Configuring ospf sham links – Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Configuring BGP VPNs on a PE
6
Configuring OSPF sham links
OSPF can be used to propagate links between a Customer Edge device (CE) and a Provider Edge
device (PE). Normal operation of this type of network assumes that the only connections between
CEs pass through the provider network. However, when other links or routes between the CEs exist
within the same area, problems can arise due to the OSPF preference for Intra-area links over
Inter-area links.
Problems can be avoided by creating a virtual intra-area OSPF link between two PEs. This virtual
link is called a sham link. When the OSPF instances exist in the same area, a sham link causes
OSPF to treat the route through the service provider network as an intra-area link instead of an
inter-area link.
NOTE
When no backdoor link exists, no purpose exists for creating a sham link.
A cost is assigned to the sham link to help the OSPF network determine when to route over the
sham link and when to route over the backdoor link. Because this virtual link (sham-link) appears
as an intra-area link, the OSPF areas in which each of the PEs reside must be the same.
To configure an OSPF sham link, use the command for creating a sham link on both the local
device and the remote PE device. Before attempting to create a sham link, note the following
important information:
•
For sham links to work, OSPF cannot be configured on the loopback interface in the applicable
area.
•
The redistribution of BGP to OSPF must be configured.
•
A BGP VPN4 route to the loopback address must exist in both of the pertinent VRFs’ routing
tables.
•
After the BGP VPN4 route exists in the VRF IP route table, the hello (and other) packet
exchanges can go through for sham links even when the backdoor CE link does not exist.
The first example that follows illustrates the command for creating an OSPF sham link between PE
devices. The command shows the command entry on one device with a source IP address of
10.2.2.1 and destination address of 10.2.2.2. The second example shows the complete
configuration sequence (from both PE devices) and shows the command for viewing the sham link.
(Refer to
702 for the display contents.)
Use this command in the OSPF VRF configuration level.
Brocade(config-ospf-router)# area 1 sham-link 10.2.2.1 10.2.2.2 cost 10
Syntax: [no] area area_id sham-link source_address / destination_address cost cost_value
Possible values:
The area_id variable is the ID number of the OSPF area assigned to the sham ink being defined in
this command.
The source_address variable is the IP address of the source PE device.
The destination_address variable is the IP address of the destination PE device.
The cost_value variable sets the OSPF cost for sending packets over the sham link. This parameter
can be a numeric value in the range 1 – 65535.