Two Uplinks to Distribution Switch - Total Bandwidth ?

[Rajan R]

Hello,

Have a couple of 9500 Distribution Switch. For any 9300 access switch, there is a 25 Gig Uplink which goes from each distribution switch to the 9300 3nos-Switch stack. Does this mean the total bandwidth is 50Gig

Below is the config I see on one of the 9300. I can see traffic on both interfaces. But not sure if the combined bandwidth is 50Gig.

interface TwentyFiveGigE1/1/1
description Fabric Physical Link
no switchport
dampening
ip address 10.0.XX.XX 255.255.255.254
no ip redirects
ip lisp source-locator Loopback0
ip pim sparse-mode
ip router isis
load-interval 30
speed 25000
bfd interval 100 min_rx 100 multiplier 3
clns mtu 1400
isis network point-to-point
end

!

sh run in Twe2/1/1
Building configuration...

Current configuration : 275 bytes
!
interface TwentyFiveGigE2/1/1
no switchport
dampening
ip address 10.0.XX.XX 255.255.255.252
no ip redirects
ip pim sparse-mode
ip router isis
load-interval 30
speed 25000
bfd interval 100 min_rx 100 multiplier 3
clns mtu 1400
isis network point-to-point
end

Thanks

[Giuseppe Larosa · Accepted solution]

Hello @Rajan R ,

both the uplinks are 25 Gbps link that are L3 and they have isis configured

>> ip router isis

if both ISIS adjacencies are up your device will use ECMP Equal Cost Multi Path , the total bandwidth will be 50 Gbps . However, for a single IP flow with a specific source IP address and speciifc destination IP address the max available bandwidth is still 25 Gbps because ECMP performs flow based load balancing.

A single flow is mapped to single uplink all packets of the flow are sent on the same L3 uplink. For this reason the limit for a single flow is still 25 Gbps.

Hope to help

Giuseppe

[Rajan R]

Thanks a ton.

[Joseph W. Doherty]

BTW, in addition to the information @Giuseppe Larosa has provided, assuming you have a mix of flows that "randomly" use both paths, your average effective bandwidth will be about 37.5 Gbps.

Why?  Because normally flows are not distributed based on link loading.  For example, with two flows there's a 50/50 chance they will use both links vs. both using the same link.

Also BTW, 100% of both link's bandwidth might be used if something like CEF's per packet distribution is used (i.e. a single flow's packets will round robin across both links).  Generally this is NOT recommended, as it can cause out of sequence packet delivery.

[Rajan R]

Thanks. You mean 37.5 Gbps ?

[Joseph W. Doherty]

Correct, I did.