ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1821 –
◆ LOCAL_PREF – This local preference attribute is similar to that of the
MED, but within an AS. It sets a metric which is used between BGP
speakers within an AS. It can help in selecting an outgoing BGP when
an AS has connectivity to multiple ASes or multiple BGP routes even
with the same next hop AS.
◆ ATOMIC_AGGREGATE – This attribute indicates that the routes were
created by aggregating more specific routes. More specific routes may
exist for some the these longer prefixes, but the router chose not to
send them, so as to reduce the size for the AS path parameters.
◆ AGGRATOR – This is an optional attribute that identifies the AS and
router that originally aggregated the routes.
◆ COMMUNITY – This attribute associates routing information with a
community of users. These communities share a common property, and
tagging routes with a community makes it easier for routers to identify
that property and enforce appropriate routing policies.
◆ ORIGINATOR_ID – This attribute is included when a route reflector
reflects a route. Then if the reflector later receives a route with its own
originator ID, a potential routing loop can be broken.
◆ CLUSTER_LIST – This attribute is of a list of the clusters through which
a route has been reflected. Every route reflector adds its own cluster ID
to the list. If the reflector receives a route with its own cluster ID, a
potential routing loop can be broken.
◆ MP_REACH_NLRI – This attribute describes routes for network
protocols other than IPv4. The attribute identifies the protocol with an
address family identifier (AFI) and a subsequent address family
identifier (SAFI). It contains the address of the next hop router for the
destinations, as well as the link level (e.g., Ethernet) addresses for that
next hop. It concludes with the destinations expressed as prefixes.
◆ MP_UNREACH_NLRI – This attribute withdraws non-IPv4 routes. It
includes the route’s AFI, SAFI, and network address prefixes.
◆ EXTENDED-COMMUNITIES – This attribute provides a mechanism for
labeling various information carried in route advertisements. It
provides an extended type field to ensure that communities can be
assigned for a broad range of uses, without fear of overlap.
PATH SELECTION
When there are multiple paths to the same prefix (with the same prefix
length), the information included in route advertisement is used to select
the best path to a destination following the rules shown below.
1. Choose the path with the highest WEIGHT. If the value of this attribute
is the same for more than one candidate, go to the next step.
2. Choose the path with the highest LOCAL-PREF. If the value of this
attribute is the same for more than one candidate, go to the next step.
3. Choose the path that was generated by the local router with the
network or aggregate-address command. If the value of this criteria is
the same for more than one candidate, go to the next step.