OSPF Sham Links Lab Tips

  • Without Sham Links – If two CE sites that are in the same OSPF area, in addition to the MPLS, are also connected by another link i.e. R3 is connected to Area 1 in Site 1 and Area 1 in Site 2, then this is known as the BACKDOOR link.
  • Traffic will be preferred over the backdoor link as routes will appear as INTRA-area routes and therefore will not prefer the MPLS link as those routes are seen as INTER-area or External routes (see below).
  • If both CE sites are in the same area, routes via MPLS will still be seen as Inter-Area routes due to the redistribution between MP-BGP and IGP into the VRF.
  • domain-id needs to match on both ends for routes, as per above, to be seen as Inter-Area routes otherwise it will be seen as External routes when redistribution occurs between MP-BGP and IGP.
  • With Sham Links will create a ‘virtual link’ between PE routers so that it is an extension of that area, i.e. say area 1. This now means the MPLS routes are now seen as INTRA area routers and or we need to do to prefer the MPLS link is to potentially manipulate the OSPF Cost (depending on link speed).
  • Building the Sham Link needs two /32 loopback addresses that has to be advertised by BGP inside the vrf.  This loopback must always be part of the vrf (ip vrf forwarding VRFNAME).
Click here for an excellent short video that explains sham-links and the above very well

IP Bridging Lab Tips

  • Need to enable this at the global config line with bridge irb
  • bridge 111 protocol ieee needed so that spanning-tree can function for the bridge-group.
  • Any protocol that needs to be routed, e.g. IP, needs to be specified with bridge 111 route ip

EIGRP Lab Tips

  • Big Large Dogs eat Red Meat – Easy way to remember to EIGRP metrics; Bandwidth, Load, Delay, Reliability and MTU

Policy Based Routing Lab Tips

  • Works only on the ingress of an interface.
  • ip policy route-map under the interface is PBR for transit traffic.
  • ip local policy route-map under global config is PBR for traffic sourced/destined by the router itself.

BGP Lab Tips

  • unsuppress -map – use this with a route-map that contain specific routes you wish a neighbour to know in case you are summarising using, say, the aggregate-address command.
  • Weight and Local Preference will affect how traffic leaves your AS
  • MED and AS-PREPEND will affect how traffic enters your AS
  • send-community both is needed when setting BGP attributes like no-export
  • Condition-map uses reverse logic. We check to see if a route does NOT exist before we advertise.


RIP Lab Tips

  • Show key chain and show ip protocols to see authenentication stuff.
  • Watch for spaces / trailing spaces e.g. show run | inc Cisco$ or Cisco $
  • Show ip protocols to check for filters and timers etc and passive interfaces.
  • Debug ip rip may show auth and version mismatches ete.

Frame Relay Lab Tips

  • Shut down port first and then configure – this can help against unwanted dynamic maps.
  • Set the hub as the DR for everything e.g. PIM, OSPF.
  • Inverse arp and no arp frame to avoid dynamic entries.
  • Show frame map and make sure nothing says ‘dynamic’. If it does, do ‘clear frame map’, reload or remove the encapsulation frame-relay command which removes everything!
  • No Need to do ‘BROADCAST’ from spoke to spoke if going via hub.
  • enable frame-relay switching in global configuration for switching.
  • frame-relay intf-type dce required on the FR Switch to bring the line protocol up.
  • RIP –

– Check hub has broadcast option on map to each spoke.

– Disable split horizon on hub for spoke to spoke comms or – Neighbour statements or GRE tunnel!