summary route distribution (LSA Type 5)

I have a question on how summary route distribution works from an external source (say, BGP to OSPF as in the example on page 362 of the CCNP Route guide). I mean the "area range" and/or "summary-address" OSPF commands.

Say BGP connects to only Area 0 of OSPF via ASBR1 and ASBR2.

Then let's say Area 0 also connects to Area 1 (via ABR1) and Area 2 (via ABR2).

Let's say we want Areas 0, 1, and 2, to have a default route for internet traffic, but we also want the OSPF areas to speak with each other.

I THINK, that if you use summary-address to redistribute 0.0.0.0/0 into OSPF from ASBR1 and ASBR2 that no other routes would come from BGP (since all, by definition, are subnets of 0.0.0.0/0). So far, this is fine.

What I want to know, is how then is this handled on ABR1 and ABR2 when they send Type 3 LSAs to Area 1 and Area 2 from Area 0?

I THINK that unless you summarize again on ABR1 and ABR2 (and I'm assuming we're not), that subordinate routes of 0.0.0.0/0 will still be sent as an Type 3 LSAs and all will be fine.

Because if they are not, then this would cut off Area 1 and Area 2 from each other and from any Area 0 routes.

I hope my question makes sense.

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Comments

Phiers

I don't believe you can use summary-address or area range commands with a default route. IOS will reject the command if I'm remembering this correctly. The simplest solution is to use default-information originate instead of trying to reinvent the wheel. Then the Type 3s come from the different areas, and everyone has a default to the internet.

smcclenaghan

Phiers wrote: »

I don't believe you can use summary-address or area range commands with a default route. IOS will reject the command if I'm remembering this correctly. The simplest solution is to use default-information originate instead of trying to reinvent the wheel. Then the Type 3s come from the different areas, and everyone has a default to the internet.

Hey, thanks.

Page 361 in the 300-101 cert guide seems to say you use summary-address on the ASBR and area range for the ABR (at least in the example they're using).

Phiers

I saw it in the book. However, neither works with defaults according to my gear and a few threads when I googled the topic. Area range errors out, and summary-address goes in with a "not-advertise".

I'm reading the OCG now as my final prep, and there seems to be a few examples where the technology doesn't work exactly like the book says it does. DMVPN's nhrp resolution request and response for spoke to spoke are the first that come to mind. The book says the hub sends back a response to the nhrp request. However, if you watch a packet capture, the hub forwards to request to the destination spoke and that spoke replies back to the original spoke directly which brings up the S2S tunnel. I call it the NHRP triangle to help me remember the data flow for standing up spoke to spoke.

Here's some screenshots from my testing regarding the ospf topic.

summary-address 0.0.0.0 0.0.0.0 not-advertise

R2(config-router)#area 0 range 0.0.0.0 0.0.0.0 advertise
% OSPF: Cannot add this range as 0.0.0.0/0 represents default

OfWolfAndMan

Just an FYI. You cannot redistribute a default route into OSPF. OSPF does not support that function. Instead, you need to have a default route in the routing table prior and inject the default route using the following command: default-information originate If there is no default route in the routing table, then you can have ospf originate one by typing the following command instead: default-information originate always Also, if the Areas outside of 0 are a stub or total stub area, the ABR will generate a default route anyway. Source: https://www.rfc-editor.org/rfc/rfc2328.txt (Section 3.6)