Question about distribute lists and route summarization

broberts

Hey guys, I'm working through the 101 CCNP labs workbook and looking at one of the labs, the task is:

Configure routers R2 and R3 to advertise ONLY the default route to R1 and R4.

To do this I just configured a summary-address to be advertised to R1 and R4:

ip summary-address eigrp 1 0.0.0.0 0.0.0.0

And that seemed to work just fine to advertise just a default route. But the solution shows that they used distribute lists in addition to summary addresses to permit only a default route to the R1 and R4 neighbors... Question is, would just the summary-address work? I am trying to think of scenarios where other routes may be advertised through as well, but can't seem to think of any. Are the distribute lists necessary?

This is the solution they proposed:

R2(config)#ip prefix-list DEFAULT-ONLY seq 5 permit 0.0.0.0/0
R2(config)#router eigrp 1
R2(config-router)#redistribute connected
R2(config-router)#distribute-list prefix DEFAULT-ONLY out serial 0/0
R2(config-router)#exit
R2(config)#interface serial 0/0
R2(config-if)#ip summary-address eigrp 1 0.0.0.0 0.0.0.0
R2(config-if)#exit

fredrikjj

broberts wrote: »

Question is, would just the summary-address work?

Yes, I believe that the default behaviour for EIGRP summaries is to suppress the prefixes that make up the summary so their filtering with distribute list is unnecessary.

I am trying to think of scenarios where other routes may be advertised through as well, but can't seem to think of any.

You can for example leak certain prefixes to do traffic engineering using the longest match rule. If I remember correctly (it's been over a year since I did that workbook), there's an EIGRP lab where you asked to do just that. This can be done with an extra feature called leak map.

Are the distribute lists necessary?

In this case it seems like they simply forgot that the default behaviour is to suppress the component prefixes (probably because in BGP the components are advertised unless you actively suppress), but distribute lists could be used for other types of filtering where summary routes would be unsuitable.