Bgp-4 Routing; Overview Of Bgp Support - Cisco ASR 5500 Administration Manual

Routing
The following command configures the maximum number of equal cost paths that can be submitted by a
routing protocol:
config
context context_name
Notes:
• max_num is an integer from 1 through 10 (releases prior to 18.2) or 1 through 32 (release 18.2+).
• Save your configuration as described in the Verifying and Saving Your Configuration chapter.

BGP-4 Routing

The Border Gateway Protocol 4 (BGP-4) routing protocol is supported through a BGP router process that is
implemented at the context level.
Border Gateway Protocol (BGP) is an inter-AS routing protocol. An Autonomous System (AS) is a set of
routers under a single technical administration that use an interior gateway protocol and common metrics to
route packets within the AS. The set of routers uses an exterior gateway protocol to route packets to other
autonomous systems.
BGP runs over TCP. This eliminates the need for the BGP protocol to implement explicit update fragmentation,
retransmission, acknowledgement, and sequencing information. Any authentication scheme used by TCP may
be used in addition to BGP's own authentication mechanisms.
BGP routers exchange network reachability information with other BGP routers. This information builds a
picture of AS connectivity from which routes are filtered and AS-level policy decisions are enforced.
BGP-4 provides classless inter-domain routing. This includes support for advertising an IP prefix and eliminates
the concept of network class within BGP. BGP-4 also allows the aggregation of routes, including the aggregation
of AS paths.
Important

Overview of BGP Support

Mobile devices communicate to the Internet through Home Agents (HAs). HAs assign IP addresses to the
mobile node from a configured pool of addresses. These addresses are also advertised to Internet routers
through an IP routing protocol to ensure dynamic routing. The BGP-4 protocol is used as a monitoring
mechanism between an HA and Internet router with routing to support Interchassis Session Recovery (ICSR).
(Refer to Interchassis Session Recovery for more information.)
The objective of BGP-4 protocol support is to satisfy routing requirements and monitor communications with
Internet routers. BGP-4 may trigger an active to standby switchover to keep subscriber services from being
interrupted.
The following BGP-4 features are supported:
• Exterior Border Gateway Protocol (EBGP) multi-hop
ip routing maximum-paths [ max_num ]
On the ASR 5000, BGP routes with IPv6 prefix lengths less than /12 and between the range of /64 and
/128 are not supported.
ASR 5500 System Administration Guide, StarOS Release 19
BGP-4 Routing
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