Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 620)

596 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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BGP MPLS metric follow IGP
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The RFE 3053 is mainly for BGP to set MED value as IGP cost and advertise out. This feature has
the foundation for this for BGP routes resolving next hop to MPLS LSP tunnel. Then, a route-map is
required to set BGP MED value to IGP metric by set metric-type internal.
Feature information
• The two options compare-lsp-metric and follow-igp are mutually exclusive. Because one option
uses MPLS metric value, the other uses IGP metric.
• This command takes effect immediately and automatically. No clear BGP session or clear
routes command is required.
• show ip bgp nexthop command displays the current BGP next hop IGP cost. When that is using
MPLS LSP as outgoing interface, the value reflects MPLS LSP metric when using
compare-lsp-metric and IGP metric when using follow-igp.
• Combined with another BGP command install-igp-cost under router bgp, user can observe that
IGP cost installed with BGP routes in RTM.
• Combined with BGP outbound policy for set metric-type internal, we can now set Layer-3 VPN
and IP over MPLS routes using IGP metric to send out as MED.
Limitation
• When the user is running IGP throughout the network, and the IGP metric is trustable in the
entire domain, the user may want to rely on this IGP metric to make a best path and forwarding
decision, regardless of whether the forwarding happens in native IP or MPLS encapsulation.
• The current implementation on MPLS metric is manually configured in each LSP. There is no
dynamic way to tie MPLS metric with IGP metric. Thus, when using MPLS LSP as BGP route
outgoing interface, the user loses the ability to tie the forwarding decision with unified IGP
metric.
Configuring BGP next-hop IGP cost
When this command is issued, BGP decides purely based on IGP cost to BGP next hop. After the
decision is made, BGP checks when an MPLS LSP is present, and totally ignores the LSP metric.
compare-lsp-metric Compare metric value among LSP ECMP paths
follow-igp Use IGP metric and ignore LSP metric
Syntax: [no] next-hop-mpls follow-igp
This command becomes mutually exclusive with next-hop-mpls compare-lsp-metric command,
since it is designed by having BGP to ignore all LSP metric value, whether it is specified or by
default. The reason is simple, when this is configured, LSP metric is totally ignored; and when
compare-lsp-metric is configured, the LSP metric can't be ignored.
Also, this new CLI command takes effect immediately and automatically. There is no clear
command required, and BGP automatically re-scans its next hop and changes the decision process
outcome accordingly.