ISP BGP default route advertisement- Use case

[nwekechampion]

Hi all,

What is the best use case for an ISP to advertise a

defaulte-route

to a customer? 

What are the benefits of this, as opposed to advertising a

full-routing table

to customer?

Regards

[Flavio Miranda]

Hi

 Benefit is simplicity and cost. 

 If you receive only a

default route

you need one router with given capacity but to receive the full routing table you are going to need a "how konws"  router capacity. 

 Why would I want to receive the whole ISP routing table if I can receive only one route? 

[MHM Cisco World · Accepted solution]

it not ISP issue it costumer side issue, 
it can receive 
1-full <<-if have dual ISP. full to make load balance 
2-partial  <<- if have dual ISP but there is specific public IP you want to connect 
3- only default <<- there is only one ISP 

[nwekechampion]

Can you please explain further when you say it is not isp issue?

Why do we need full table for dual isp? why cant we have or use floating default  back to ISP?

To be clear, I was talking about ISP advertising routes vis bgp to customer not the other way around.

Thanks

[MHM Cisco World]

ISP have already FULL table, it issue of costumer to order full/partial/only default 

[Richard Burts]

From the customer perspective there are costs associated with receiving partial routing table or full routing table from ISP. These costs include requires more memory to contain the larger routing table, consumes bandwidth to transmit routing updates, and increased cpu usage to process the routing updates.

If a customer has a single ISP there is no advantage in receiving partial routing table or full routing table. A

default route

is the only route a customer needs if there is a single ISP. If a customer has multiple ISP then there can be some advantage in having partial routing table or full routing table from ISP. The advantage is that it can be more efficient to forward some traffic using ISP A if those destinations are more closely connected to A and to forward other traffic using ISP B if other destinations are more closely connected to B.

[paul driver]

Hello
apart from the financial side of things - from a networking perspective the benefits would be the amount of rtr resource you save - as then your wan rtrs will not need to process a full or partial internet route table as the only route you would be receiving are those defaults and then you could easily traffic engineer your outbound traffic to utilise either isp link.

[nwekechampion]

Thanks guys!

So what would be a use-case for having a full or partial routing table?

[Joseph W. Doherty · Accepted solution]

Best use case for ISP to provide only a

default route?

For a single (customer) egress path (to Internet), as a full route table provides no benefit selecting egress port beyond dropping packets with unknown destination addresses AND you want confirmation that ISP is up.

Another benefit of a

default route vs

the full Internet table, the former is much, much less resource demanding (as also noted by other posters).

Real case example - worked at a firm that installed a pair of 3660 ISRs, each with a single DS3 to one of two ISPs.  Each ISR took a full Internet table from its directly connected ISP and from the other ISP via the peer ISR.  Firm also had its own AS.

A text book implementation.

Unfortunately, both ISRs were crushed by CPU load - principally by the BGP scanner!

Converting to just taking a

default route

from each ISP, dropped CPU load by about half.

BTW, also enabled (then new) OER.  The latter only slightly increased CPU load, but appeared to "route" better than having full Internet tables.

[nwekechampion]

Thanks Guys