Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

An EIGRP question

I have an interface as follows:

int fa0/0

ip summary-address eigrp 1 0.0.0.0 0.0.0.0 5

router eigrp 1

passive-interface default

no passive-interface vlan1

network 10.0.0.0

no auto-summary

Because I'm advertising a default-route using the summary-address command will I still advertise other routes over thaat interface or am I just limited to advertising that default route?

Just curious.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: An EIGRP question

Mitch

I appreciate wanting to understand as much as possible about EIGRP. I have spent quite a bit of time working toward that objective myself. In my experience when I am trying to understand a particular topic I find it useful to simplify and look closely at that topic by itself. When I get more comfortable with it, then I add complexity and combine topics. But you are certainly free to approach it any way that you wish.

So if you are trying to understand passive-interface I would offer this explanation: passive-interface deals with intefaces that have matched a network statement. The default is for every interface that matches the network statement to be active and to send hello messages looking for a neighbor. In some cases you may not want the interface to send hello messages and can make it passive (for example an interface onto a segment where there are only end stations and no other router).

That default works well if most of the interfaces are active and you want only a few interfaces to be passive. But what if most of the interfaces will be passive (for example a router/switch with 10 interfaces to user VLANs and 2 uplinks). If you want the active interfaces to be only the 2 uplinnks and the 10 user VLAN interfaces to be passive, it is easier to make the default passive and then activate the interfaces that you want.

So you can establish the default to be the behavior that matches the greatest number of interfaces (active or passive) and then individually set the other interfaces.

Thanks for rating posts that are helpful.

I encourage you to continue to be active on the NetPro forums.

HTH

Rick

7 REPLIES
Hall of Fame Super Silver

Re: An EIGRP question

Mitch

In general when you configure a summary address such as this you would expect that any more specific routes within the summary would not be advertised.

What you have posted is a bit unusual. You have posted interface fastethernet0/0 with a summary address and router eigrp 1 which makes all interfaces passive by default and then makes vlan 1 not passive. It is not clear what the relationship is between vlan 1 and fastethernet 0/0. Given this I am not sure that anything is being advertised out fastethernet 0/0.

Perhaps you can provide a bit more detail on how the router is configured?

HTH

Rick

New Member

Re: An EIGRP question

I'm trying to sort out this eigrp passive interfaces in my head. With my example I'm assuming that the advertised default route is the only routing information that the neighbor will see, otherwords, is it saying, the attached networks to fa0/0 can have me as a default route if it doesn't have a more specific route in it's routing table. I'm not trying to do best practices here, I'm trying to understand what eigrp can do with every possible command.

Hall of Fame Super Silver

Re: An EIGRP question

Mitch

I believe that you are making things more complicated than they need to be, especially when you combine passive interface with summary-address in the same question. Looking at the explanation in this post it seems like you believe that they are related and they are quite distinct.

Passive interface controls whether EIGRP will send hello messages on an interface which matches a network statement (and indirectly controls whether EIGRP will send or learn routes on that interface). If the interface is active then EIGRP will send hello messages and if there is an active EIGRP router on that segment then they will form a neighbor relationship (assuming other parameters match such as AS number) and will exchange routes. If an interface is passive then EIGRP will not send hello on that interface and therefore will not discover any neighbors and will not send nor learn routes on that interface.

All of that is quite distinct from the behavior of summary-address. The way that you have constructed your example the router will advertise the 0.0.0.0 and not advertise any more specific routes. (assuming that the interface is active and not passive)

You might also consider the alternative that EIGRP learns and advertises a default route along with the more specific routing entries in its routing table.

HTH

Rick

New Member

Re: An EIGRP question

Rick,

thanks for your explanations. All I'm really trying to do is understand as much as I can about EIGRP. The topic I was playing with was the topic of Passive-interface default. When I'm trying to learn something like this I try my best to throw as much as I can at the topic. While I understand the summary-address quite well, I don't fully understand the passive-interface command, especially if you have a switch with many vlan interfaces that you don't want to be passive. Honestly, I really don't know why someone would announce passive-interface default then place a no-passive in front of an interface. That's the part I was trying to understand. For some odd reason I decided to throw the summary-address in there also.

Thanks for all your explanations.

Hall of Fame Super Silver

Re: An EIGRP question

Mitch

I appreciate wanting to understand as much as possible about EIGRP. I have spent quite a bit of time working toward that objective myself. In my experience when I am trying to understand a particular topic I find it useful to simplify and look closely at that topic by itself. When I get more comfortable with it, then I add complexity and combine topics. But you are certainly free to approach it any way that you wish.

So if you are trying to understand passive-interface I would offer this explanation: passive-interface deals with intefaces that have matched a network statement. The default is for every interface that matches the network statement to be active and to send hello messages looking for a neighbor. In some cases you may not want the interface to send hello messages and can make it passive (for example an interface onto a segment where there are only end stations and no other router).

That default works well if most of the interfaces are active and you want only a few interfaces to be passive. But what if most of the interfaces will be passive (for example a router/switch with 10 interfaces to user VLANs and 2 uplinks). If you want the active interfaces to be only the 2 uplinnks and the 10 user VLAN interfaces to be passive, it is easier to make the default passive and then activate the interfaces that you want.

So you can establish the default to be the behavior that matches the greatest number of interfaces (active or passive) and then individually set the other interfaces.

Thanks for rating posts that are helpful.

I encourage you to continue to be active on the NetPro forums.

HTH

Rick

Bronze

Re: An EIGRP question

I have never seen that passive command before but this link seems to explain it well in case you haven't seen it. It explain's the reason and its in line with Rick's comments.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_guide09186a008008784e.html#wp7841

Daniel

New Member

Re: An EIGRP question

I've been looking for a worthy document that showed the passive-interface default in all its glory, and you found it. Thank-you and Rick for your help in this topic.

243
Views
10
Helpful
7
Replies