Dell Force10 S55T S55 Configuration Guide FTOS 8.3.5.3 - Page 137

AS1 and AS2, it is called External BGP EBGP, When BGP operates

Page 137 highlights

A transit AS is one that provides connections through itself to separate networks. For example as seen in Figure 8-1, Router 1 can use Router 2 (the transit AS) to connect to Router 4. ISPs are always transit ASs, because they provide connections from one network to another. The ISP is considered to be "selling transit service" to the customer network, so thus the term Transit AS. When BGP operates inside an Autonomous System (AS1 or AS2 as seen in Figure 8-1), it is referred to as Internal BGP (IBGP Interior Border Gateway Protocol). When BGP operates between Autonomous Systems (AS1 and AS2), it is called External BGP (EBGP Exterior Border Gateway Protocol). IBGP provides routers inside the AS with the knowledge to reach routers external to the AS. EBGP routers exchange information with other EBGP routers as well as IBGP routers to maintain connectivity and accessibility. Figure 8-1. BGP Autonomous Zones lpbgp1111 Router 3 Router 5 Router 1 Router 2 Router 4 Exterior BGP (EBGP) Router 7 Router 6 AS 1 Interior BGP (IBGP) AS 2 Interior BGP (IBGP) BGP version 4 (BGPv4) supports classless interdomain routing and aggregate routes and AS paths. BGP is a path vector protocol - a computer network in which BGP maintains the path that update information takes as it diffuses through the network. Updates traveling through the network and returning to the same node are easily detected and discarded. BGP does not use traditional Interior Gateway Protocol (IGP) matrix, but makes routing decisions based on path, network policies and/or rulesets. Unlike most protocols, BGP uses TCP as its transport protocol. Since each BGP routers talking to another router is a session, a BGP network needs to be in "full mesh". This is a topology that has every router directly connected to every other router. For example, as seen in Figure 8-2, four routers connected in a full mesh have three peers each, six routers have 5 peers each, and eight routers in full mesh will have seven peers each. Border Gateway Protocol IPv4 (BGPv4) | 137

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Border Gateway Protocol IPv4 (BGPv4)
|
137
A
transit
AS is one that provides connections through itself to separate networks. For example as seen in
Figure 8-1
, Router 1 can use Router 2 (the transit AS) to connect to Router 4. ISPs are always transit ASs,
because they provide connections from one network to another. The ISP is considered to be “selling transit
service” to the customer network, so thus the term Transit AS.
When BGP operates inside an
Autonomous System
(AS1 or AS2 as seen in
Figure 8-1
), it is
referred to as Internal BGP (IBGP
Interior Border Gateway Protocol
). When BGP operates
between
Autonomous Systems (
AS1 and AS2), it is called External BGP (EBGP
Exterior Border
Gateway Protocol
).
IBGP provides routers inside the AS with the knowledge to reach routers external to
the AS. EBGP routers exchange information with other EBGP routers as well as IBGP routers to maintain
connectivity and accessibility.
Figure 8-1.
BGP Autonomous Zones
BGP version 4 (BGPv4) supports classless interdomain routing and aggregate routes and AS paths.
BGP
is a
path vector protocol
- a computer network in which BGP maintains the path that update
information takes as it diffuses through the network. Updates traveling through the network and
returning to the same node are easily detected and discarded.
BGP does not use traditional Interior Gateway Protocol (IGP) matrix, but makes routing decisions based
on path, network policies and/or rulesets. Unlike most protocols, BGP uses TCP as its transport protocol.
Since each BGP routers talking to another router is a session, a BGP network needs to be in “full mesh”.
This is a topology that has every router directly connected to every other router. For example, as seen in
Figure 8-2
, four routers connected in a full mesh have three peers each, six routers have 5 peers each, and
eight routers in full mesh will have seven peers each.
AS 1
AS 2
Router 2
Router 1
Router 3
Router 4
Router 7
Router 6
Router 5
Interior BGP (IBGP)
Interior BGP (IBGP)
Exterior BGP (EBGP)
lpbgp1111