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406 Nokia Network Voyager for IPSO 4.0 Reference Guide
attachment or made a policy decision to prefer another route to a network destination. Route
withdrawals are sent when a router makes a new local decision that a network is no longer
reachable.
BGP Multi-Exit Discriminator
Multi-exit Discriminator (MED) values are used to help external neighbors decide which of the
available entry points into an AS are preferred. A lower MED value is preferred over a higher
MED value and breaks the tie between two or more preferred paths.
Note
A BGP session does not accept MEDs from an external peer unless the Accept MED field is
set for an external peer.
BGP Interactions with IGPs
All transit ASes must be able to carry traffic that originates from locations outside of that AS, is
destined to locations outside of that AS, or both. This requires a certain degree of interaction and
coordination between BGP and the Interior Gateway Protocol (IGP) that the particular AS uses.
In general, traffic that originates outside of a given AS passes through both interior gateways
(gateways that support the IGP only) and border gateways (gateways that support both the IGP
and BGP). All interior gateways receive information about external routes from one or more of
the border gateways of the AS that uses the IGP.
Depending on the mechanism used to propagate BGP information within a given AS, take
special care to ensure consistency between BGP and the IGP, since changes in state are likely to
propagate at different rates across the AS. A time window might occur between the moment
when some border gateway (A) receives new BGP routing information (which was originated
from another border gateway (B) within the same AS) and the moment the IGP within this AS
can route transit traffic to the border gateway (B). During that time window, either incorrect
routing or black holes can occur.
To minimize such routing problems, border gateway (A) should not advertise to any of its
external peers a route to some set of exterior destinations associated with a given address prefix
using border gateway (B) until all the interior gateways within the AS are ready to route traffic
destined to these destinations by using the correct exit border gateway (B). Interior routing
should converge on the proper exit gateway before advertising routes that use the exit gateway to
external peers.
If all routers in an AS are BGP speakers, no interaction is necessary between BGP and an IGP. In
such cases, all routers in the AS already have full knowledge of all BGP routes. The IGP is then
only used for routing within the AS, and no BGP routes are imported into the IGP. The user can
perform a recursive lookup in the routing table. The first lookup uses a BGP route to establish
the exit router, while the second lookup determines the IGP path to the exit router.