FastEthernet Secondary addresses

Unanswered Question


I have a quick question. I have 7206 router in Production. Currently i have 2 FastEthernet interfaces. But if i wanted to use the Secondary addresses on these interfaces. How many secondary addresses i can have. ??? I have got ibgp/bgp and ospf running on the backbone router....Just wondering if any of you have seen any issues with Routing protocol if there is One Primary ip address defined under FastEthernet and couple of Secondary addresses. Any issues or concern with the routing protocol or router itself...if anyone has seen......Input will be apprecaited..


Hassan, Syed

interface FastEthernet0/1

ip address secondary

ip address secondary

ip address secondary

ip address

duplex auto

speed auto


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Richard Burts Mon, 08/20/2007 - 10:53

There is not any limitation to the number of secondary addresses that you can configure (other than the limit on the size of the config file).

The dynamic routing protocols can handle secondary addresses. There are a few restrictions to observe such as the fact that the routing protocols will form neighbor relationships only on the primary address. Otherwise it should work fine for you to go ahead and configure secondary addresses.



Thanks alot Rick,

I am planning to move the Firewall's to the interface. We have 3 Firewall's. So on my FastEthernet i already have Primary address defined. I was thinking of putting 3 different subnets under FastEthernet as secondary. Make a static route pointing for subnets sitting behind the Firewall and perform " redistribute static" under BGP. So i can make these networks be injected into BGP so it can send these networks to IBGP or EBGP peers through "Route-map"...

Rick...just wondering ....would the scenario work..or i am missing something...thanks alot man...



Richard Burts Mon, 08/20/2007 - 11:37


I think that the scenario should work ok.

One option to think about might be after you get the static routes configured would be to use network statements (perhaps with mask specified) to get those subnets into BGP rather then doing redistribute static. But it should work either way.



sundar.palaniappan Mon, 08/20/2007 - 11:03

I just would like to add something to Rick's post. It isn't an issue directly related to use of secondary address but ethernet in general. If all 3 ./24 subnets are populated fully, approx. 750 hosts, you may have an issue with high number of broadcasts that could slow down LAN connectivity considerably. Through use of switches/pruning etc. you can mitigate the impact of this but if you can then separate the different IP subnet traffic by use of VLANs etc.




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