Salman left an interesting comment on my Running BGP on Servers blog post:
My prior counterparts thought running OSPF on Mainframes was a good idea. Then we had a routing blackhole due to misconfiguration on the server. Twice! The main issue was the Mainframe admins lack of networking/OSPF knowledge.
Well, there’s a reason OSPF is called Interior Routing Protocol.
With symmetric fabric… does it make sense for a node to know every bit of fabric info or is reachability information sufficient?
Let’s ignore for the moment that large non-redundant layer-3 fabrics where BGP-in-Data-Center movement started don’t need more than endpoint reachability information, and focus on a bigger issue: is knowledge of network topology (as provided by OSPF and not by BGP) beneficial?
Carlos Mendioroz sent me an interesting question about unnumbered interfaces in Cumulus Linux and some of the claims they make in their documentation.
TL&DR: Finally someone got it! Kudos for realizing how to use an ancient trick to make data center fabrics easier to deploy (and, BTW, the claims are exaggerated).
One of the Expert Express sessions focused on an MPLS/VPN-based WAN network using OSPF as the routing protocol. The customer wanted to add DMVPN-based backup links and planned to retain OSPF as the routing protocol. Not surprisingly, the initial design had all sorts of unexpectedly complex kludges (see the case study for more details).
Having a really smart engineer on the other end of the WebEx call, I had to ask a single question: “Why don’t you use BGP everywhere” and after a short pause got back the expected reply “wow ... now it all makes sense.”
VMware gave me early access to NSX hands-on lab a few days prior to VMworld 2013. The lab was meant to demonstrate the basics of NSX, from VXLAN encapsulation to cross-subnet flooding, but I quickly veered off the beaten path and started playing with routing protocols in NSX Edge appliances.
The true OpenFlow zealots would love you to believe that you can drop whatever you’ve been doing before and replace it with a clean-slate solution using dumbest (and cheapest) possible switches and OpenFlow controllers.
In real world, your shiny new network has to communicate with the outside world … or you could take the approach most controller vendors did, decide to pretend STP is irrelevant, and ask people to configure static LAGs because you’re also not supporting LACP.
… The OSPF route selection rule is that intra-area routes are preferred over inter-area routes, which are preferred over external routes. However, this rule should apply to routes learned via the same process …
Let’s see what’s going on behind the scenes.
A while ago I got an interesting question:
Let's say that due to circumstances outside of your control, you must have stretched data center subnets... What is the best method to get these subnets into OSPF? Should they share a common area at each data center or should each data center utilize a separate area for the same subnet?
Assuming someone hasn’t sprinkled the application willy-nilly across the two data centers, it’s best if the data center edge routers advertise subnets used by the applications as type-2 external routes, ensuring one data center is always the primary entry point for a specific subnet. Getting the same results with BGP routing in Internet is a much tougher challenge.
When testing the OSPF graceful shutdown feature, I've encountered an interesting OSPF feature: if you force a change in LAN DR router (other than rebooting the current DR), you'll end up with two network LSAs describing the same LAN.
This blog has been sitting in my Draft folder for years, so Cisco IOS behavior might have changed in the meantime, or it might have been a transient and/or race condition. Nonetheless, I still find it interesting.
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.
Assume we have a simple triangular network:
Now imagine the A-to-C link fails. How will OSPF react to the link failure as compared to EIGRP? Which one will converge faster? Try to answer the questions before pressing the Read more link ;)
Most of the DMVPN-related questions I get are a variant of the “how many tunnels/hubs/interfaces/areas do I need for a redundant DMVPN design?” As always, the right answer is “it depends” (and I can always help you with your design if you’d like to get a second opinion), but here’s what I’ve learned so far.
A reader of my blog planning to migrate his network from a traditional BGP-everywhere design to a BGP-over-MPLS one wondered about potential unexpected consequences. The MTU implications of introducing MPLS in a running network are usually well understood (even though you could get some very interesting behavior); if you can, increase the MTU size by at least 16 bytes (4 labels) and check whether MTU includes L2 header. Another somewhat more mysterious beast is the interaction between IGP and LDP that can cause traffic disruptions after the physical connectivity has been reestablished.
One of my readers sent me the following question a few days ago:
Do you have a webinar that covers Dual DMVPN HUB deployment using OSPF? If so which webinar covers it?
I told him that the DMVPN: From Basics to Scalable Networks webinar covers exactly that scenario (and numerous others), describing both Phase 1 DMVPN and Phase 2 DMVPN design and implementation guidelines. Interestingly, he replied that the information on this topic seems to be very scant:
SK left a long comment to my More OSPF-over-DMVPN Questions post describing a scenario I find quite often in enterprise networks:
- Primary connectivity is provided by an MPLS/VPN service provider;
- Backup connectivity should use DMVPN;
- OSPF is used as the routing protocol;
- MPLS/VPN provider advertises inter-site routes as external OSPF routes, making it hard to properly design the backup connectivity.
If you’re familiar with the way MPLS/VPN handles OSPF-in-VRF, you’re probably already asking the question “how could the inter-site OSPF routes ever appear as E1/E2 routes?”