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Category: IP routing

Unexpected Interactions Between OSPF and BGP

It started with an interesting question tweeted by @pilgrimdave81

I’ve seen on Cisco NX-OS that it’s preferring a (ospf->bgp) locally redistributed route over a learned EBGP route, until/unless you clear the route, then it correctly prefers the learned BGP one. Seems to be just ooo but don’t remember this being an issue?

Ignoring the “why would you get the same route over OSPF and EBGP, and why would you redistribute an alternate copy of a route you’re getting over EBGP into BGP” aspect, Peter Palúch wrote a detailed explanation of what’s going on and allowed me to copy into a blog post to make it more permanent:

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Deploying Plug-and-Pray Software in Large-Scale Networks

One of my readers sent me a sad story describing how Chromium service discovery broke a large multicast-enabled network.


The last couple of weeks found me helping a customer trying to find and resolve a very hard to find “network performance” issue. In the end it turned out to be a combination of ill conceived application nonsense and a setup with a too large blast radius/failure domain/fate sharing. The latter most probably based upon very valid decisions in the past (business needs, uniformity of configuration and management).

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Unnumbered Ethernet Interfaces, DHCP Edition

Last week we explored the basics of unnumbered IPv4 Ethernet interfaces, and how you could use them to save IPv4 address space in routed access networks. I also mentioned that you could simplify the head-end router configuration if you’re using DHCP instead of per-host static routes.

Obviously you’d need a smart DHCP server/relay implementation to make this work. Simplistic local DHCP server would allocate an IP address to a client requesting one, send a response and move on. Likewise, a DHCP relay would forward a DHCP request to a remote DHCP server (adding enough information to allow the DHCP server to select the desired DHCP pool) and forward its response to the client.

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Unequal-Cost Multipath with BGP DMZ Link Bandwidth

In the previous blog post in this series, I described why it’s (almost) impossible to implement unequal-cost multipathing for anycast services (multiple servers advertising the same IP address or range) with OSPF. Now let’s see how easy it is to solve the same challenge with BGP DMZ Link Bandwidth attribute.

I didn’t want to listen to the fan noise generated by my measly Intel NUC when simulating a full leaf-and-spine fabric, so I decided to implement a slightly smaller network:

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Routing Protocols: Use the Best Tool for the Job

When I wrote about my sample katacoda hands-on lab on LinkedIn (mentioning how easy it is to set up an OSPF+BGP network), someone couldn’t resist asking:

I’m still wondering why people use two routing protocols and do not have clean redistribution points or tunnels.

Ignoring for the moment the fact that he missed the point of the blog post (completely), the idea of “using tunnels or redistribution points instead of two routing protocols” hints at the potential applicability of RFC 1925 rule 4.

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Unnumbered Ethernet Interfaces

Imagine an Internet Service Provider offering Ethernet-based Internet access (aka everyone using fiber access, excluding people believing in Russian dolls). If they know how to spell security, they might be nervous about connecting numerous customers to the same multi-access network, but it seems they have only two ways to solve this challenge:

  • Use private VLANs with proxy ARP on the head-end router, forcing the customer-to-customer traffic to pass through layer-3 forwarding on the head-end router.
  • Use a separate routed interface with each customer, wasting three-quarters of their available IPv4 address space.

Is there a third option? Can’t we pretend Ethernet works in almost the same way as dialup and use unnumbered IPv4 interfaces?

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Single-Metric Unequal-Cost Multipathing Is Hard

A while ago we discussed whether unequal-cost multipathing (UCMP) makes sense (TL&DR: rarely), and whether we could implement it in link-state routing protocols (TL&DR: yes). Even though we could modify OSPF or IS-IS to support UCMP, and Cisco IOS XR even implemented those changes (they are not exactly widely used), the results are… suboptimal.

Imagine a simple network with four nodes, three equal-bandwidth links, and a link that has half the bandwidth of the other three:

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Packet Forwarding and Routing over Unnumbered Interfaces

In the previous blog posts in this series, we explored whether we need addresses on point-to-point links (TL&DR: no), whether it’s better to have interface or node addresses (TL&DR: it depends), and why we got unnumbered IPv4 interfaces. Now let’s see how IP routing works over unnumbered interfaces.

The Challenge

A cursory look at an IP routing table (or at CCNA-level materials) tells you that the IP routing table contains prefixes and next hops, and that the next hops are IP addresses. How should that work over unnumbered interfaces, and what should we use for the next-hop IP address in that case?

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Back to Basics: Unnumbered IPv4 Interfaces

In the previous blog post in this series, we explored some of the reasons IP uses per-interface (and not per-node) IP addresses. That model worked well when routers had few interfaces and mostly routed between a few LAN segments (often large subnets of a Class A network assigned to an academic institution) and a few WAN uplinks. In those days, the WAN networks were often implemented with non-IP technologies like Frame Relay or ATM (with an occasional pinch of X.25).

The first sign of troubles in paradise probably occurred when someone wanted to use a dial-up modem to connect to a LAN segment. What subnet (and IP address) do you assign to the dial-up connection, and how do you tell the other end what to use? Also, what do you do when you want to have a bank of modems and dozens of people dialing in?

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Back to Basics: The History of IP Interface Addresses

In the previous blog post in this series, we figured out that you might not need link-layer addresses on point-to-point links. We also started exploring whether you need network-layer addresses on individual interfaces but didn’t get very far. We’ll fix that today and discover the secrets behind IP address-per-interface design.

In the early days of computer networking, there were three common addressing paradigms:

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Back to Basics: Do We Need Interface Addresses?

In the world of ubiquitous Ethernet and IP, it’s common to think that one needs addresses in packet headers in every layer of the protocol stack. We have MAC addresses, IP addresses, and TCP/UDP port numbers… and low-level addresses are assigned to individual interfaces, not nodes.

Turns out that’s just one option… and not exactly the best one in many scenarios. You could have interfaces with no addresses, and you could have addresses associated with nodes, not interfaces.

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Building Unnumbered Ethernet Lab with netsim-tools

Last week I described the new features added to netsim-tools release 0.4, including support for unnumbered interfaces and OSPF routing. Now let’s see how I used them to build a multi-vendor lab to test which platforms could be made to interoperate when running OSPF over unnumbered Ethernet interfaces.

I needed to define an unnumbered addressing pool first:

addressing:
  core:
    unnumbered: true

I wanted to run OSPF on all devices in the lab:

module: [ ospf ]
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