EIGRP go from Single AS to Multiple AS

Unanswered Question
Feb 19th, 2010

Hi Folks, 

I have design problem for EIGRP. Looking to switch from a Single AS to multiples Autonomous Systems.  What are some gothcas to look out for? 

a) have AS for Backbone

b) have AS for Each hub Site with Localy connected hub sites 

c) have AS for  VPN tunnel  for backup connectivty to a Hub site. 

d) Do I need filter at Hub Sites? Redistributing at Hub sites only?


1) Back Bone is  MPLS connecting  Hub sites

2) Connections to HUB site can be via VPN-IPSEC Tunnel, MAN Link using Ethernet

3)  Traffic Carried across is netowrk, Data, VOIP etc...

4) Only TCP/IP Protocol used

5) Different Domestic carrier. Backbone supplied by one vendor

6) One HUB site will have a VPN router providing Backup Connectivty if a Hub Site gets cut from  MPLS backbone. Providing VPN access   all individual sites

7) Currently the Routing protocol used is EIGRP with a Single  AS  

              HUB-A=======MPLS==========HUB-B ==========MPLS===========HUBC      

                 l                                                       l                                                           l                                                                                                  LSW3                                               LSW3                                                   LSW3     

     VPN Router                                VPN Router     \                                                   VPN Router

       /     \                                             /              MAN-link                                           /   \                     

     /        \                                          /                /            \                                          

VPN      VPN                                   Site B-1       LSW3      LSW3

  /                  \                                               SiteB-2     Site  B3                                                           l                                                      

SiteA1..10     Domestic MAN 


              Site A1B    Site A2B

Thank you for any help in advance

I have this problem too.
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Giuseppe Larosa Sun, 02/21/2010 - 23:40

Hello Rshocker99,

in your complex network scenario the need to use multiple EIGRP AS numbers may come if primary links are MPLS L3 VPN links using BGP as PE-CE protocol.

If so the risk is to see the desired backup routes, those via VPN/IPSec tunnels, to be considered the primary routes for the fact that using a single EIGRP AS domain would make those routes internal routes and could be preferred over the external routes derived from redistribution of BGP into EIGRP.

Under this hyphotesis, the use of a second EIGRP AS extending over VPN connections to make them the secondary routes.

I would not see the need for more then two EIGRP AS numbers unless you have two different backup networks.

note: if you are using EIGRP on primary backbone links you may want to consider the use of EIGRP tools and to use only one AS.

That is, playing with delay or offset-lists or also with ip eigrp summary-addess can be a way to deal with this.

Most specific routes are used first, so a way to make secondary paths to be backup is to advertise over them less specific routes.

This may be feasible or not depending on your address plan (if it allows for route summarization at remote site).

Hope to help



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