Partial BGP routes in Cat 3650?

[Kevin-H]

Has anyone done BGP peering w/ an ISP, and received prtial routes w/o any problems?

We're thinking to do this on a 3650 switch, and weren't sure if it has enough resources to hold the partial routes.

How much memory does a route take?

3650 has 4GB of RAM on its datasheet, which seems adequate, but it also says it's limited to 24,000 IPv4 routes, which is probably not enough.

TIA

[Georg Pauwen]

Hello,

the Catalyst 3650 with IP Services has full BGP support. Where did you see the 14,000 route limit ?

For the full BGP table, you need approximately 1GB, so 4GB is plenty.

[Kevin-H]

Thanks for the quick reply.

The 24,000 v4 route capacity was taken from the datasheet:

https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3650-series-switches/datasheet-c78-729449.html

Total number of IPv4 routes (ARP plus learned routes)

24,000

DRAM

4 GB

[Georg Pauwen]

Hello,

odd, as in general, the size of the routing table is limited only by memory. Maybe they are referring to static routes ? Either way, full BGP support should mean full BGP routing table support...

[Predrag Jovic]

Size of routing table is not limited only by memory. Since 3650 is the swich, the problem is that routes need also to be loaded into line cards (IP cef) otherwise CPU will not be able to keep up, that's why there is limitation for number of routes.

[Peter Paluch]

Hi Georg,

Predrag is right. On switches, the routing table (what you see in "show ip route") actually has to be downloaded into the TCAM as all forwarding decisions are done using the data in the TCAM. In switches, TCAM is always a scarce and limited resource, and is much smaller - in terms of the number of elements it can hold - than the amount of RAM. This is where the limit for a maximum of 24K routes on a Catalyst 3650 comes from. RAM can obviously hold many more routes, but they won't all make it into the TCAM.

Catalyst 3650 is positioned as an access layer switch, and that pretty much defines its intended use case. It is possible to run "full BGP" in terms of all BGP bells and whistles you may be accustomed to, but the switch won't be able to handle the full BGP table in its hardware. If the datasheet specifies that the maximum number of routes is about 24K, then that is the true limitation, and is unlikely to ever be lifted, as it is derived from the TCAM resources the switch has available. Once again, this is an access layer switch, and 24K routes are a lofty reserve for an access switch; positioning it as an edge router with either a partial or a full BGP feed might be stretching it beyond its limits.

My two cents...

Best regards,
Peter

[Georg Pauwen]

Peter, Pedrag,

good stuff, thanks for that info, I didn't know that.

That said, I have never seen a (3650) switch on the edge of a large enough network to need the full BGP table. 

Which warrants the question to the OP: do you really need the full table...

[Kevin-H]

Thank you all for your participation in this thread - didn't anticipate so many people helping out in such a short time period :-)

Minimal requirement is just to receive the default route from two ISP's.

We never planned to receive full routes, knowing a switching platform is not going to be powerful enough, but was hoping to receive partial / customer routes.

One of the ISP's said they had over 80,000 routes just for partial, so if the C3K is limited to only 24K, then we'll just stick w/ default-only.