Number of BGP routes a large ISP sees in total

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1

Hi,

Could anyone give me a sense how many BGP routes a large ISP typically sees in total?
Here by "in total", I mean if the RIB of all routers in the ISP were merged, how many distinctive
routes would there be?

Thanks,
Yi

2

Yi Wang wrote:

Hi,

Could anyone give me a sense how many BGP routes a large ISP typically
sees in total?
Here by "in total", I mean if the RIB of all routers in the ISP were
merged, how many distinctive
routes would there be?

Google(route bgp) "I am feelink lucky"

aka first hit, and just look around there.

Greets,
Jeroen

3

I should have said I'm interested in the merged size of RIB-In (which contains all the raw
routes received).

I couldn't find information about the number of different routes for the same prefix
a (large) AS typically receives/learns. Hints?

Yi

4

I'd suggest taking a look at the RIBs from routeviews.org or RIPE and performing an analysis on same. Some SPs also offer public routeservers.

A few minutes with a search engine should prove fruitful in this regard.

5

Telnet to any of these route servers:
http://www.bgp4.net/wiki/doku.php?id=tools:ipv4_route_servers

and do "show ip bgp"

--Ricardo

6

I guess what I see there is the lower bound of the path diversity? Because even
though an edge router received more than one path for a prefix, it'll only export
the best route to the other edge routers of the ISP.

Yi

7

Depends on the route server. If the route server has sessions with lots of edges, it will have lots of prefixes. If not, then it's generally got the same number of prefixes as you see in the CIDR report.

But you said: "if the RIB of all routers in the ISP were merged, how many distinctive routes would there be?". Define "distinctive"? Are you including things like same prefix & path, but different next hop?

If my guess is correct, the answer is "it varies". Some networks have literally a dozen or more interconnection points. If the networks are large, then you have 10s of 1000s of prefixes, with dozens of next hops, and perhaps multiple that by multiple networks. Then realize that many of the prefixes are duplicated across multiple peers and ....

Anyway, the number is very difficult to determine. And it is highly dependent on the network you look at.

Mind if I ask why you want to know? Perhaps there are some simplifying assumptions we can make, depending upon your application?

8

Hi Patrick,

I guess what I see there is the lower bound of the path diversity? Because even
though an edge router received more than one path for a prefix, it'll only export
the best route to the other edge routers of the ISP.

Depends on the route server. If the route server has sessions with lots of edges, it will have lots of prefixes. If not, then it's generally got the same number of prefixes as you see in the CIDR report.

But you said: "if the RIB of all routers in the ISP were merged, how many distinctive routes would there be?". Define "distinctive"? Are you including things like same prefix & path, but different next hop?

Exactly. Because this may be useful for traffic engineering.

If my guess is correct, the answer is "it varies". Some networks have literally a dozen or more interconnection points. If the networks are large, then you have 10s of 1000s of prefixes, with dozens of next hops, and perhaps multiple that by multiple networks. Then realize that many of the prefixes are duplicated across multiple peers and ....

Anyway, the number is very difficult to determine. And it is highly dependent on the network you look at.

Mind if I ask why you want to know? Perhaps there are some simplifying assumptions we can make, depending upon your application?

OK, let me try to put it this way:
If there was a single router (despite the scalability for a moment) that maintains
eBGP sessions with all the ISP's neighbors, how many routes (i.e., the "prefix, AS path,
next-hop" three-tuple) would it learn on average for each prefix?
I'm interested in the number of *raw* routes per prefix an ISP can possibly learn, before any policy filters are applied. I know there is no single number. Just want to have a rough
sense about the average (e.g., about 5? 10? 20?), as for a "large" ISP.

Thanks,
Yi

9

Well, if you're interconnecting with other large ISPs in 5 places then you'll get each prefix at least 5 times. Having 5 eBGP sessions between two ASes is quite common if both are large ISPs. So yes, I'd say that between 5-10 is quite common.

10

At least 5, and more than 10 for many prefixes, inside very, very large networks.