Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 652)

628 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Configuring BGP VPNs on a PE
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When configuring ECMP hardware forwarding, all outgoing paths must configured in the same VRF.
A hash value is computed for a packet when it is received by XPP. XPP uses the hash value to select
a PRAM that is used to forward the packet to its destination. An ECMP PRAM block consists of 8
PRAMs. The hash value for each outgoing packet on a Customer Edge device interface is calculated
based on the source MAC address, destination MAC address, VLAN ID, source IP address,
destination IP address, IP protocol field, TCP or UDP source port, and TCP or UDP destination port.
The hash value for each incoming packet on the route target is calculated based on the source
MAC address, destination MAC address, VLAN ID, source IP address, destination IP address, IP
protocol field, TCP or UDP source port, TCP or UDP destination port, and a VC label for an MPLS
packet.
Configuring autonomous system number override
There are some situations where a customer wants to connect to a service provider’s BGP or MPLS
VPN network using the same AS number at more than one site. This can create a problem because
it is the default BGP procedure to reject routes from the same AS. One solution to this problem is to
configure a PE router to override the AS_PATH attribute of its BGP neighbor. This is accomplished by
configuring the neighbor ip_address as-override command on the PE. When this is enabled, the PE
device determines when the AS_PATH attribute in a route intended for a neighbor CE contains the
same AS number as the CE. When this is determined, the PE device substitutes its own AS number
for the CEs in the AS_PATH attribute. The CE is then able to receive the route. The following
additional conditions apply when this feature is in effect:
• In a situation where the AS_PATH attribute contains more than one occurrence of the CEs AS
number in the initial sequence, the PE device replaces all those occurrences with its own AS
number.
• The PE device adds its own AS number to the AS_PATH attribute just as it would normally.
The following command configures the PE device to replace its attached CEs AS number with its
own AS number. BGP neighbor at IP address 10.33.36.2 the configuration of PE 2 required to
enable Autonomous System number override for the BGP neighbor CE 2.
To configure a PE device to replace its attached CEs AS number with its own AS number, enter the
following commands at the VRF level of the BGP Config level.
Brocade(config-bgp-vpnv4u)# neighbor 10.33.36.2 as-override
Syntax: [no] neighbor IPaddress as-override
The IPaddress variable is the IP address of the CE whose AS number is being replaced with the PEs
AS number.
Configuring a PE to allow routes with its AS number
BGP rejects routes that contain its own AS number within its AS_PATH attribute to prevent routing
loops. In an MPLS or VPN hub and spoke topology this can stop legitimate routes from being
accepted. The allowas-in command fixes this problem by allowing the user to set a parameter that
disables the AS_PATH check function for routes learned from a specified location.
To configure a PE to disable the AS_PATH check function for routes sent to it by its BGP neighbor (a
CE device with the IP address 10.33.36.2) for a maximum limit of three occurrences of the route,
enter the following command at the BGP VRF configuration level.