Network connectivity issue when connected to cable modem

[Richard Rosenheim]

If I connect a Raspberry Pi 4B to a router which is then connected to a cable modem, no problems. It doesn't matter which version of Raspberry OS the Pi is running -- Buster, Bullseye or Bookworm.

But, if I connect a Raspberry Pi 4B (specifying a static IP address) and running either Bullseye or Bookworm version of Raspberry OS directly to a cable modem (i.e, no router), then there's network issues. The problem shows up mainly when you attempt to ping, especially an IPv4 address. First of all, it can take 5+ seconds for ping to even display the header line. And if you are pinging either a name that resolves to a IPv4 address, or specified an IPv4 address, (for example, cox.com or 1.1.1.1) then you will get "Destination Host Unreachable" error messages. If you specify a name that resolves to a IPv6 address, (like ibm.com) then ping will eventually start pinging the specified address successfully. I can still connect into the Pi via SSH from an external address, but the connection tends to drop after a little bit.

If the Pi is connected directly to the cable modem (like above) and I'm running the Buster version of Raspberry OS, I have no problems what-so-ever. I can ping both IPv4 and IPv6 addresses with no problem. Ping returns and display the header line without any delay. And I can connect into the Pi via SSH with no problems, and the connection doesn't drop.

It appears that when the version of Raspberry OS moved from Buster to Bullseye, something got changed. I'm guessing in the code for the network stack.

In the case of Buster and Bullseye, I'm specifying the static IP address in the file /etc/dhcpcd.conf by adding these lines:

    interface eth0
    static ip_address=xxx.xxx.xxx.89/24
    static routers=xxx.xxx.xxx.84
    static domain_name_servers=yyy.yyy.105.15 yyy.yyy.106.15

where xxx.xxx.xxx.89 is the static IP assigned to me by the cable company, xxx.xxx.xxx.84 is the IP address of the cable modem, and yyy.yyy.105.15 and yyy.yyy.106.15 are the IP addresses of the cable company's DNS servers.

In the case of Bookworm, I'm specify the static IP addresses by using nmtui.

Here's some sample output of pinging when the Pi is running either Bullseye or Bookworm:

pi@test1:~ $ ping cox.com -c 4
PING cox.com (45.60.45.167) 56(84) bytes of data.
From wsip-xxx.xxx.xxx-89.lv.lv.cox.net (xxx.xxx.xxx.89) icmp_seq=2 Destination Host Unreachable
From xxx.xxx.xxx.89 icmp_seq=3 Destination Host Unreachable
From xxx.xxx.xxx.89 icmp_seq=4 Destination Host Unreachable

--- cox.com ping statistics ---
4 packets transmitted, 0 received, +3 errors, 100% packet loss, time 3073ms
pipe 3

pi@test1:~ $ ping ibm.com -c 4
PING ibm.com(g2600-1406-3400-038d-0000-0000-0000-3831.deploy.static.akamaitechnologies.com (2600:1406:3400:38d::3831)) 56 data bytes
64 bytes from g2600-1406-3400-038d-0000-0000-0000-3831.deploy.static.akamaitechnologies.com (2600:1406:3400:38d::3831): icmp_seq=1 ttl=58 time=14.8 ms
64 bytes from g2600-1406-3400-038d-0000-0000-0000-3831.deploy.static.akamaitechnologies.com (2600:1406:3400:38d::3831): icmp_seq=2 ttl=58 time=14.8 ms
64 bytes from g2600-1406-3400-038d-0000-0000-0000-3831.deploy.static.akamaitechnologies.com (2600:1406:3400:38d::3831): icmp_seq=3 ttl=58 time=14.2 ms
64 bytes from 2600:1406:3400:38d::3831: icmp_seq=4 ttl=58 time=14.3 ms

--- ibm.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 9218ms
rtt min/avg/max/mdev = 14.248/14.532/14.837/0.269 ms

If you have any other question, feel free to contact me.