Dell S4048T-ON, Configuration Manual

Page: 186 / 1146

MBGP uses either an IPv4 address configured on the interface (which is used to establish the IPv6 session) or a stable IPv4 address that is
available in the box as the next-hop address. As a result, while advertising an IPv6 network, exchange of IPv4 routes does not lead to
martian next-hop message logs.
NOTE: It is possible to configure BGP peers that exchange both unicast and multicast network layer reachability information
(NLRI), but you cannot connect multiprotocol BGP with BGP. Therefore, you cannot redistribute multiprotocol BGP routes into
BGP.
Implement BGP with Dell Networking OS
The following sections describe how to implement BGP on Dell Networking OS.
Additional Path (Add-Path) Support
The add-path feature reduces convergence times by advertising multiple paths to its peers for the same address prefix without replacing
existing paths with new ones. By default, a BGP speaker advertises only the best path to its peers for a given address prefix. If the best
path becomes unavailable, the BGP speaker withdraws its path from its local RIB and recalculates a new best path. This situation requires
both IGP and BGP convergence and can be a lengthy process. BGP add-path also helps switchover to the next new best path when the
current best path is unavailable.
Advertise IGP Cost as MED for Redistributed Routes
When using multipath connectivity to an external AS, you can advertise the MED value selectively to each peer for redistributed routes. For
some peers you can set the internal/IGP cost as the MED while setting others to a constant pre-defined metric as MED value.
Use the
set metric-type internal
command in a route-map to advertise the IGP cost as the MED to outbound EBGP peers when
redistributing routes. The configured
set metric
value overwrites the default IGP cost.
By using the
redistribute
command with the
route-map
command, you can specify whether a peer advertises the standard MED or
uses the IGP cost as the MED.
When configuring this functionality:
• If the
redistribute
command does not have
metric
configured and the BGP peer outbound route-map does have
metric-
type internal
configured, BGP advertises the IGP cost as MED.
• If the
redistribute
command has
metric
configured (
route-map set metric
or
redistribute route-type
metric
) and the BGP peer outbound route-map has
metric-type internal
configured, BGP advertises the metric configured
in the
redistribute
command as MED.
• If BGP peer outbound route-map has
metric
configured, all other metrics are overwritten by this configuration.
NOTE: When redistributing static, connected, or OSPF routes, there is no
metric
option. Simply assign the appropriate route-
map to the redistributed route.
The following table lists some examples of these rules.
Table 10. Redistributed Route Rules
Command Settings BGP Local Routing Information
Base
MED Advertised to Peer
WITH route-map metric-type
internal
MED Advertised to Peer
WITHOUT route-map metric-
type internal
redistribute isis (IGP cost = 20) MED: IGP cost 20 MED = 20 MED = 0
redistribute isis route-map set
metric 50 MED: IGP cost 50 MED: 50 MED: 50 MED: 50 MED: 50
redistribute isis metric 100 MED: IGP cost 100 MED: 100 MED: 100
186 Border Gateway Protocol IPv4 (BGPv4)