Mixing IPv4 and IPv6 in BGP

▼ When I first started running IPv6 over BGP, I ran into an interesting problem. The Cisco router I was working on had four full BGP feeds over IPv4. But when did show ip bgp, I noticed the router had five copies of each prefix. One of these paths had a really weird next hop address and was marked as unreachable.

Turned out those were IPv4 prefixes learned over an IPv6 BGP session.

What had happened was that when adding an IPv6 BGP session, I didn't specifically tell the router to not do IPv4 over that session. So despite the session being between two IPv6 addresses, the routers on both sides negotiated the use of the IPv4 unicast address family over that session, and thus exchanged IPv4 prefixes.

The problem with that is that BGP's automatic next hop processing then doesn't work: normally, the next hop is set to the IP address the router uses for the TCP session that BGP runs over. So when router 192.0.2.1 sends IPv4 prefixes to router 192.0.2.2, it sets the next hop attribute in those updates to 192.0.2.1. That doesn't work so well when the IP addresses for that BGP session are 2001:db8::1 and 2001:db8::2. What actually seems to happen is that the routers set the IPv4 next hop address to the first 32 bits of the IPv6 address used for the BGP TCP session...

So this is why for eBGP, we recommend using an IPv4 BGP session to exchange IPv4 prefixes and an IPv6 BGP session to exchange IPv6 prefixes. As the default is IPv4, for IPv6 BGP sessions you usually need to disable IPv4 and enable IPv6 (unless the defaults have been changed at some point, but you really don't want to depend on that):

!
router bgp 65550
 neighbor 2001:db8::2 remote-as 65551
!
 address-family ipv4
 no neighbor 2001:db8::2 activate
!
 address-family ipv6
 neighbor 2001:db8::2 activate
!

Or you can make a route map that sets the next hop address to an address that does work.

For iBGP, it's often easier to also use the IPv4 BGP sessions for IPv6, as the number of iBGP sessions adds up quickly and iBGP doesn't modify the next hop address, so the issue above doesn't come up.

However, there's an additional complication when mixing IPv4 and IPv6 in BGP: debugging gets much harder.

If you want to see what's happening with your BGP sessions, you can use the show bgp ipv4 unicast summary and show bgp ipv6 unicast summary commands:

# show bgp ipv4 unicast summary Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 223.223.223.81 4 81 23 24 0 0 0 00:20:28 16 2001:1c00:161d:df00::1 4 81 24 24 0 0 0 00:20:27 19

# show bgp ipv6 unicast summary Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 2001:1c00:161d:df00::1 4 81 26 26 0 0 0 00:22:01 1

So with these commands, we get to see the BGP sessions for which IPv4 or IPv6 is activated, regardless of whether the session runs over IPv4 or IPv6. So far, so good. And in later Cisco IOS versions, I believe this works the same for other show bgp commands. But under older IOS versions the IPv4 commands won't always take an IPv6 address and vice versa.

But at least under Cisco IOS, the commands now make a reasonable amount of sense: rather than show ip bgp ... for IPv4 and show bgp ... for IPv6, we can now use the much more consistent show bgp ipv4 unicast ... and show bgp ipv6 unicast ... commands.

Not so much under Quagga, though. At least, under version 0.99.22.4. The good news is that you can actually use show ip bgp neighbors <IPv6 address> and show bgp neighbors <IPv4 address>. The bad news is that even though Quagga understands the show bgp ipv4 unicast summary command, it doesn't understand the show bgp ipv4 unicast neighbors ... command. You have to use show ip bgp neighbors ... instead.

So the moral of the story is that you make life a lot easier if you don't mix IPv4 and IPv6 in BGP. If you do, it helps to use recent software, but make sure you check all the possible variations of the different commands, because they're not necessarily used consistently.

Permalink - posted 2016-03-18

20 years of BGP

▼ 20 years ago today, I got my first autonomous system (AS) number, marking my entry in the BGP business. (5399, if you're wondering.)

To quote Ferris Bueller: "Life moves pretty fast. If you don't stop and look around once in a while, you could miss it." So let's look back and see what has happened over those 20 years.

Back in 1995, BGP had been around for about six years: BGP versions 1 - 3 were published as RFCs in 1989, 1990 and 1991, respectively. And then in 1994, the BGP-4 spec was published and that's still the version we use today. Crazy, right? I remember when I was writing my BGP book (published by O'Reilly in 2002), my editor asked whether there'd be a new version of BGP soon. I told him not to worry about it.

bART

However, pretty much all other aspects of the internet have changed in those two decades. The first version of the IPv6 specifications wouldn't be published until several months later. I hadn't heard of Network Address Translation (NAT) yet—RFC 1918 hadn't even been published! So we had to use RFC 1597 to get our private addresses. But IPv4 addresses were plentiful those days, so why would we? DHCP had been out for a couple of years, but I'm pretty sure I hadn't used it by then: stuff either was configured manually, or through PPP.

Yes, those were the days of screeching modems. AS5399 was the AS number for bART Internet Services, a small ISP in The Hague. When I first got in touch with bART they had a 28 kbps link to one ISP and a 128 kbps link to another. But all the traffic flowed over the slow line. I rerouted most of it over the faster line, which landed me a job. But obviously being able to use both lines together would be even better, but to do that right, you need BGP to do proper "multihoming".

One of the perks of working at an ISP was that I was able to get my own leased line. Back in those days, you could lease a regular copper phone wire between two locations, and this was relatively cheap (55 guilders (25 euros) a month, IIRC) if both locations were served by the same telephone exchange. As one third of The Hague is connected to the really, really huge exchange in the Marnixstraat, that applied to me. However, my leased line was only 28k, so around that same time I got my ISDN line (which I would keep until 2007), which was much faster at 64 kbps, or even 128 kbps if you bundled both channels. But remember, those channels were metered so that could get expensive, even for local calls. Today, I have a 120 Mbps cable connection at home, which is a thousand times faster. That makes for a bandwidth increase of about 41% per year. I remember signing a contract for 2 Mbps internet service for 30,000 guilders (13,000 euros) per month a year later. So the wholesale price of bandwidth has also gone down by about a factor 1000 in 20 years.

Size of the BGP table

We initially ran BGP on a Cisco 2501 router , maxed out with16 MB RAM. Back then, the BGP table was around 30,000 prefixes, which just about fit in that 16 MB. I don't remember what kind of router we switched to as the table size outgrew that 16 MB. Today, there are about 560,000 IPv4 prefixes in the BGP table, so roughly a factor 20 increase in 20 years, or about 16% per year.

Back in 2006, the Internet Architecture Board held a "routing and addressing" workshop in Amsterdam, where the issue of the growth of the BGP table was discussed. For a long time, this was considered a serious problem. We even came up with Shim6 in the IETF to allow for multihoming without using BGP with IPv6, but that never went anywhere as people just used BGP to multihome with IPv6. So, 16% annual growth of the BGP table is inconvenient, but apparently not as inconvenient as the alternatives. (Of course, if you run BGP to multihome, that solves your problem immediately while increasing the BGP table for everyone. Waiting for the whole world to adopt Shim6 so you can multihome without BGP doesn't provide that instant gratification even though it keeps the routing table small and thus the routers cheap(er).)

BGP's evolution

I still have a Cisco 2503 (that's the model with an ISDN port) lying around. I think we used IOS 10.3 or so back in 1995. If I load that IOS version on the 2503, I'm pretty sure that 2503 with the 20-year-old software could still talk BGP with a current router. (Well, until that 16 MB RAM is full.) However, that is not to say that nothing has changed in intermediate decades. A whole bunch of capabilities were added over the years, including:

• communities, for tagging routes with additional information
• flap damping, to keep unstable prefixes from being propagated
• the TCP MD5 option to protect BGP sessions against spoofed packets
• multiprotocol extensions, to support IPv6, multicast, VPNs and more
• route refresh, to allow sessions to be reset non-destructively
• the 32-bit AS number capability
• and in the not-too-distant future: BGPsec, which will make BGP more secure

The creators of BGP-4 did a great job with the extension mechanism, allowing the protocol to be extended as needed with very few backward compatibility issues. So future network protocol creators, take heed. And as a result, we'll probably be running BGP-4 forever, although it's hard to say what will be bolted onto it in the future.

BGP and me

Over the past 20 years, I have configured BGP for dozens of clients on about a dozen different types of routers, I wrote a book about BGP, started bgpexpert.com, wrote a master's thesis on a crazy way to do multipath in BGP and proposed an improvement to the protocol. Since my book came out, I've been doing BGP training courses, having trained at least 200, but probably more than 300 people. And I'm sure BGP and me haven't seen the last of each other. But right now, I have a movie to watch, candles to blow out and pie to eat!

Permalink - posted 2015-08-15

5 minutes of BGP instability after leap second

▼ This July 30th, at 23:59:60, a leap second was added to Coordinated Universal Time (UTC). Dyn Research posted the following graph on Twitter that shows there was significant BGP update instability for five minutes after the leap second occurred:

Unfortunately, it's not clear why this happened. However, leap seconds have triggered all kinds of mishaps in the past. They're basically miniature Y2K problems. Time and time again, software engineers show that they can't be trusted to take corner cases into account properly.

This does remind me of a situation about a decade ago, where I had a customer that experienced BGP instability every night at the same time. They used Quagga running on Linux machines. We couldn't figure out what the problem was, until we realized that at that very moment, the ntpdate command was run from the cron. ntpdate synchronizes the system clock with an NTP server. As the machine in question had a very poor system clock, this meant that the system's time was adjusted a lot every night, I think a minute or more, but definitely more than 30 seconds.

Which meant that if Quagga had gotten a BGP keepalive message 8 seconds earlier, it now thought that was 38 seconds ago. If BGP is configured with a hold time of 30 seconds, this means that Quagga now thinks the other side has been quiet for longer than the hold time and it'll tear down the BGP session. This is what happened every night for a bunch of BGP sessions. We solved this by running the NTP daemon continuously, so there was never a big adjustment in system time. (Alternatively, just letting the time drift would also have worked.)

The minimum BGP hold time is 3 seconds, so adjusting for an (improperly handled) leap second shouldn't be able to make BGP think the hold time for a session is expired. However, there could be bug somewhere else that impacted BGP.

I'm not sure whether these kinds of issues are a good argument in favor of abandoning leap seconds, as the bugs won't go away, they'll just show up at a less predictable time. But I don't like the current leap second practice, as they're unpredictable, and you can't calculate the time difference in seconds between two dates without taking the entire list of leap seconds into account. I think it would be better to save the leap seconds up and apply them all at the end of a century.

Permalink - posted 2015-07-06