What are ATM and BVI interfaces?

[gmail2]

We're about to put in a VPN to a new site over a PIX. There is already a router at the office which was provided by a local ISP (who have kindly given us access to the router). We've done some trace route tests to the router and the results are generally quiet good (app 85ms). However, between the next hope of the router and the actual router there's a massive jump (app 140ms). I loged onto the rotuer and done a ping to the next hop router. The response time was about 58ms. Surely this should be virtually instantaneous seing that it's the next hop?

This brings me onto my next question. There are the following interfaces:

Interface                  IP-Address      OK? Method Status                Protocol
ATM0                       unassigned      YES NVRAM  up                    up
BVI1                       <REMOVED>   YES NVRAM  up                    up
Ethernet0                  <REMOVED>   YES NVRAM  up                    up

So initially I thought that this was using an ATM connection (which I know very little about, apart from the fact that they can be quiet slow). However, I've looked up ATM and BVI and they always seem to be linked - but as yet I haven't found out exactly what they are. Apparently the line is 256Kbps



Irish Poetry - Karen O'Connor
Get your Irish Poetry Published
Garten und Landschaftsbau

 

[UnaBomber]

ATM is *not* slow, its a packet switch technology and can be very fast.

BVI is a bridged virtual interface, which is used to tie your PPPoE(/A) to the ATM interface in this situation. The config you are describing fits a xDSL connection

 

[gmail2]

Oh right, OK then. Reason I thought it was slow was because when I looked it up I found this:

http://www.cisco.com/en/US/tech/tk39/tk371/technologies_tech_note09186a00802878e7.shtml

OK, it would probably make sense that it's a DSL connection. But ATM isn't a "physical" connection as such is it (like ethernet, etc). It's just a technology that runs over something else, right? Does that make sense? Any ideas why the reponse time to the next hop is so slow then?

Irish Poetry - Karen O'Connor
Get your Irish Poetry Published
Garten und Landschaftsbau

 

[UnaBomber]

ATM is a layer2 technology, it has definitions for layer 3 and layer 1, but primarily it is used at layer2

its a packet switch technology and can be very fast.

Actually this is not 100% correct ATM is a "cell relay" technology.

I have no idea why the next hop is slow, perhaps you dont have a very good upstream dsl connection. Anyway typically the hop from a DSL router to the central office will be relatively slow compared to router to router over a LAN technology.

 

[gmail2]

Actually this is not 100% correct ATM is a "cell relay" technology.

I knew that was what you meant as I'd read up a bit on ATM already. The DSL connection is 512Kbps - I still would have thought it would be quicker than 85ms. I can hop all the way to

http://www.google.com

in less time than that on our ADSL line (admittedly ours is 2Mbps, but even so. Is it possible that it's being caused by mis-configuration on the router?

Irish Poetry - Karen O'Connor
Get your Irish Poetry Published
Garten und Landschaftsbau

 

[UnaBomber]

I think you are confusing 'speed' and 'delay' (And I dont think i helped by using the terms interchangeably in my previous post).

Delay is the time lag
Speed is the throughput

Simply because you have a lot of speed doesnt mean the delay will be small. For example using satellite to access the internet can give you huge bandwidth but loads of delay because of the huge distances involved.

Anyway, that aside, if you are using a normal dsl connection as your last mile technology, I would think 85ms is to high for latency between your router and the central office. Have you spoke to your provider?

UnaBomber
ccnp mcse2k

 

[lui3]

BVI is bridge virtual interface

it represents the interfaces configured with a bridge group. On switches these interfaces are normally called SVI (switched virtual interfaces)

ex.

interface vlan 1 (switched virtual interface)
ip address 10.1.1.1 255.255.255.0


int fa 0/1
switchport access vlan 1
int fa 0/2
switchport access vlan 1

devices on vlan will be able to communicate directly (without a default gateway) to the svi represented by interface vlan 1


on a router we use IRB or integrated routing and bridging with bridge groups

the BVI (bridge virtual interface) represents the bridge group - very similar to the SVI on the switch

ex

bridge 1 protocol ieee (spanning tree)
bridge 1 route ip (tell it to route on the bvi for ip)

interface BVI1 (1 represents the bridge group)

interface fa 0/0
bridge group 1
int fa 0/1
bridge group 1

devices on fa0/0 and fa 0/1 will bridge and be able to communicate with other routed interfaces via the BVI interface


with ATM its used because they are using RFC1483 PVCs with bridging using PPPoE for authentication at the link level. THey have to have something to assign the ip address to if its not already statically defined

Lui3
CCNP,CCDA,A+/Net+
Cisco Wireless Specialization

 

[plshlpme]

atm is not slow but due to overhead you should make sure the telco is providing you with more bandwidth then you order..

ie. when we want 10mb service the telco puts a 12mb pvc in.

also with bridged pvs's over dsl there is no pppoe. the pvc is permanently added to their network and you have your own virtual link from your office to your isps office.

the bvi is essentially what a dsl modem does...
takes the atm sinal over the rj11 phone wire and bridges it to ethernet where you can assign your layer 3 address and plug in via rj45.

However, between the next hope of the router and the actual router

here are you referring to the hop between your router and its gateway? typically 140ms is very high but youll need to have your telco tell you what your line looks like as far as distance, signal strength and noise.

also was pots splitter installed on this line? or if not have you filtered all other jacks that this phone line appears on?

 

[fartingfrog]

Try adjusting the tcp mtu size, too. This will only make a difference when trying to load web pages that are large, not necessarily on a ping.

router(config)#ip tcp adjust-mss 1492

Also, on any computer trying to get out to the inet, use tcpdoctor to adjust the mtu's to 1452(equal to a router's 1492). Default for a Cisco is 1500, which means that servers that are not configured to handle these sizes or do not answer pings will stop loading the web page (like USBank.com). This only applies to dsl connections, especially authentication over ATM (PPPoA), rather than PPPoE. By the way, I can tell you're a programmer---you think too damn much! Try telnetting into the router and post a "sh run".

Tim