Category: you've asked for it

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.

Jernej sent me an interesting question: “does Cisco IOS have an equivalent to the Extremeware’s show bgp neighbor a.b.c.d rejected-routes command which displays all routes rejected by inbound filters?”

Short answer: it doesn’t.

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).

Shahid wrote me an e-mail asking about local command authorization. He would like to perform it within the AAA model, but while AAA local authorization works, it only allows you to specify user privilege level (and autocommand), not individual commands (like you can do on a TACACS+ server).

A reader who prefers to remain anonymous has reported an interesting observation: autocommands configured on local usernames do not work after configuring aaa new-model.

I've got the following question from Matthew: »how would one go about matching the default route for filtering using standard ACLs?«

In all routing protocols but EIGRP (which can carry the »default candidate« flag on any IP prefix), the default route has IP address 0.0.0.0 and subnet mask 0.0.0.0.

To match the default route with a standard ACL, use access-list x permit 0.0.0.0. To match it with an extended ACL (which matches the IP address and the subnet mask portions), you have to use access-list y permit ip host 0.0.0.0 host 0.0.0.0. And finally, to match the default route in a prefix list, use ip prefix-list z permit 0.0.0.0/0.

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

The readers preparing for various certification exams are a constant source of amazing details, including this one:

I have configured ip ssh timeout 60 and exec-timeout 5 on VTY lines. Preferred input connection is ssh. How much time can I be idle?

According to the IOS documentation, the ip ssh timeout detects the problems in SSH negotiation phase (including user authentication) and the exec-timeout detects user inactivity after the user has logged in.

Do not set ip ssh timeout to a very low value or you won’t be able to type your password before the router disconnects the session.

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

Ali sent me a question that should bother every networking engineer:

Could you explain how Cisco [or another vendor] comes up with the throughput parameters in a products datasheet? For example if a vendor says that "if IPSec is turned on the throughput is 20Mpps", exactly what does it mean? What is the packet size he is referring to and what are the implications here, because very seldom do we have fixed packet sizes in a traffic flow.

The answer, as always, is "it depends". If you're reading a serious performance analysis report, it should document the test procedures, including the packet sizes. If you're getting a "marketing" figure with no further explanation, you can be sure it's been cooked as much as possible. For example, a Gigabit Ethernet link sometimes has 2 Gbps performance (in-and-out) and in case of IPSec packet-per-second values, they are most probably measured with optimal (in this case low) packet size.

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

I’ve got this question from Pete:

Which community will be sent if only "neighbor {ip-address} send-community" is configured?

Quick answer: only the standard BGP communities are propagated.

A member of the cisco-nsp mailing list asked an interesting question a while ago: he tried to test his EEM applet with the event manager run command and got the “Embedded Event Manager policy not registered with event none Event Detector” message.

An EEM applet (until EEM 3.02.4) can be triggered only by a single condition. If you want to trigger the applet from the command line (with the "event man run" command), it cannot be triggered by anything else. Such an applet must have "event none" pseudo-trigger.

The event none is used to indicate that "no trigger" is actually what you want to do (as opposed to "I forgot to specify the trigger").

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

One of the readers of my “When would an MPLS LSR have Untagged output label?” post made an interesting comment:

When a loopback network is advertised as 1.1.0.0/16, it's seen as »pop tag« on the neighboring router and I can't see it in the »show mpls forwarding« printout on the local router. What's going on?

As explained in the “When would an MPLS LSR have Untagged output label?” post, the Untagged (also displayed as No label in recent IOS releases) value means that the Label Switch Router (LSR) cannot use the inbound label to decide what to do with the packet and has to perform layer-3 lookup.

A reader sent me an interesting question:

Do you have any advice for resetting/logging into a router (2821) where the one time user of cisco:cisco has already been used?

I couldn't offer any better advice than performing the regular password recovery procedure. Is there another solution?

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

What would you think if you’d receive three queries about the same (somewhat obscure) feature within six hours? It started with a nice e-mail from an engineer that I’ve corresponded with in the past. He wanted to send a Wake-on-LAN packet to a PC in a remote office. Usually you could use the ip directed-broadcast feature, but he wanted to use the remote office router to generate the packet.

One of my readers sent me an interesting question a while ago:

I reviewed one of your blog posts "Per-Destination or Per Packet CEF Load Sharing?" and wondered if you had investigated previously on how MQC QoS worked together with the CEF load-sharing algorithm (or does it interact at all)? For example, let's say I have two equal cost paths between two routers and the routing table (as well as CEF) sees both links as equal paths to the networks behind each router. On each link I have the same outbound service policy applied with a simple LLQ, BW, and a class-default queues. Does CEF check each IP flow and make sure both link's LLQ and BW queues are evenly used?

Unfortunately, packet forwarding and QoS are completely uncoupled in Cisco IOS. CEF performs its load balancing algorithm purely on source/destination information and does not take in account the actual utilization of outbound interfaces. If you have bad luck, most of the traffic ends on one of the links and the packets that would easily fit on the other link will be dropped by the QoS mechanisms.

You could use multilink PPP to solve the problem in low-speed environments. With MLPPP, CEF sends the traffic to a single output interface (the Multilink interface) and the queuing mechanisms evenly distribute packet fragments across the links in the bundle.

In high-speed environments, you can only hope that the number of traffic flows traversing the links will be so high that you’ll get a good statistical distribution (which is usually the case).