Bgp Sessions (Internal And External); Bgp Path Attributes - Nokia IPSO 4.0 Reference Manual

9
IPv4 unicast and IPv6 unicast
For peering to be established, the routers must share a capability.
If your system is exchanging IPv4 routes over IPv6 or vice versa, use route map commands to
set nexthop to match the family of the routes being exchanged. If they do not match, the routes
will not be active.
Note
Do not use the route redistribution and inbound filter pages of Network Voyager to configure
routing policies for BGP-4++. Instead use the route map commands in the CLI.

BGP Sessions (Internal and External)

BGP supports two basic types of sessions between neighbors: internal (sometimes referred to as
IBGP) and external (EBGP). Internal sessions run between routers in the same autonomous
systems, while external sessions run between routers in different autonomous systems. When
sending routes to an external peer, the local AS number is prepended to the AS path. Routes
received from an internal neighbor have, in general, the same AS path that the route had when
the originating internal neighbor received the route from an external peer.
BGP sessions might include a single metric (Multi-Exit Discriminator or MED) in the path
attributes. Smaller values of the metric are preferred. These values are used to break ties between
routes with equal preference from the same neighbor AS.
Internal BGP sessions carry at least one metric in the path attributes that BGP calls the local
preference. The size of the metric is identical to the MED. Use of these metrics is dependent on
the type of internal protocol processing.
BGP implementations expect external peers to be directly attached to a shared subnet and expect
those peers to advertise next hops that are host addresses on that subnet. This constraint is
relaxed when the multihop option is enabled in the BGP peer template during configuration.
Type internal groups determine the immediate next hops for routes by using the next hop
received with a route from a peer as a forwarding address and uses this to look up an immediate
next hop in IGP routes. Such groups support distant peers, but they need to be informed of the
IGP whose routes they are using to determine immediate next hops.
Where possible, for internal BGP group types, a single outgoing message is built for all group
peers based on the common policy. A copy of the message is sent to every peer in the group,
with appropriate adjustments to the next hop field to each peer. This minimizes the
computational load of running large numbers of peers in these types of groups.

BGP Path Attributes

A path attribute is a list of AS numbers that a route has traversed to reach a destination. BGP
uses path attributes to provide more information about each route and to help prevent routing
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