ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1818 –
RELATED COMMANDS
area virtual-link (1802)
BORDER GATEWAY PROTOCOL (BGPV4)
BGP OVERVIEW An autonomous system (AS) functions as a separate routing domain under
one administrative authority, which implements its own routing policies. An
AS exchanges routing information within its boundaries using Interior
Gateway Protocols (IGPs) such as RIP or OSPF, and connects to external
organizations or to the Internet using an Exterior Gateway Protocol (EGP).
BGP version 4 is the primary EGP deployed on the Internet today.
A communication session must be maintained between bordering ASs to
support the periodic exchange of routing information. One of the major
design choices for BGP is the use of a TCP connection to exchange routing
information between peers. Exchanging connectivity information over a
reliable transport mechanism effectively delegates all error control
functions to TCP.
The other major innovation for BGP is the use of path vectors which carry
the full list of transit networks, or ASs, traversed between the source and
destination. Loops are prevented simply by checking the path vector to see
if same AS is listed twice. This approach solves many of the scalability
problems encountered when applying distance-vector or link-state
methods to make routing decisions in complex topologies.
EXTERNAL AND
INTERNAL BGP
When connecting to the Internet, external BGP (eBGP) is used. Although
BGP is widely used as an exterior gateway protocol (EGP), it is also used in
many organizations with complex internal networks. Internal networks can
be simplified by exchanging routing information among BGP peers within
the same organization through internal BGP (iBGP) peering sessions.
Hello due The timeout for the next hello message from the neighbor
Adjacency state The adjacency state between these neighbors:
Down – Connection down
Attempt – Connection down, but attempting contact (for non-broadcast
networks)
Init – Have received Hello packet, but communications not yet established
Two-way – Bidirectional communications established
ExStart – Initializing adjacency between neighbors
Exchange – Database descriptions being exchanged
Loading – LSA databases being exchanged
Full – Neighboring routers now fully adjacent
Table 269: show ipv6 ospf virtual-links - display description
Field Description