Category: GRE

The last bits of updated Never-Ending Story of IP Fragmentation were published a few days ago: IP fragmentation and tunnels and summary and related blog posts, RFCs and other articles.

Two weeks after VXLAN (backed by VMware, Cisco, Citrix and Red Hat) was launched at VMworld, Microsoft, Intel, HP & Dell published NVGRE draft (Arista and Broadcom are cleverly sitting on both chairs) which solves the same problem in a slightly different way.

If you’re still wondering why we need VXLAN and NVGRE, read my VXLAN post (and the one describing how VXLAN, OTV and LISP fit together).

It’s obvious the NVGRE draft was a rushed affair, its only significant and original contribution to knowledge is the idea of using the lower 24 bits of the GRE key field to indicate the Tenant Network Identifier (but then, lesser ideas have been patented time and again). Like with VXLAN, most of the real problems are handwaved to other or future drafts.

Short answer: yes, it does.

During the geeky chat we had just after we’d finished recording the Data Center Fabric Packet Pushers podcast, Kurt (@networkjanitor) Bales asked me whether the MPLS/VPN-over-DMVPN scenarios I’m describing in Enterprise MPLS/VPN Deployment webinar really work (they do seem a bit complex).

I always test the router configurations I use in my webinars and I usually share them with the attendees. Enterprise MPLS/VPN Deployment webinar includes a complete sets of router configurations covering 10 scenarios, including five different MPLS/VPN-over-DMVPN designs, so you can easily test them in your lab and verify that they do work. But what about a live deployment?

More than five years after the MPLS/VPN-in-mGRE encapsulation was standardized (add a few more years for the work-in-progress and IETF draft stages), it finally debuted in a mainstream-wannabe IOS release running on ISR routers (15.1(2)T), making it usable for the enterprise WAN designers, who are probably its best target audience.

I was writing about the two conflicting MPLS/VPN over mGRE implementations a while ago and got the impression the Service Providers aren’t too excited about this option. No wonder – most of them use full-blown MPLS backbones, so they have no need for GRE tunnels.

RFC 4364 (also known as RFC 2547bis from its draft days) specifies two methods of transporting VPN packets between PE-routers: well-known MPLS transport and GRE transport. The GRE transport idea is extremely simple: you take the labeled VPNv4 packet, put it into a GRE envelope, set the GRE Ethertype field to MPLS Unicast (0x8847) and send the GRE packet to the IP address of the egress PE-router.

This is not the same mechanism as running MPLS over point-to-point LDP-enabled GRE tunnels or running MPLS over DMVPN tunnels; this one does not require multiple tunnels or LDP/NHRP to work.

Unveiling of the Cisco IOS release 15.1(1)T was the extreme opposite of the CRS-3 and Catalyst 3750-X splashes; the next release of one of the foundations of Cisco’s core business deserved a modest two-paragraph mention in the What's New in Cisco Product Documentation page.

If you’re a voice guru, you’ll probably enjoy the list of 20+ voice-related new features, including the all-important Enhanced Music on Hold. For the rest of us, here’s what I found particularly interesting:

The “GRE keepalives or EIGRP hellos” discussion has triggered another interesting question:

Is there a good rule-of-thumb for setting hold-down timers in respect to the bandwidth/delay of a given link? Perhaps something based off of the SRTT?

Routing protocol hello packets or GRE keepalive packets are small compared to the bandwidths we have today and common RTT values are measured in milliseconds while the timers’ granularity is usually in seconds.

It looks like everyone who’s not using DMVPN is running IPSec over GRE these days, resulting in interesting questions like »should IP use EIGRP hellos or GRE keepalives to detect path loss?«

Any dedicated link/path loss detection protocol should be preferred over tweaking routing protocol timers (at least in theory), so the politically correct answer is »use GRE keepalives and keep EIGRP hellos at their default values«. Even better, use BFD over GRE (if your device supports it) instead of a hodgepodge of point technologies.

The IP-over-IP (usually GRE) tunnels (commonly in combination with IPSec to provide security) are frequently used when you want to transport private IP traffic over public IP network that does not support layer 3 VPNs. If you use the GRE tunnels in combination with default routing (or route summarization), you can get serious routing issues when the tunnel destination disappears, but a default (or summary) route in the IP routing table still covers it. You could work around this issue by deploying a routing protocol over the GRE tunnel (which could lead to hard to diagnose routing loops if you're not careful) or by using GRE keepalives introduced in IOS release 12.2(8)T.

The implementation of the GRE keepalives is amazing: the router sending the keepalive packet constructs a GRE packet that would be sent from the remote end back to itself (effectively building a GRE reply), sets the GRE protocol type to zero (to indicate the keepalive packet) and sends the whole packet through the tunnel (effectively encapsulating GRE reply into another GRE envelope). The receiving router strips the GRE envelope and routes the inside packet … which is the properly formatted GRE keepalive reply.

This trick allows you to implement different GRE keepalive timers on each end of the link. For example, the remote site might use fast keepalive timers to detect loss of primary link and switch over to a backup link, while the central site would use less frequent keepalive tests to detect failed remote site (if there is a single path to the remote site, you don't care too much when you detect it's down).

Every ingenious solution has its drawbacks and this one is no exception: if the receiving router protects its IP addresses (to stop spoofing attacks), it will drop the incoming GRE keepalive packet. Furthermore, a document available on Cisco's web describes the issues of using GRE keepalives in IPSec environment.

I wanted to get in-depth details on how various MTU parameters interact in GRE/IPSec/MPLS environment. Before going into router configuration details, I wanted to have a tool that would reliably measure actual path MTU between the endpoints. After a while, Google gave me a usable link: supposedly the tracepath program on Linux does what I needed. As I'm a purely Windows user (for me, PCs are just a tool), I needed a Windows equivalent … and found mturoute, the utility that does exactly what I was looking for.