Building network automation solutions

9 module online course

Start now!

Category: you've asked for it

Some more QoS basics

I got a really interesting question from one of my readers (slightly paraphrased):

Is this a correct statement: QoS on a WAN router will always be on if there are packets on the wire as the line is either 100% utilized or otherwise nothing is being transmitted. Comments like “QoS will kick in when there is congestion, but there is always congestion if the link is 100% utilized on a per moment basis” are confusing.

Well, QoS is more than just queuing. First you have to classify the packets; then you can perform any combination of marking, policing, shaping, queuing and dropping.

2011-06-23: Added description of various link efficiency mechanisms.

read more see 14 comments

What exactly is a Nexus 1000V?

Nexus 1000V makes a lot of people unfamiliar with the virtualization intricacies mightily confused (more so as Cisco usually discusses it together with hardware-based switches like Nexus 5000 and Nexus 7000). These are the typical questions I get from my readers:

What exactly is the Nexus 1000V? It sits in the VMware host, but how do the servers connect into it? Is it a software connection spilled out into hardware at the 1000V egress? Do you somehow track what traffic belongs to each server and put polices against them like a normal switch would?

Nexus 1000V is a replacement for VMware’s distributed switch; it’s a software-only layer-2 switch sitting inside the VMware hypervisor kernel.

read more see 8 comments


I got a long list of MPLS-related follow-up questions from one of the attendees of my Enterprise MPLS/VPN Deployment webinar (register for a live session or buy a recording) and thought it might be a good idea to share them (and the answers) with you.

You said that the golden rule in simple VPN topologies is RD = export RT = import RT. Are there any other “generic rules”? How would you setup this RD&RT association for hub&spoke VPN scenario?

Common services VPN topologies could be implemented in two ways (on top of existing simple VPN topology):

read more see 1 comments

Editing AS-path access lists

Jerry sent me an interesting question:

I was wondering if there's a way to modify an as-path access-list much like we do with regular access lists, simply by adding/ removing lines according to their sequence numbers.

I'm not aware of any such mechanism in Cisco IOS (but then maybe I’m missing something), but his question made me wonder: if you’re maintaining large AS-path access lists, do you edit them on the router (I guess not) or off-line (on a NMS platform) and download them when they need to be changed?

see 4 comments

Report interface loss based on OSPF neighbor loss

Nicolas sent me an interesting problem: he has numerous point-to-point GRE-over-IPSec tunnels on his core router and detects remote site failure with OSPF neighbor loss events. He would like to receive an e-mail when an OSPF neighbor goes down (quite easy to do with EEM), but would also like to receive interface description in the e-mail subject to simplify his troubleshooting.

With the regular expressions available in EEM 3.0 you can extract interface name from syslog message, execute show interface command and extract the interface description from it.

The EEM applet source code is available in the CT3 wiki

This article is part of You've asked for it series.

add comment

Encrypting P-to-P-router traffic

Rob sent me a really good question:

I have an enterprise MPLS network. Two P routers are connected via carrier point-to-point Gigabit Ethernet and I would like to encrypt the MPLS traffic traversing the GE link. The PE-routers don't have hardware crypto accelerators, so I would like to keep the MPLS within the buildings running in cleartext and only encrypt the inter-site (P-to-P) MPLS traffic.

The only solution I could imagine would nicely fit the motto of one of our engineers: »Any time you have a problem, use more GRE tunnels« (if you have a better solution, please post it in the comments).

read more see 11 comments

Carrier Ethernet service from customer’s perspective

As the Carrier Ethernet services are becoming more popular, people are starting to wonder how to use it in a router-based network. I’ve got the following question from one of my readers:

I was wondering if it was possible to design a redundant network where the core uses L2 MPLS, the provider edge uses L2 for access but the customer edge equipment uses L3 Routers. We don't want to customer to see any STP at their routers.

Of course you can do that. There are two scenarios to consider:

(A) The Service Provider is offering point-to-point Ethernet service (pseudowire). In this case, two of the customer routers would be connected with what looks like a point-to-point Ethernet link. Usually the remote site would have just one "outside" Ethernet connection while the hub site would have numerous links bundled in a trunked (VLAN) link.

(B) The SP is offering VPLS service. In this case, all customer routers appear as being connected to the same Ethernet segment.

In both cases, the customer edge (CE) routers should treat the SP Ethernet link as a simple LAN segment, in case (A) connecting two routers, in case (B) connecting many routers.

see 2 comments

What went wrong: SCTP

Someone really wants to hear my opinion on SCTP (RFC 4960); he’s added a “what about SCTP” comment to several Internet-related posts I wrote in the last weeks. So, here are my totally unqualified (I have no hands-on experience) thoughts about SCTP.

Let me reiterate: I’m taking a 30,000-foot perspective here and whatever I’m writing could be completely wrong. If that’s the case, please point out my mistakes in your comments.

From the distance, the protocol looks promising. It provides datagram (unreliable messages), reliable message (record) and stream transport. Even more important, each connection can run across multiple IP addresses on each endpoint, providing native support for scalable IP multihoming (where each multihomed host resides in multiple PA address blocks from various Service Providers).

read more see 15 comments

Questions on BGP AS-path filters in non-transit networks

I’ve sent a link to my “Filter excessively prepended AS-paths” wiki article as an answer to a BGP route-map question to the NANOG mailing list and got several interesting questions from Dylan a few hours later. As they are pretty common, you might be interested in them as well.

In my environment, we are not doing full routes. We have partial routes from AS X and then fail to AS Y. Is their any advantage for someone like me to do this, as we are not providing any IP transit so we are not passing the route table to anyone else?
read more see 2 comments

Trigger EEM applet with SNMP

Anderson sent me an interestion question:

I'd like to use the snmpset command to get my router to execute an EEM script. Are there OIDs that are associated with EEM scripts that could help me achieve this?

Although EEM has associated MIB, it has a single read-write variable: the size of the history table. It's thus not possible to use EEM MIB to trigger EEM events. However, EEM 2.4 added support for SNMP notification events, which you can use to trigger EEM applets based on incoming SNMP traps/informs.

You can therefore use the event snmp-notification command on a router and the snmptrap command on a Linux host to remotely trigger EEM applets.

Read more in the Trigger EEM applets with SNMP Informs article in the CT3 wiki.

This article is part of You've asked for it series.

add comment

Inter-VRF static routes

Swapnendu was trying to implement inter-VRF route leaking in multi-VRF environment without using route targets. He decided to use inter-VRF static routes, but got concerned after reading the following paragraph from Cisco’s documentation:

You can not configure two static routes to advertise each prefix between the VRFs, because this method is not supported. Packets will not be routed by the router. To achieve route leaking between VRFs, you must use the import functionality of route-target and enable Border Gateway Protocol (BGP) on the router. No BGP neighbor is required
read more see 8 comments

Recommendations for keepalive/hello timers

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.

read more see 3 comments