Category: BGP

I’m exposed to an incredible variety of topics in my ExpertExpress engagements, but there are always a few recurring themes, one of them being “we’re experiencing long convergence times and high packet loss after our primary Internet link fails.” Almost always the root cause turns out to be full Internet routing table being received on inadequate hardware.

A reader of my blog experienced problems setting no-export BGP community. Here’s a quick how-to guide (if you’re new to BGP, you might want to read BGP Communities and BGP and route maps posts first).

One of my readers sent me an interesting problem a few days ago: the BGP process running on a PE-router in his MPLS/VPN network preferred an iBGP route received from another PE-router to a locally sourced (but otherwise identical) route. When I looked at the detailed printout, I spotted something “interesting” – the pre-bestpath cost extended BGP community.

During the February Packet Party someone asked the evergreen question: “Is it safe to run Internet services in a VRF?” and my off-the-cuff answer was (as always) “Doing that will definitely consume more memory than having the Internet routes in the global routing table.” After a few moments Derick Winkworth looked into one of his routers and confirmed the difference is huge ... but then he has a very special setup, so I decided to do a somewhat controlled test.

Whenever I’m explaining MPLS/VPN technology, I recommend using the same route targets (RT) and route distinguishers (RD) in all VRFs belonging to the same simple VPN. The Single RD per VPN recommendation doesn’t work well for multi-homed sites, so one might wonder whether it would be better to use a different RD in every VRF. The RD-per-VRF design also works, but results in significantly increased memory usage on PE-routers.

My tweet about the latest proof of my layer-2 = single failure domain claim has raised numerous questions about the use of bridging (aka switching) within Internet Exchange Points (IXP). Let’s see why most IXPs use L2 switching and why L2 switching is the simplest solution to the problem they’re solving.

Jerome has just published the second version of our BGP operations and security Internet draft. Most of the typos and obvious blunders have been fixed (or so we hope) and we’ve incorporated numerous comments received online or during the Paris IETF meeting. Feedback is (as always) highly welcome.

The latest draft is available here.

While I was discussing the intricacies of Cisco’s IPv6 implementation with Gunter Van de Velde a while ago, he suddenly changed hats and asked me whether I would be willing to contribute to a BGP filtering best practices draft. I’m still too young to realize it’s not a good idea to say YES every time you see something interesting and immediately accepted the challenge.

Did you rush to try OSPF Loop-Free Alternate on a Cisco 7200 after reading my LFA blog post… and disappointedly discovered that it only works on Cisco 7600? The reason is simple: while LFA does add feasible-successor-like behavior to OSPF, its primary mission is to improve RIB-to-FIB convergence time.

I got plenty of responses to the How could we filter extraneous BGP prefixes post, some of them referring to emerging technologies and clean-slate ideas, others describing down-to-earth approaches. Thank you all, you’re fantastic!

Almost everyone in the “down-to-earth” category suggested a more or less aggressive inbound filter combined with default routing toward upstream ISPs. Ideally the upstream ISPs would send you responsibly generated default route, or you could use static default routes toward well-known critical infrastructure destinations (like root name servers).

Did you know that approximately 40% of BGP prefixes polluting your RIB and FIB are not needed, as they could be either aggregated or suppressed (because an aggregate is already announced)? We definitely need “driver’s license for the Internet”, but that’s not likely to happen, and in the meantime everyone has to keep buying larger boxes to cope with people who cannot configure their BGP routing correctly.

I got a follow-up question to the Should I use 6PE or native IPv6 post:

Am I remembering correctly that if you run IPv6 native throughout the network you need to enable BGP on all routers, even P routers? Why is that?

I wrote about BGP-free core before, but evidently wasn’t clear enough, so I’ll try to fix that error.

Imagine a small ISP with a customer-facing PE-router (A), two PE-routers providing upstream connectivity (B and D), a core router (C), and a route reflector (R). The ISP is running IPv4 and IPv6 natively (no MPLS).

This is a nice email I got from an engineer struggling with multi-homing BGP setup:

We faced a problem with our internet routers a few days back. The engineer who configured them earlier used the syntax: network x.x.x.x mask y.y.y.y route-map PREPEND to influence the incoming traffic over two service-providers.

... and of course it didn’t work.

Chris sent me the following question a while ago:

I've got a full Internet BGP table, and want to [responsibly]{.emphasis} send a default route to a downstream AS. It's the "responsibly" part that's got me frustrated: How can I judge whether the internet is working and make the origination of the default conditional on that?

He’d already figured out the neighbor default-originate route-map command, but wanted to check for more generic conditions than the presence of one or more prefixes in the IP routing table.

Daniel left a very relevant comment to my convoluted BGP session shutdown solution:

What I am currently doing is using EEM to watch my tracked objects and then issuing a neighbor shutdown command. Is there a functional reason I would not want to do it that way, and use the method you prescribe?

As always, the answer is “it depends.” In this case, the question to ask yourself is: “do I track configuration changes and react to them?”