Users Guide

Autonomous systems
BGP autonomous systems are a collection of nodes under a single administration with shared network routing policies. Each AS
has a number, which an Internet authority assigns—you do not assign the BGP number.
The Internet Assigned Numbers Authority (IANA) identifies each network with a unique AS number (ASN). AS numbers 64512
through 65534 are reserved for private purposes. AS numbers 0 and 65535 cannot be used in a live environment. IANA assigns
valid AS numbers in the range of 1 to 64511.
Multihomed AS Maintains connections to more than one other AS. This group allows the AS to remain connected to the
Internet if a complete failure occurs to one of their connections. This type of AS does not allow traffic
from one AS to pass through on its way to another AS.
Stub AS Connected to only one AS.
Transit AS Provides connections through itself to separate networks. For example, Router 1 uses Router 2—the
transit AS, to connect to Router 4. Internet service providers (ISPs) are always a transit AS because they
provide connections from one network to another. An ISP uses a transit AS to sell transit service to a
customer network.
When BGP operates inside an AS - AS1 or AS2, it functions as an Internal Border Gateway Protocol (IBGP). When BGP operates
between AS endpoints - AS1 and AS2, it functions as an External Border Gateway Protocol (EBGP). IBGP provides routers
inside the AS with the path to reach a router external to the AS. EBGP routers exchange information with other EBGP routers
and IBGP routers to maintain connectivity and accessibility.
Classless interdomain routing
BGPv4 supports classless interdomain routing (CIDR) with aggregate routes and AS paths. CIDR defines a network using a
prefix consisting of an IP address and mask, resulting in efficient use of the IPv4 address space. Using aggregate routes reduces
the size of routing tables.
Path-vector routing
BGP uses a path-vector protocol that maintains dynamically updated path information. Path information updates which return
to the originating node are detected and discarded. BGP does not use a traditional Internal Gateway Protocol (IGP) matrix but
makes routing decisions based on path, network policies, and/or rule sets.
Full-mesh topology
In an AS, a BGP network must be in full mesh for routes received from an internal BGP peer to send to another IBGP peer.
Each BGP router talks to all other BGP routers in a session. For example, in an AS with four BGP routers, each router has three
peers; in an AS with six routers, each router has five peers.
Configuration notes
All Dell EMC PowerSwitches except MX-Series, S4200-Series, S5200 Series, and Z9332F-ON:
● If you use eBGP to exchange routes with switches in an SFS environment, the router must directly connect to the switch or
switches present. You must use the interface IP to set up BGP peering.
NOTE: This behavior is applicable only to the S4100-ON series of switches.
● By default, routes that are learned on multiple paths to eBGP peers are advertised to IBGP peers with the next-hop local IP
address. This behavior allows for local repair of atomic failure of any external peers.
● Fast external failover is enabled by default. To disable or re-enable fast external failover, use the [no] fast-externalfallover
command. For the fast-external-fallover command to take effect on an established BGP session, you must reset the session
using the clear ip bgp {* | peer-ipv4-address | peer-ipv6-address} command.
● Enabling the BGP add-paths globally for all BGP neighbors is not supported (the add-path command in ROUTER-BGPv4-AF
or ROUTER-BGPv6-AF mode). To enable the BGP add-path for one neighbor, use the add-path command in ROUTERBGP-
NEIGHBOR-AF mode.
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Layer 3