Testing E1 links with BERT, instead of installing a loopback and furiously refreshing to check on the counters.
If you can loop back the E1 link, a more thorough test is to run a Bit Error Rate Test or BERT. This is available in both older and new generation Cisco VMIC and NIM WAN cards.
Config terminal
Controller E1 Shelf/slot/port
bert pattern {0s| 1s| 2^15| 2^20| 2^23| alt-0-1| qrss} interval minutes
exit
exit
To view the test performance, use the show controller command. The first few lines contain the important summary information about the number of bits and the error bits if any.
show controller E1 shelf/slot/port