Leased Line - Outdated Firmware

[davidenco]

We have a 100Mbit over 1Gbit bearer leased line (United Kingdom) terminated into an ADVA FSP 150-GE102 Pro, connected to a Cisco C1117-4P. The only equipment we manage is everything behind the Cisco, including our DrayTek 2927 router which connects into LAN1 on the Cisco.

The firmware version running on the Cisco is 17.5.1a:

https://software.cisco.com/download/home/286315006/type/282046477/release/Bengaluru-17.5.1a

Since the service went live, our ISP has never pushed software updates to the Cisco and from what I can see, Cisco are promoting version 17.6.5 as being the better firmware. We're having on-going issues with certain websites and services running unbearably slow, that run perfectly fine over any other connection, including 1-bar 4G mobile data.

Speed tests show full speeds, but I believe there's some sort of routing issue happening, and I wondered if these sorts of issues can happen with certain firmware, or even be improved/resolved with newer firmware?

[balaji.bandi]

Do you have any monitoring system in place ? what is the usage of Link at the time of slow web links ?

what timing you having issue ? (may be peak time i am thinking this link overloading i guess ?)

Upgrading the IOS XE helps, but you need to do some testing before you upgrade.

can you post show run / show interface x/x (connected to adva)

[Rich R]

It's possible but unlikely that upgrading IOS will magically fix your issue.
Has the ISP checked all interface stats, CPU levels and logs on the router?
Have you done a packet capture (Wireshark) on an affected device to see what this "unbearably slow" looks like at a packet level?  That may show packet drops/retransmissions or fragmentation.  It will be useful to compare a site that works well to a site that doesn't work well.
Without knowing what network the ISP is using hard to say whether there will be any MTU limitation but if there is then TCP traffic is easily fixed by using TCP MSS adjust somewhere - easily done on Cisco IOS interface - for Draytek will be specific to your model and software version - some guidance I found: https://draytek.co.uk/support/guides/kb-vigor-mtu and https://faq.draytek.com.au/2021/03/15/adjusting-vpn-mss-mtu-settings/
Browsers using QUIC over UDP should automatically deal with MTU limitations or switch to TCP.