Let’s get an IPv4 default route into EIGRP.  There are a few methods to do it.  I hate most of them, though.  I think it will be obvious which one I like.

Here’s the lab I have set up to test everything.  I want R4 to generate the default in each case.

Default Network - Candidate default.  I don’t think I’ve ever used that all my years in networking, but here’s how to use it in EIGRP for a default route.  You basically say “If you don’t know where to send a packet, send it to where network X lives.”  We’re going to set the 192.168.1.0/24 as the default network, so, in our case X = 192.168.1.0. R4 will tag that route as a default candidate when it advertises it to the rest of the network.  The config is easy but requires a classful (yes, classful) network to be configured as the default.

For both OSPF and EIGRP routers to become neighbors, their interface’s primary IP address must be on the same subnet. That statement is true. There is a difference in the definition of “same subnet”, though.

In OSPF, both routers have to be configured to be on the same subnet with the same mask or else they won’t neighbor up.  When an hello packet is sent, the subnet mask is sent embedded in there.  The router does a quick look to be sure the subnets are defined the same way on both ends.  If everything doesn’t match, they don’t neighbor. Here’s a Wireshark screenshot to show you the OSPF hello.  Note: See edit below.

I wanted to do some analysis of the EIGRP topology table last night, so I fired up a small lab. I was especially interested in how external routes appear there and compare to internal entries. Like all good scientific endeavors, the whole thing got derailed when I made a realization.

Here’s the lab I set up. You can ignore the IPv6 info for this exercise.

It’s a simple little thing.  All the networks you see are included in EIGRP 100 for simplicity.  I limited the network statements to 192.0.2.0/24 to keep my options open. I went ahead and added Loopback100 on R3 with an address of 3.3.3.3/32 and added a redistribute connected with a route-map to get the route out in the wild.  Here’s what I had.

Per the standard rules, please correct anything that’s wrong.

One of EIGRP’s big features is the ability to use unequal cost paths for load balancing.  This is done with the variance command.

variance : A multiplier used to calculate which feasible successors can be used as active routes.  The router takes integer and multiplies it by the successor’s feasible distance, and any FS with a an FD less than this new number gets submitted to the routing table manager.

I’m not going to go all out like Jeremy over at Packetlife.net has, but I’m going to start to discuss a few IPv6 topics.  In time (like in September when APNIC runs out of IPv4 addresses), I’m sure I’ll ramp up the IPv6 talk, but let’s start easy and get EIGRP for IPv6 up and running.  

Configuration

There are quite a few differences between EIGRP for IPv6 (yes, that’s an official name) and the IPv4 version.  First of all, all IPv6 routing is disabled by default on a Cisco router, so, if you’re doing any routing in IPv6, you’ll want to enable it or risk smashing your head into the desk trying to figure out what’s going on.

As always, corrections are requested.

Study Questions

• I’ve got IGRP and EIGRP both configured with the same AS number.  What’s special about this configuration?

If both use the same AS number, then they automatically redistribute their routes into each other without using the redistribute command.

• When redistributing one IGP into another, where’s a good place to filter routes?

There’s no one good place, but at the router(s) that’s doing the redistribution is a good start.  There’s no need to send an IGP a bunch of routes it doesn’t need.

I didn’t do so well on IGP redistribution the last time out, so here’s some more stuff to study.  As always, feel free to correct.

Study Questions

• What three things are needed to be able to redistribute one routing protocol into another?

1. One or more links into each routing protocol 2. A proper, working config for each protocol 3. The addition of the redistribute command to one or more of the protocols

Study Questions

• Why would anyone develop a version of RIP that supports IPv6?

I have no idea.  Boredom, maybe.  Whatever the case, it works just like RIPv2, which is pretty scary.

• In EIGRP for IPv4, there are several requirements for two routers to neighbor up.  Which of those is not true for EIGRP for IPv6?

The two routers don’t need to be in the same subnet.  The concept of the link local address takes care of that need since neighbors always share a common medium like an Ethernet segment or a serial link.

As always, feel free to correct.

Study Notes

• When a router redistributes from one routing protocol to another, where does the router get the list of routes to redistribute?

From the routing table.  Only IGP A’s routes (not topology or successors) are redistributed into IGP B’s domain.

• What are two methods of filtering redistributed routes?

Use a route-map in the redistribute line or a distribute-list.

• Of the two methods for filtering, which one has more options?

The route-map method has more options.  You can match on all sorts of stuff, including an ACL or interface, and filter based on that.

As always, feel free to correct.

Study Questions

• When you redistribute OSPF into EIGRP, what are you really redistributing?

Routes knows via OSPF Networks of OSPF-enabled interfaces

• What’s the default cost of an EIGRP route redistributed into OSPF?

20

• What’s the default metric of an OSPF route redistributed into EIGRP?

There is none since EIGRP has all those nifty k-values that have to be processed.  Routes actually won’t redistribute without them.

Corrections welcome.

Study Questions

• Why would you ever want to summarize routes?

Summarizing routes minimizes the routes advertised to the network.  For example, instead of advertising 192.168.0.0/24, 192.168.1.0/24…192.168.n.0/24, a router can advertise a single route to 192.168.0.0/16.  Keeping routing tables small saves hardware resources, minimizes convergence times, helps avoid route flapping, and makes the routing table easier to read for humans.

• When will an EIGRP router auto-summarize a route?

If a router has interfaces that that are in different classes of network (Class A, B, C), then that router will auto-summarize those routes up to the classful boundary.  For example, if you have a 10.0.0.1/24 and a 192.168.100.1/30, the router will advertise 10.0.0.0/8 and 192.168.100.0/24.

Or neighborships, as they call it in the book.  What a terrible word.

Study Questions

• What settings must match between two routers in order to become EIGRP neighbors?

Both routers must be in the same primary subnet Both routers must be configured to use the same k-values Both routers must in the same AS Both routers must have the same authentication configuration (within reason) The interfaces facing each other must not be passive

Study Questions

• How do you keep EIGRP from killing your WAN?

You can use the ip bandwidth-percent eigrp AS X command to limit the amount of bandwidth that EIGRP uses to update neighbors.

• How does EIGRP calculate how much bandwidth it can use for each frame relay PVC?

By default, EIGRP takes 50% of the (sub)interface’s configured bandwidth (with the bandwidth command) to use for updates on NBMA (non-broadcast mutliaccess) networks like frame relay.  This value is divided equally among all the PVC configured on that interface.

I’ve seen and used the command before, but I’ve never really seen any use of the show ip protocols command until tonight while reading up for my ROUTE test.  There’s a lot of good information in the output, and, from the way the book is reading, this is a great candidate for use in a lab question.

To check it out a bit, I set up a small network with four routers connected only to a single Ethernet segment.  I set up one router to run EIGRP, OSPF, and BGP to each one of the other routers just so I could see the output for the different routing protocols.  Here’s what puked out after struggling with GNS for a few minutes.

Last time, we talked about a nifty little lab I set up for redistribution and how the OSPF ASBRs acted a little differently than I expected.  This time, let’s look at how changing external OSPF routes to a metric-type of 1 (E1) affects the routing tables.

Here’s the network again.

The static routes are being redistributed into their respective IGPs, and EIGRP is being redistributed into OSPF.  Let’s look at the routing table on R1.

I just got back from Global Knowledge’s ROUTE class, and I must say that it was a great class.  John Barnes puts on quite the show and is the best instructor I’ve ever had.  I digress, though.

One of the topics we covered was route redistribution, so I went back to the hotel one night and fired off this network in GNS3 to study a bit.

The object was to see how redistributing statics into OSPF and into EIGRP differ.  It was also an opportunity to see how EIGRP redistributes into OSPF (and OSPF into EIGRP, but I didn’t make it that far).  To do that, I redistributed 10.10.10.0/24 from R1 into OSPF and 10.10.20.0/24 from R4 into EIGRP.  I then had R2 and R5 redistribute all EIGRP routes into OSPF.  It’s a nice mix, but I saw some weirdness in the paths to 10.10.20.0/24.

Here’s a handy list of ACL entries to allow your devices to speak routing protocols, availability protocols, and some other stuff. We’ll assume you have ACL 101 applied to your Ethernet inbound; your Ethernet has an IP of 192.168.0.1.

• BGP : Runs on TCP/179 between the neighbors

access-list 101 permit tcp any host 192.168.0.1 eq 179

• EIGRP : Runs on its own protocol number from the source interface IP to the multicast address of 224.0.0.10

access-list 101 permit eigrp any host 224.0.0.10

I realized the other day that I haven’t mentioned EIGRP once. As a Cisco guy, I think I’m required to do at least one article on it, so here it goes.

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol. Routing protocols share routes, right, but “interior” is the keyword here; it’s used to distribute routes on your internal network (Contrast that with BGP, which is allows you to share your routes with others). In a nutshell, each router in the EIGRP cloud tells everyone what subnets it has connected to him.  A receiving router then combines that information with everything that it already knows and passes on any new information.  Do that recursively for a while, and, eventually, every routers knows all the subnets in the network.