BGP Help

[Amin Shaikh]

Hello,

Our HQ is connected to branch office with point to point connection but soon will use service provider MPLS cloud.

At HQ we use OSPF routing protocol on LAN and BGP will be for MPLs.

on Router which is terminating service provider connection for MPLS do I need redistribution of OSPF to BGP and vice-versa.

[Peter Paluch · Accepted solution]

Hello,

Writing "network 0.0.0.0 0.0.0.0 area 0" in OSPF configuration will include all directly connected interfaces into OSPF area 0. It is a cheat trick to quickly include all configured interfaces - present and future - into a routing protocol.

That being said, I would never use this in a production network. It is far too dangerous - it runs OSPF on all interfaces including those which you perhaps don't want to advertise and which may not be well secured.

Regarding the "network" command - personally, in a production network, I think it is better to be more specific than less specific.

Best regards,

Peter

[Giuseppe Larosa · Accepted solution]

Hello Amin,

as Peter I see many possible disavantages in using a command like

network 0.0.0.0 255.255.255.255 area 0

sooner or later you may want to move to a multi-area OSPF design.

A command like that is like a permit any any in an ACL.

network area commands can be seen as ACL statements where the order of commands care.

So if you add later a command like

network 10.10.10.0 0.0.0.255 area 1

it would be not effective and you would need to remove

network 0.0.0.0 255.255.255.255 area 0

to change the order of commands.

For this reason I would use a network command for each subnet or groups of subnets leaving alone the links to the core so that you can easily change without losing ip connectivity to device.

Hope to help

Giuseppe

[Peter Paluch]

Giuseppe,

Just a single correction: if there are overlapping "network" statements in OSPF configuration then the rule of the closest match applies. If you have two interfaces, Fa0/0=10.0.1.1/24 and Fa0/1=10.0.10.10/24, and you configured:

network 10.0.0.0 0.0.0.255 area 0

network 0.0.0.0 0.0.0.0 area 1

then Fa0/0 will be in Area 0, the Fa0/1 will be in Area 1.

Best regards,

Peter

[Peter Paluch]

Oops, a slight typo: I should have written "network 0.0.0.0 255.255.255.255 area 0"... Sorry.

[Amin Shaikh]

Thank You Both Gentlemen

[Giuseppe Larosa]

Hello Peter,

I've probably have in mind the OSPF behaviour of some years ago that we experimented in lab.

It is possible that this has been changed later, but I never tried again.

Actually, in 12.4 Command Reference

http://www.cisco.com/en/US/partner/docs/ios/iproute/command/reference/irp_osp2.html#wp1013362

says that most specific statement is used but there is also the following note:

>>Any individual interface can only be attached to a single area. If the address ranges specified for different areas overlap, the software will adopt the first area in the network command list and ignore the subsequent overlapping portions. In general, we recommend that you configure address ranges that do not overlap in order to avoid inadvertent conflicts.

that is an ACL logic like first match out.

Hope to help

Giuseppe

[Peter Paluch]

Hi Giuseppe,

Thanks again for your kind advice. I have checked the command reference but I must admit that it is not clear for me what does it mean to "ignore the subsequent overlapping portions". The page is not very clear on this.

Nevertheless, I gave it a try right now. No matter which order of "network" commands I used, the router always took the most specific match.

But this all is just an implementation issue... The bottom line is that using overlapping network statements is a Bad Thing (TM) :)

Best regards,

Peter