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.

• Tagging provides a way to mark common or similar routes to manipulate later.
• In redistribution scenarios with mutual redistribution on two different routers, any routes that gets redistributed from one route process to another are tagged.
  • When the other router sees those tags on the route, that route to keep from adding non-optimal routes to its routing table.
• Tags can also be used to do other manipulation such as setting higher metrics or changing ADs.

OSPF

EIGRP allows you to tag external routes.  That is, any route redistributed into EIGRP can be tagged with a numeric descriptor from 0 to 4294967295.

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.

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.