
Configuring IP Services
6-10
Although the Routing Update Response is typically sent as a response, each
router is allowed to send one Unsolicited Routing Update packet between Poll
intervals. This Unsolicited Routing Update is sent either upon a transition to
the UP state, or when there is a neighbor reachability change.
The Poll message and Routing Update Response both use an IP Source Network
Field. The IP Source Network Field contains the IP address of the network to
which both EGP neighbors have an interface. From this network, all distances to
reachable networks (contained in Routing Update Responses) are measured.
Figure 6-5 shows the typical Network Reachability sequence between two routers
that have established an EGP neighbor relationship, and have determined
neighbor reachability through the exchange of Hello and I-H-U messages.
Figure 6-5. Network Reachability Sequence between Two EGP Neighbors
Modes
The EGP router can be configured to operate in one of two gateway modes for any
given IP interface:
• Noncore
When the router is configured as a noncore gateway, the AS to which it
belongs acts as a stub AS. It advertises and forwards only traffic that
originated or is destined for a network within its AS.
ASN = 50 Hello = 30 Poll = 120
Mode = Passive
Router A
ASN = 51 Hello = 30
Poll = 240
Mode = Active
Router B
State = UP
State = UP
Update
ASN = 50 seq = 4
status = UP, list of gate-
ways, networks reachable
through the gateways and
associated distances.
Poll Command
ASN = 51 seq = 4
status = UP
ASN = 51 seq = 4
status = UP, list of gate-
ways, networks reachable
through the gateways and
associated distances.
Update
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