Brocade Communications Systems ICX 7250 series, Руководство по настройке

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BGP4+ next hop recursion
A device can find the IGP route to the next-hop gateway for a BGP4+ route.
For each BGP4+ route learned, the device performs a route lookup to obtain the IPv6 address of the next hop for the route. A BGP4+
route is eligible for addition in the IPv6 route table only if the following conditions are true:
• The lookup succeeds in obtaining a valid next-hop IPv6 address for the route.
• The path to the next-hop IPv6 address is an IGP path or a static route path.
By default, the software performs only one lookup for the next-hop IPv6 address for the BGP4+ route. If the next hop lookup does not
result in a valid next hop IPv6 address, or the path to the next hop IPv6 address is a BGP4+ path, the BGP4+ route destination is
considered unreachable. The route is not eligible to be added to the IPv6 route table.
The BGP4+ route table can contain a route with a next hop IPv6 address that is not reachable through an IGP route, even though the
device can reach a hop farther away through an IGP route. This can occur when the IGPs do not learn a complete set of IGP routes, so
the device learns about an internal route through IBGP instead of through an IGP. In this case, the IPv6 route table will not contain a route
that can be used to reach the BGP4+ route destination.
To enable the device to find the IGP route to the next-hop gateway for a BGP4+ route, enable recursive next-hop lookups. With this
feature enabled, if the first lookup for a BGP4+ route results in an IBGP path that originated within the same AS, rather than an IGP path
or static route path, the device performs a lookup on the next hop IPv6 address for the next hop gateway. If this second lookup results in
an IGP path, the software considers the BGP4+ route to be valid and adds it to the IPv6 route table. Otherwise, the device performs
another lookup on the next hop IPv6 address of the next hop for the next hop gateway, and so on, until one of the lookups results in an
IGP route.
You must configure a static route or use an IGP to learn the route to the EBGP multihop peer.
BGP4+ NLRIs and next hop attributes
BGP4+ introduces new attributes to handle multiprotocol extensions for BGP.
Multiprotocol BGP (MBGP) is an extension to BGP that enables BGP to carry routing information for multiple address families.
BGP4+ introduces new attributes to handle multiprotocol extensions for BGP:
• Multiprotocol reachable Network Layer Reachability Information (MP_REACH_NLRI): Used to carry the set of reachable
destinations, together with the next hop information, to be used for forwarding to these destinations.
• Multiprotocol unreachable NLRI (MP_UNREACH_NLRI): Used to carry the set of unreachable destinations.
MP_REACH_NLRI and MP_UNREACH_NLRI are optional and non-transitive, so that a BGP4+ speaker that does not support the
multiprotocol capabilities ignores the information carried in these attributes, and does not pass it to other BGP4+ speakers. A BGP
speaker that uses multiprotocol extensions for IPv6 uses the capability advertisement procedures to determine whether the speaker can
use multiprotocol extensions with a particular peer.
The next hop information carried in the MP_REACH_NLRI path attribute defines the network layer address of the border router that will
be used as the next hop to the destinations listed in the MP_NLRI attribute in the UPDATE message.
MP_REACH_NLRI and MP_UNREACH_NLRI carry IPv6 prefixes.
BGP4+ next hop recursion
Brocade FastIron Layer 3 Routing Configuration Guide
400 53-1003903-04