Matching ISP Bandwidth Rate on a Cisco 3800 Series Router

[dlofstrom1]

We have a 50Mbps Verizon Business circuit that connects to a Cisco 3800 router (3845, specifically, running IOS 12.4(18a)) via a subinterface. We recently had a ticket open with them for something unrelated but they ran a performance analysis and indicated that they were seeing an increasing number of dropped packets on their equipment and asked us to verify the interface configurations on our end. The configuration of the interface that connects to the Verizon equipment is (incidentally, there is only the one subinterface configured on Gi0/1):

interface GigabitEthernet0/1
 description connected to ISP
 no ip address
 duplex full
 speed 100
 media-type rj45
!
interface GigabitEthernet0/1.15
 description connected to ISP
 encapsulation dot1Q 15
 ip address 152.179.192.46 255.255.255.252
!

Our network is pretty much a straight shot out to the internet, though here's a summary of the devices in the chain:

LAN  --------  ASA-5520  -------  GeoLB -------- (Gi0/0) Cisco-3845 (Gi0/1, 100Mbps) -------  (50Mbps) Verizon

I can see where packets may be getting dropped as they traverse between Gi0/1 on the 3800 and the 50Mbps link due to a mismatch in link speeds at both ends. The problem is, I'm not entirely certain of the appropriate way it needs to be configured. It seems the 'rate-limit' command would continue dropping non-conforming packets rather than tune bandwidth, while it seems the 'bandwidth' command is only used for route selection and doesn't impact link speed. We've used the 'srr-queue bandwidth limit' command on some layer 3 switches with connecitivity to similar services, but I'm not certain what the equivalent command is for router code. I've also read some information that alludes to using policy maps, though I'm not certain if this is appropriate or overkill given our scenario. Any help would be appreciated!

[Joseph W. Doherty · Accepted solution]

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Posting

Right, you don't want to rate-limit (or police), what you want to do is shape for the 50 Mbps (on your subinterface).  (NB: BTW, my belief is most shapers don't account for L2 overhead, but a vendor's 50 Mbps usually does.  So, you need to shape slower than vendor's bandwidth.  I find 10 to 15% usually works well.)

For your version of IOS, something as simple as traffic shape average 45000000, on your subinteface may be all you need.  You can also do shaping via CBWFQ, which would be the choice if you want to "manage" any congestion.

[dlofstrom1]

Thanks for pointing me in the right direction Joseph; reading through that info sounds like exactly what we're looking to achieve. I'll test out the suggestions and let you know.