Posts from ‘IGP’
Optimizing IP Event Dampening
The purpose of event dampening is reducing the effect of oscillations on routing systems. In general, periodic process that affect the routing system as a whole should have the period no shorter than the system convergence time (relaxation time). Otherwise, the system will never stabilize and will be constantly updating its state. In reality, complex system have multiple periodic processes running at the same time, which results is in harmonic process interference and complex process spectrum. Considering such behavior is outside the scope of this paper. What we want to do, is finding optimal settings to filter high-frequency events from the routing system. In our particular case, events are interface flaps, occurring periodically. We want to make sure that oscillations with period T or less are not reported to the routing system. Here T is found empirically, based on observed/estimated convergence time as suggested above.
Event dampening uses exponential back-off algorithm to suppress event reporting to the upper level protocols. Effectively, every time an interface flaps (goes down, to be accurate) a penalty value of P is added to the interface penalty counter. If at some point the accumulated penalty exceeds the “suppress” value of S, the interface is placed in the suppress state and further link events are not reported to the upper protocol modules. At all time, the interface penalty counter follows exponential decay process based on the formula P(t)=P(0)*2^(-t/H) where H is half-life time setting for the process. As soon as accumulated penalty reaches the lower boundary of R – the reuse value, interface is unsuppressed, and further changes are again reported to the upper level protocols.
Tags: bgp, event dampening, exponential backoff, ip event dampening, optimal values
In this blog post we are going to discuss some OSPF features related to convergence and scalability. Specifically, we are going to discuss Incremental SPF (iSPF), LSA group pacing, and LSA generation/SPF throttling. Before we begin, let’s define convergence as the process of restoring the stable view of network after a change, and scalability as the property of the routing protocol to remain stable and well-behaving as the network grows. In general, these two properties are reciprocal, i.e. faster convergence generally means less stable and scalable network and vice versa. The reason for that is that faster convergence means that the routing protocol is “more sensitive” to oscillating or “noisy” processes, which in turn makes it less stable.
Incremental SPF
Tags: ccie, incremental spf, ispf, lsa group pacing, lsa throttling, ospf, spf throttling
This is the follow up discussion for the post titled, “Have you seen my Router ID?”
The underlying issue here was trying to get OSPF to bypass the usual selection process for Router ID. The normal selection order is:
Manual router ID configured under ospf process
Highest IP address of a loopback in the up state in the respective routing table
Highest IP address of an interface of an up state in the respective routing table
If there are no up interfaces and you have not manually configured a router ID, you will get an error when you try to configure an OSPF process.
In general, most solutions focused around either using the OSPF selection process to one’s advantage, or trying to “hide” the loopback from OSPF, so that it would select something else.
In yesterday’s post, titled “Have you seen my Router ID?”, a challenge section was provided. This post will focus on scrutinizing the section itself, from a strategy / analysis point of view.
From a high level overview, we have two devices peering OSPF over a FastEthernet link, with some loopback networks advertised by one side, and received on the other router. If that was all that the section was asking for, then it should be a task that anyone at CCNA level could complete. When looking at the higher levels of certification, to some point you’re still configuring some of the same items, but you are expected to know more and more about the underlying technologies, theories, and interactions.
Just like other practice lab sections, we are provided with a list of bullet points. In order to get full credit for the section, we need to make sure that we are meeting ALL of the stated requirements. In the event that you don’t know how to complete ALL the items, you are usually better off skipping the section, unless it is needed for core connectivity. When you’re in the studying phase, it’s also a good idea to play “what if”, and ask yourself how you would achieve the task if the section were worded slightly differently.
Make sure that you take the time to read carefully, rather than just diving into a configuration. Make sure that you are carefully taking the time to understand exactly what is being asked.
Taking a look at the individual bullet points:
There is more than one possible solution for this challenge. Feel free to post your proposed answer in the comments section. We will try to keep comments hidden from public view, so that the fun isn’t spoiled for others. Also, don’t feel bad if the answer(s) aren’t immediately apparent. A number of very bright people have been puzzled by this scenario. Answers will be posted on Friday, September 18th.
Scenario:
R1 and R2 are configured with their FastEthernet interfaces on the same subnet. R1 will be forming an OSPF neighbor adjacency to R2 over the FastEthernet interface, and will also be advertising some loopback networks into OSPF.
R1’s Relevant Configuration:
interface Loopback1 ip address 1.1.1.1 255.255.255.255 interface Loopback11 ip address 11.11.11.11 255.255.255.255 interface Loopback111 ip address 111.111.111.111 255.255.255.255 interface FastEthernet0/0 ip address 150.10.12.1 255.255.255.0 no shut
R2’s Relevant Configuration:
interface FastEthernet0/0 ip address 150.10.12.2 255.255.255.0 no shut router ospf 1 network 150.10.12.0 0.0.0.255 area 0
Challenge:
Your task is to configure R1 while meeting all of the following criteria for requirements and restrictions:
- R2 should see the networks 1.1.1.1/32, 11.11.11.11/32, and 111.111.111.111/32 as OSPF routes in R2’s routing table, but they should not appear as IA, E2, or E1.
- The output of “show ip ospf neighbor” on R2 should show 11.11.11.11 as the Neighbor ID for the adjacency to R1, even if R1 is reloaded. No other Neighbor IDs should show up on R2.
- You are not allowed to use the “router-id” command on R1.
- You are not permitted to shut down any interfaces on R1, or remove any of the existing configuration on R1.
- No additional configuration may be added to R2, all configuration for this challenge is done on R1.
Loading ...
