Exercise: duplex mismatch

Importance

A duplex mismatch can cause packet loss of 1-5%, sufficient to reduce TCP performance signficantly without being obviously visible in a traceroute.

• All connections into a hub must be half duplex
• Connections from router to switch, or switch to switch, or router to router, can be full duplex or half duplex. Full duplex gives you more bandwidth for free.
• Many devices auto-negotiate duplex, but it doesn't always work
• If you only set one end to a fixed setting, the other end will fail to autonegotiate. It's therefore vital to set both ends to a fixed setting, if you are going to override auto-negotiation.

Testing for mismatch

Send a burst of pings to the nearest adjacent device. (It's no use sending them to a remote device on the Internet; if it shows a problem, it could be anywhere between here and there. It also wastes bandwidth)

The Unix flood ping is not very good for this, but the 'ping ip' command in enable mode on a Cisco does the job well.

cape-border-1#ping ip
Target IP address: 137.158.216.254
Repeat count [5]: 500
Datagram size [100]: 1400
Timeout in seconds [2]: [Enter]
Extended commands [n]: [Enter]
Sweep range of sizes [n]: 
Type escape sequence to abort.
Sending 500, 1400-byte ICMP Echos to 137.158.216.254, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!
Success rate is 100 percent (500/500), round-trip min/avg/max = 8/11/28 ms
cape-border-1#

Exercise: HSRP

In pairs on the same desk, one of you will provide backup routing for the other person's PC - without making any config changes on the PC itself, which still only has a single default route

• Make sure the network is up and running - i.e. your PCs both have connectivity to the outside world.
• Add a link from E1/0 on the 3600/2600 to the other person's hub

        Backbone                      Backbone
           ^                             ^
           |  3600/2600                  |  2500
        +-------------------+          +------------+
        | E0/1  E0/0   E1/0 |          | E0   E1    |
        +-------------------+          +------------+
                 |       |                    |
                 |       +-----------------+  |
                 |                         |  |
             +-------+                    +-------+
             | hub X |                    | hub Y |
             +-------+                    +-------+
                 |                            |
                 PC                           PC
  

• On the 2500's network (E1 on hub Y), allocate TWO additional IP addresses at the top of the subnet, below the original router.

  e.g. if the PC was .1 and the router was .6, then the two new addresses would be .5 and .4

• On the 2500 port E1, configure one of those addresses as the main interface address, and HSRP for the original router address

  e.g. if the router's E1 address was 137.158.218.6 originally:

  t2-router2#conf t
  t2-router2(config)#int e1
  t2-router2(config-if)#ip address 137.158.218.5 255.255.255.248
  t2-router2(config-if)#no ip proxy-arp
  t2-router2(config-if)#no ip redirects
  t2-router2(config-if)#no shutdown
  t2-router2(config-if)#standby ip 137.158.218.6
  t2-router2(config-if)#standby priority 20
  

  Don't save your config

• On the 3600/2600 port E1/0, configure the other one of those addresses and HSRP for the original router address

  t2-router1#conf t
  t2-router1(config)#int e1/0
  t2-router1(config-if)#ip address 137.158.218.4 255.255.255.248
  t2-router1(config-if)#standby ip 137.158.218.6
  t2-router1(config-if)#standby priority 10
  

  Don't save your config

• Run a continuous ping from the PC on hub Y to 137.158.216.254, and check it is getting answers.
• Unplug the connection into hub Y from each of the two routers in turn. Check that connectivity returns after a short break.

  In each case, also do a traceroute to the outside world and look at the IP address of the first hop; it should show which router is actually handling the outgoing traffic.

• Reload your routers to get back to the previous saved configuration.