix:

• Traditional networking operating systems enter only the OSPF route into the IP routing table.
• FRRouting enters OSPF and IBGP routes into the IP routing table.
• On all platforms I’ve tested, only the OSPF route gets into the forwarding table¹.

One could conclude that it’s perfectly safe to advertise the same prefixes in OSPF and IBGP. The OSPF routes will be used within the autonomous system, and the IBGP routes will be propagated over EBGP to adjacent networks. Well, one would be surprised 🤦‍♂️

Last time, we discussed the first line of defense against fat finger incidents: limiting the number of BGP prefixes your router accepts from a BGP neighbor. However, you can do much more without deploying customer-specific filters (which might require a customer database) or ROV/RPKI.

You can practice the default filters you should always deploy on EBGP sessions with your customers in the Stop the Propagation of Configuration Errors lab exercise.

This is how we described the interactions between routing protocol tables, RIB, and FIB in the ancient times:

• Routing protocols compute the best paths to all known prefixes.
• These paths compete for entry in the routing table. The path(s) with the lowest administrative distance win.
• The entries from the routing table are fully evaluated (in particular, their next hops) and entered in the forwarding table.

Let’s use a simple BGP+OSPF network to illustrate what I’m talking about:

Arista EOS and Cisco Nexus OS got interface EBGP sessions years after Cumulus Linux. While they’re trivially easy to configure on FRRouting (the routing daemon used by Cumulus Linux), getting them to work on Arista EOS is a bit tricky.

To make matters worse, my Google-Fu failed me when I tried to find a decent step-by-step configuration guide; all I got was a 12-minute video full of YouTube ads. Let’s fix that.

I usually say that you cannot run netlab on Apple Silicon because the vendors don’t provide ARM images. However, when I saw an ARM version of the FRRouting container, I wondered whether I could run the BGP labs (admittedly only on FRR containers) on my M2 MacBook Pro.

TL&DR: Yes, you can do that.

Now for the recipe:

One of my readers was building a private MPLS/VPN backbone and wondered whether they should use their public AS number or a private AS number for the backbone. Usually, it doesn’t matter; the deciding point was the way they want to connect to the public Internet:

We also plan to peer with multiple external ISPs to advertise our public IP space not directly from our PE routers but from dedicated Internet Routers, adding a firewall between our PEs and external Internet routers.

They could either run BGP between the PE routers, firewall, and WAN routers (see BGP as High-Availability Protocol for more details) or run BGP across a bump-in-the-wire firewall:

Here’s an easy way to stop fat-finger incidents in which an end-user autonomous system redistributes IGP into BGP or advertises the whole DFZ routing table from affecting the entire Internet: limit the number of BGP prefixes your routers accept from your customers. You can practice this nifty feature in the next BGP lab exercise.

One of the previous BGP labs explained how you can use session templates to configure common TCP or BGP session parameters. Some BGP implementations have another templating mechanism: policy templates that you can use to apply consistent routing policy parameters to an EBGP neighbor. You can practice them in the next BGP lab exercise.

In a previous BGP lab exercise, I described how an Internet Service Provider could run BGP with a customer without the customer having a public BGP AS number. The only drawback of that approach: the private BGP AS number gets into the AS path, and everyone else on the Internet starts giving you dirty looks (or drops your prefixes).

Let’s fix that. Most BGP implementations have some remove private AS functionality that scrubs AS paths during outgoing update processing. You can practice it in the Remove Private BGP AS Numbers from the AS Path lab exercise.

I got this question after publishing the BGP Session Templates lab exercise:

Would you apply BGP route maps with a peer/policy template or directly to a BGP neighbor? Of course, it depends; however, I believe in using a template for neighbors with the same general parameters and being more specific per neighbor when it comes to route manipulation.

As my reader already pointed out, the correct answer is It Depends, now let’s dig into the details ;)

Configuring an IBGP session on a route reflector takes a half-dozen parameters, starting with the remote BGP AS number (equal to the local one), remote IP address, and the source IP address or interface. You might have to specify the propagation of BGP communities and an MD5 password, and you will definitely have to specify that the BGP neighbor is a route reflector client.

Wouldn’t it be nice if you could group those parameters into a template and apply the template to a neighbor? Most BGP implementations have something along those lines. That feature could be called a session template or a peer group, and you can practice it in the next BGP lab exercise.

One of the few beauties of most “industry standard CLI” implementations¹ is that they’re idempotent: nothing changes (apart from ACLs) if you configure the same stuff a dozen times. Most of these implementations allow you to deconfigure the same stuff multiple times; FRRouting is one of the unfortunate exceptions.

What Am I Talking About?

Imagine you have a bunch of IP prefixes you want to advertise with BGP. You could use network statements within the router bgp configuration to get that done:

Can you use BGP to connect to the global Internet without having a public BGP AS number? Of course, assuming your Internet Service Provider is willing to run BGP with a network using a private AS number. But what happens if you want to connect to two ISPs? It’s ridiculous to expect you’ll be able to persuade them to use the same private AS number.

That’s one of the many use cases for the local-as functionality available in most BGP implementations. You can practice it in the Use Multiple AS Numbers on the Same Router lab exercise.

When I described the need to turn off the BGP AS-path loop prevention logic in scenarios where a Service Provider expects a customer to reuse the same AS number across multiple sites, someone quipped, “but that should be fixed by the Service Provider, not offloaded to the customer.”

Not surprisingly, there’s a nerd knob for that (AS override), and you can practice it in the next BGP lab exercise: Fix AS-Path in Environments Reusing BGP AS Numbers.

FRR or (pre-NVUE) Cumulus Linux are the best bets if you want to run BGP labs in a resource-constrained environment like your laptop or a small public cloud instance. However, they both behave a bit differently from what one might expect from a networking device, including:

• Interfaces are created through standard Linux tools;
• You have to start the FRR management CLI from the Linux shell;
• If you need a routing daemon (for example, the BGP daemon), you must enable it in the FRR configuration file and restart FRR.

A new lab exercise covers these intricate details and will help you get fluent in configuring BGP on FRR or Cumulus Linux virtual machines or containers.