Cisco CCNP TSHOOT  Troubleshooting Switching Part II – BackboneFast Overview



Backbone fast is configured on EVERY switch in the switching fabric.  Backbone fast uses Root Link Queries (RLQ).  Here is the scenario, a switches Root Port fails and it now thinks that it is the new Root Bridge.  It promotes itself and starts sending BPDUs, but switches in the network do not recognize the switch as being the Root so they send a RLQ to the real root bridge to see if it is still alive.  If the real Root Bridge answers, then the switch send a correction to the switch who lost his old Root Port and it now creates a new Root Port.

Cisco CCNP TSHOOT Enabling and Verifying BackboneFast


Spanning tree backbonefast is configured using the spanning-tree backbonefast command.  Verification of the backbonefast feature is accomplished with the show spanning-tree backbonefast command.

Cisco CCNP TSHOOT Potential STP Problems


Spanning Tree problems occur at either Layer 1 or 2 of the OSI Reference Model.

Cisco CCNP TSHOOT Troubleshooting STP


Accurate / up-to-date network diagrams are key to troubleshooting.

Cisco CCNP TSHOOT Spanning Tree debug Commands


Use debug commands with caution as they can be very CPU intensive and can potentially cause problems in an operation environment.

Cisco CCNP TSHOOT EtherChannel


Benefits of Cisco EtherChannel:

Standards Based

Multiple Platforms

Flexible incremental bandwidth

Load balancing

Resiliency and fast convergence

Ease of management

Transparent to network applications

Supports 100 Megabit, 1 Gigabit and 10 Gigabit Ethernet

Cisco CCNP TSHOOT Troubleshooting First Hop Redundancy Protocols


Hosts are not effected by routers going down when a First Hop Redundancy Protocol like HSRP, VRRP or GLBP are used.  The default gateway on the host is set to a virtual IP Address that is tied to the “active” router.


Cisco CCNP TSHOOT Hot Standby Routing Protocol


HSRP defines a set of routers working together to represent one virtual fault-tolerant router.  HSRP is defined by RFC 2281.

Alternatives to HSRP are:

  • • ICMP Router Discovery Protocol (IRDP) defined by RFC 1256
  • • Virtual Router Discovery Protocol (VRRP) defined by 2338
  • • Gateway Load Balancing Protocol (GLBP)
  • • Routing Information Protocol (RIP)
  • • Proxy ARP
  • • Static Default Gateways

GLBP performs a similar, but not identical, function for the user as the HSRP and the VRRP. HSRP and VRRP protocols allow multiple routers to participate in a virtual router group configured with a virtual IP address. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. The other routers in the group are redundant until the active router fails. These standby routers have unused bandwidth that the protocol is not using. Although multiple virtual router groups can be configured for the same set of routers, the hosts must be configured for different default gateways, which results in an extra administrative burden. GLBP provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets.

Cisco CCNP TSHOOT Locating the Virtual Router MAC Address


Use the show ip arp command in privileged mode in order to identify the HSRP Virtual MAC Address.  Every MAC address is a unique 48 bit address represented in 12 HEX characters.  The first 6 characters identifies a vendor code.  The next 4 characters identifies the MAC address as being a Virtual Router, and the last two characters represents the HSRP group.

Cisco CCNP TSHOOT Active and Standby Router Interaction


All routers listen to Multicast address 

Cisco CCNP TSHOOT Active and Standby Router Interaction


Hello messages are used to communicate between the active and standby HSRP routers.


Cisco CCNP TSHOOT Selecting Active and Standby Routers


The output on the slide above shows state changes the router is going through when selecting the Active and Standby routers.

Cisco CCNP TSHOOT Displaying the Standby Brief Status


The above slide shows an example output from a show standby brief command.

Cisco CCNP TSHOOT Using the debug standby Command


The output from the slide above shows example output from a debug standby command.

Cisco CCNP TSHOOT HSRP Functionality and Example


Remember, the end hosts / servers point to a virtual IP Address that moves with the Active Router, hence the host is not impacted when an HSRP router fails.