Chapter 16: Dynamic Routing Protocols
STANDARD Revision 1.0
C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved.
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Loopback Interfaces
To allow BGP sessions to be established between peering routers via loopback interfaces, the routers must communicate
the reachability of the various loopback interfaces. Typically, these interfaces have a network mask of /32. Advertisement
of loopback interfaces is accomplished using an overlay OSPF network.
Multiprotocol BGP (MP-BGP)
BGP was initially developed for IPv4 Internet Classless Inter-Domain Routing (CIDR). Thus, by default, BGP carries IPv4
routing information (via Network Layer Reachability Information (NLRI) together with a number of specific path attributes
like ORIGIN, AS_PATH, NEXT_HOP (as an IPv4 address), etc.
MP-BPG extensions, as described in RFC4760, will enable BGP to carry routes for other Network Layer protocols or Address
Families (AF) like IPv6, VPN IPv4, VPN IPv6, L2VPN, etc. MP-BGP also introduces two new attributes: Multiprotocol
Reachable NLRI (MP_REACH_NLRI) and Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI).
The introduction of MP-BGP will allow for IPv6 BGP peers along with the sharing of IPv6 NLRI information. The introduction
of IPv6 will change BGP behavior in the cases of Prefix Lists, Route Maps, Peer Addressability and General Setup.
Transport Network Layer Address
BGP is based on sessions between peers and uses TCP for transport. The TCP session is formed between two BGP systems
(usually routers) and the two systems would be called BGP Peers or BGP Neighbors. The two systems must be reachable
via IP so, unless they are locally inter-connected, an IGP must run to provide the base for IP connectivity for BGP. The BGP
peers can be internal (IBGP) when the peering session is between two systems within the same ASN, or external (EBGP),
when a router inter-connects with other AS on the Internet.
A BGP router (or speaker) is always identified by a 4 byte integer number, the BGP Identifier. It identifies a BGP speaker
and is the same for every peer and on any interface. The BGP ID has to be unique within an ASN, and it also cannot overlap
with any other BGP speaker within an AS. This is why one of the IP addresses of the BGP system is usually assigned as a
BGP ID.
A BGP speaker also needs a transport IP address to establish BGP peering sessions. This address can be different for each
peer or interface. However, for simplicity and reliability, it is usually recommended that the transport address is the same
for all peers and is set to the IP address of the loopback interface of the BGP speaker. This way, the state of the BGP
speaker is not dependent on the state of an interface and is always up.
