Configuring a Cisco 877 in full bridge mode

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Accessing the device

When sshing into this device with a recent version of ssh you may need to use this command:

  ssh -o HostKeyAlgorithms=ssh-rsa,ssh-dss -o KexAlgorithms=diffie-hellman-group1-sha1 \
-o Ciphers=aes128-cbc,3des-cbc -o MACs=hmac-md5,hmac-sha1 admin@

As there is a bug here and here with new versions of OpenSSH and Cisco's old sshd.

Enabling SSH

If you do not have SSH access to your modem you may need to enable it. The > represents regular "user" mode while # on the terminal represents "privileged". Think of it like "su".

router#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
router(config)#ip domain-name <YOUR DOMAIN>

Make sure to say you want 2048 bits or the router will fall back to ssh 1 and won't allow you to have ssh 2

router(config)#crypto key generate rsa
The name for the keys will be: router.<YOUR DOMAIN>
Choose the size of the key modulus in the range of 360 to 2048 for your
  General Purpose Keys. Choosing a key modulus greater than 512 may take
  a few minutes.
How many bits in the modulus [512]: 2048
% Generating 2048 bit RSA keys, keys will be non-exportable...
router(config)#ip sh version 2

Uploading a configuration file to the router

There are a number of ways of doing this. You can use tftp (trivial ftp) or ssh.

From a server running a sshd

If you have a server running a sshd you can copy the configuration file from that server over your running-config

copy running-config scp://user:password@ip/filename

Enabling the scp server on the modem

router(config)#ip scp server enable
router(config)#aaa new-model 
router(config)#aaa authentication login default local
router(config)#aaa authorization exec default local none
router(config)#username USER privilege 15 password <USER PASSWORD CLEARTEXT>
router(config)#ip scp server enable

Then from your Linux workstation you can upload the configuration file.

The startup-config is the one the modem loads from nvram on boot:

scp admin@ .

The running-config is the one the modem uses that is in memory:

scp admin@ .

Using minicom over serial

If you're completely locked out of your Cisco router you may need to use the serial cable.

For this you can use minicom, the settings you will need to connect to the modem

pu port             /dev/ttyUSB0
pu baudrate         9600
pu bits             8
pu parity           N
pu stopbits         1

Note you'll have to change the serial line to whatever it is on your computer. I used a USB to serial dongle because I have no serial ports.

Saving the configuration file to system

If you make minor edits to the running-config make sure to copy it to the startup-config or it will be lost if you lose power to the modem or reboot it:

copy running-config startup-config

Cisco 877 ADSL Modem in Bridge mode

version 12.4
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
service password-encryption
hostname <HOSTNAME>
logging message-counter syslog
logging buffered 4096 informational
enable secret 5 <SECRET>
aaa new-model
aaa authentication login default local
aaa authentication login local_auth local
aaa authorization exec default local none 
aaa session-id common
dot11 syslog
ip source-route
no ip routing
no ip cef
ip domain name <DOMAIN NAME>
no ipv6 cef
multilink bundle-name authenticated
username admin privilege 15 secret 5 <SECRET>
username USER privilege 15 password 7 <SECRET>
 log config
ip ssh version 2
ip scp server enable
bridge irb
interface ATM0
 no ip address
 no ip route-cache
 no atm ilmi-keepalive
 pvc 8/35 
  encapsulation aal5snap
 bridge-group 1
interface FastEthernet0
interface FastEthernet1
interface FastEthernet2
interface FastEthernet3
interface Dot11Radio0
 no ip address
 no ip route-cache
 speed basic-1.0 basic-2.0 basic-5.5 6.0 9.0 basic-11.0 12.0 18.0 24.0 36.0 48.0 54.0
 station-role root
interface Vlan1
 no ip address
 no ip route-cache
 bridge-group 1
interface BVI1
 ip address
 no ip route-cache
ip default-gateway
ip default-network
ip forward-protocol nd
no ip http server
no ip http secure-server
ip access-list standard SSH_ACCESS
!This is a list of the addresses you want to allow
 permit <IP>
 permit <IP>
bridge 1 protocol ieee
bridge 1 route ip
banner login 
Authorized access only!
Disconnect IMMEDIATELY if you are not an authorized user!

line con 0
 no modem enable
line aux 0
line vty 0 4
 access-class SSH_ACCESS in
 authorization exec local_author
 login authentication login_local
 transport input ssh
scheduler max-task-time 5000

Besides changing the obvious things such as the hostname, domain name, and permitted IPs, you'll also need to verify the ATM0 settings match your ISP's configuration. Specifically: "pvc 8/35" and "encapsulation aal5snap" works for me but may not for you. You will also need to generate the passwords and replace all the instances of <SECRET>.

Generating the passwords

Secret 5 Password

You can generate the hash for the "secret 5" passwords with this OpenSSL command:

   openssl passwd -salt `openssl rand -base64 3` -1 "<YOUR PASSWORD>"

Put the output in <SECRET>

Secret 7 Password

To encrypt the secret 7 password you can use this perl script thanks to m00nie.

# Cisco (type 7) password tool from :D 
# Will either decrypt a _TYPE 7_ password from a cisco device
# or will encrypt a string so that it can be used in a cisco
# device. 
# I made this code to learn more Perl and just out of interest 
# about the type 7 "encryption". The decryption code is already 
# and from a mailing list. The original header from that is below.
# Credits for original code and description,
# SPHiXe, .mudge et al. and for John Bashinski <jbash@CISCO.COM>
# for Cisco IOS password encryption facts.
# Use for any malice or illegal purposes strictly prohibited!

@xlat = ( 0x64, 0x73, 0x66, 0x64, 0x3b, 0x6b, 0x66, 0x6f, 0x41,
          0x2c, 0x2e, 0x69, 0x79, 0x65, 0x77, 0x72, 0x6b, 0x6c,
          0x64, 0x4a, 0x4b, 0x44, 0x48, 0x53 , 0x55, 0x42 );

$loop = 0;
while ($loop == 0) {
	print "\n\n***************************************************************\n";
	print "*    Cisco (type 7) password tool from :D      *\n";
	print "* Use for any malice or illegal purposes strictly prohibited! *\n";
        print "***************************************************************\n\n";
	print "1. Decrypt a password\n";
	print "2. Encrypt plain text\n";
	print "3. Quit\n\n";
	print "Pick either 1, 2 or 3: ";
	chomp ($choice = <STDIN>);
	if ( $choice == 1 ) {
	} elsif ( $choice == 2) {
	} elsif ($choice == 3) {
	} else {
		print "$choice is not a valid option\n";

sub decrypt {
print "Enter the encrypted password: ";
chomp ($epass = <STDIN>);

        	if (!(length($epass) & 1)) {
                	$ep = $epass; 
			$dpass = "";
                	($s, $e) = ($ep =~ /^(..)(.+)/);
                	for ($i = 0; $i < length($e); $i+=2){
                  		$dpass .= sprintf "%c",hex(substr($e,$i,2))^$xlat[$s++];
		print "\nEncrypted pass was: $epass\n";
		print "Decrypted pass is: $dpass\n";

sub encrypt {
	print "Enter the string to encrypt:\n";
	chomp ($ptext = <STDIN>);
	$pt = $ptext;
	$etext = "";
	$n = 2;
	$etext .=  sprintf("%.2o", $n);
	for ($k = 0; $k < length($pt); $k+=1){
		$tmp = ord(substr($pt,$k,1))^$xlat[$n++];
		$etext .= sprintf("%.2X", $tmp);
	print "\nPlain string was: $ptext\n";
	print "Encrypted string is: $etext\n";
# eof